I.—1. THE nervous ſyſtem has its origin from the brain, and is diſtributed to every part of the body. Thoſe nerves, which ſerve the ſenſes, principally ariſe from that part of the brain, which is lodged in the head; and thoſe, which ſerve the purpoſes of muſcular motion, principally ariſe from that part of the brain, which is lodged in the neck and back, and which is erroneouſly called the ſpinal marrow. The ultimate fibrils of theſe nerves terminate in the immediate or⯑gans of ſenſe and muſcular fibres, and if a ligature be put on any part of their paſſage from the head or ſpine, all motion and perception ceaſe in the parts beneath the ligature.

2. The longitudinal muſcular fibres compoſe the locomotive muſ⯑cles, whoſe contractions move the bones of the limbs and trunk, to which their extremities are attached. The annular or ſpiral muſcu⯑lar [8] fibres compoſe the vaſcular muſcles, which conſtitute the inteſ⯑tinal canal, the arteries, veins, glands, and abſorbent veſſels.

3. The immediate organs of ſenſe, as the retina of the eye, pro⯑bably conſiſt of moving fibrils, with a power of contraction ſimilar to that of the larger muſcles above deſcribed.

4. The cellular membrane conſiſts of cells, which reſemble thoſe of a ſponge, communicating with each other, and connecting toge⯑ther all the other parts of the body.

5. The arterial ſyſtem conſiſts of the aortal and the pulmonary ar⯑tery, which are attended through their whole courſe with their cor⯑reſpondent veins. The pulmonary artery receives the blood from the right chamber of the heart, and carries it to the minute extenſive ramifications of the lungs, where it is expoſed to the action of the air on a ſurface equal to that of the whole external ſkin, through the thin moiſt coats of thoſe veſſels, which are ſpread on the air-cells, which conſtitute the minute terminal ramifications of the wind-pipe. Here the blood changes its colour from a dark red to a bright ſcarlet. It is then collected by the branches of the pulmonary vein, and con⯑veyed to the left chamber of the heart.

6. The aorta is another large artery, which receives the blood from the left chamber of the heart, after it has been thus aerated in the lungs, and conveys it by aſcending and deſcending branches to every other part of the ſyſtem; the extremities of this artery termi⯑nate either in glands, as the ſalivary glands, lacrymal glands, &c. or in capillary veſſels, which are probably leſs involuted glands; in theſe ſome fluid, as ſaliva, tears, perſpiration, are ſeparated from the blood; and the remainder of the blood is abſorbed or drank up by branches of veins correſpondent to the branches of the artery; which are furniſhed with valves to prevent its return; and is thus carried back, after having again changed its colour to a dark red, to the right chamber of the heart. The circulation of the blood in the liver differs from this general ſyſtem; for the veins which drink up the re⯑fluent [9] blood from thoſe arteries, which are ſpread on the bowels and meſentery, unite into a trunk in the liver, and form a kind of ar⯑tery, which is branched into the whole ſubſtance of the liver, and is called the vena portarum; and from which the bile is ſeparated by the numerous hepatic glands, which conſtitute that viſcus.

7. The glands may be divided into three ſyſtems, the convoluted glands, ſuch as thoſe above deſcribed, which ſeparate bile, tears, ſa⯑liva, &c. Secondly, the glands without convolution, as the ca⯑pillary veſſels, which unite the terminations of the arteries and veins; and ſeparate both the mucus, which lubricates the cellular membrane, and the perſpirable matter, which preſerves the ſkin moiſt and flexible. And thirdly, the whole abſorbent ſyſtem, conſiſting of the lacteals, which open their mouths into the ſto⯑mach and inteſtines, and of the lymphatics, which open their mouths on the external ſurface of the body, and on the internal lin⯑ings of all the cells of the cellular membrane, and other cavities of the body.

Theſe lacteal and lymphatic veſſels are furniſhed with numerous valves to prevent the return of the fluids, which they abſorb, and terminate in glands, called lymphatic glands, and may hence be con⯑ſidered as long necks or mouths belonging to theſe glands. To theſe they convey the chyle and mucus, with a part of the perſpirable mat⯑ter, and atmoſpheric moiſture; all which, after having paſſed through theſe glands, and having ſuffered ſome change in them, are carried forward into the blood, and ſupply perpetual nouriſhment to the ſyſ⯑tem, or replace its hourly waſte.

8. The ſtomach and inteſtinal canal have a conſtant vermicular motion, which carries forwards their contents, after the lacteals have drank up the chyle from them; and which is excited into ac⯑tion by the ſtimulus of the aliment we ſwallow, but which becomes occaſionally inverted or retrograde, as in vomiting, and in the iliac paſſion.

II. 1. The word ſenſorium in the following pages is deſigned to expreſs not only the medullary part of the brain, ſpinal marrow, nerves, organs of ſenſe, and of the muſcles; but alſo at the ſame time that living principle, or ſpirit of animation, which reſides throughout the body, without being cognizable to our ſenſes, except by its effects. The changes which occaſionally take place in the ſenſorium, as during the exertions of volition, or the ſenſations of pleaſure or pain, are termed ſenſorial motions.

2. The ſimilarity of the texture of the brain to that of the pan⯑creas, and ſome other glands of the body, has induced the inquirers into this ſubject to believe, that a fluid, perhaps much more ſubtile than the electric aura, is ſeparated from the blood by that organ for the purpoſes of motion and ſenſation. When we recollect, that the electric fluid itſelf is actually accumulated and given out voluntarily by the torpedo and the gymnotus electricus, that an electric ſhock will frequently ſtimulate into motion a paralytic limb, and laſtly that it needs no perceptible tubes to convey it, this opinion ſeems not without probability; and the ſingular figure of the brain and nervous ſyſtem ſeems well adapted to diſtribute it over every part of the body.

For the medullary ſubſtance of the brain not only occupies the ca⯑vities of the head and ſpine, but paſſes along the innumerable ramifi⯑cations of the nerves to the various muſcles and organs of ſenſe. In theſe it lays aſide its coverings, and is intermixed with the ſlender fibres, which conſtitute thoſe muſcles and organs of ſenſe. Thus all theſe diſtant ramifications of the ſenſorium are united at one of their extremities, that is, in the head and ſpine; and thus theſe central parts of the ſenſorium conſtitute a communication between all the or⯑gans of ſenſe and muſcles.

3. A nerve is a continuation of the medullary ſubſtance of the brain from the head or ſpine towards the other parts of the body, wrapped in its proper membrane.

4. The muſcular fibres are moving organs intermixed with that [11] medullary ſubſtance which is continued along the nerves, as men⯑tioned above. They are indued with the power of contraction, and are again elongated either by antagoniſt muſcles, by circulating fluids, or by elaſtic ligaments. So the muſcles on one ſide of the fore⯑arm bend the fingers by means of their tendons, and thoſe on the other ſide of the fore-arm extend them again. The arteries are diſ⯑tended by the circulating blood; and in the necks of quadrupeds there is a ſtrong elaſtic ligament, which aſſiſts the muſcles, which elevate the head, to keep it in its horizontal poſition, and to raiſe it after it has been depreſſed.

5. The immediate organs of ſenſe conſiſt in like manner of moving fibres enveloped in the medullary ſubſtance above mentioned; and are erroneouſly ſuppoſed to be ſimply an expanſion of the nervous me⯑dulla, as the retina of the eye, and the rete mucoſum of the ſkin, which are the immediate organs of viſion, and of touch. Hence when we ſpeak of the contractions of the fibrous parts of the body, we ſhall mean both the contractions of the muſcles, and thoſe of the immediate organs of ſenſe. Theſe fibrous motions are thus diſtin⯑guiſhed from the ſenſorial motions above mentioned.

6. The external organs of ſenſe are the coverings of the immediate organs of ſenſe, and are mechanically adapted for the reception or tranſmiſſion of peculiar bodies, or of their qualities, as the cornea and humours of the eye, the tympanum of the ear, the cuticle of the ſingers and tongue.

7. The word idea has various meanings in the writers of metaphy⯑ſic: it is here uſed ſimply for thoſe notions of external things, which our organs of ſenſe bring us acquainted with originally; and is defined a contraction, or motion, or configuration, of the fibres, which conſtitute the immediate organ of ſenſe; which will be ex⯑plained at large in another part of the work. Synonymous with the word idea, we ſhall ſometimes uſe the words ſenſual motion in con⯑tradiſtinction to muſcular motion.

[12] 8. The word perception includes both the action of the organ of ſenſe in conſequence of the impact of external objects, and our atten⯑tion to that action; that is, it expreſſes both the motion of the organ of ſenſe, or idea, and the pain or pleaſure that ſucceeds or accompa⯑nies it.

9. The pleaſure or pain which neceſſarily accompanies all thoſe perceptions or ideas which we attend to, either gradually ſubſides, or is ſucceeded by other fibrous motions. In the latter caſe it is termed ſenſation, as explained in Sect. V. 2, and VI. 2.—The reader is in⯑treated to keep this in his mind, that through all this treatiſe the word ſenſation is uſed to expreſs pleaſure or pain only in its active ſtate, by whatever means it is introduced into the ſyſtem, without any reference to the ſtimulation of external objects.

10. The vulgar uſe of the word memory is too unlimited for our purpoſe: thoſe ideas which we voluntarily recall are here termed ideas of recollection, as when we will to repeat the alphabet back⯑wards. And thoſe ideas which are ſuggeſted to us by preceding ideas are here termed ideas of ſuggeſtion, as whilſt we repeat the alphabet in the uſual order; when by habits previouſly acquired B is ſuggeſted by A, and C by B, without any effort of deliberation.

11. The word aſſociation properly ſignifies a ſociety or convention of things in ſome reſpects ſimilar to each other. We never ſay in common language, that the effect is aſſociated with the cauſe, though they neceſſarily accompany or ſucceed each other. Thus the con⯑tractions of our muſcles and organs of ſenſe may be ſaid to be aſſociated together, but cannot with propriety be ſaid to be aſſociated with irri⯑tations, or with volition, or with ſenſation; becauſe they are cauſed by them, as mentioned in Sect. IV. When fibrous contractions ſucceed other fibrous contractions, the connection is termed aſſociation; when fibrous contractions ſucceed ſenſorial motions, the connection is termed caſſation; when fibrous and ſenſorial motions reciprocally in⯑troduce each other in progreſſive trains or tribes, it is termed catena⯑tion [13] of animal motions. All theſe connections are ſaid to be produced by habit; that is, by frequent repetition.

12. It may be proper to obſerve, that by the unavoidable idiom of our language the ideas of perception, of recollection, or of imagina⯑tion, in the plural number ſignify the ideas belonging to perception, to recollection, or to imagination; whilſt the idea of perception, of recollection, or of imagination, in the ſingular number is uſed for what is termed "a reflex idea of any of thoſe operations of the ſen⯑ſorium."

13. By the word ſtimulus is not only meant the application of ex⯑ternal bodies to our organs of ſenſe and muſcular fibres, which ex⯑eites into action the ſenſorial power termed irritation; but alſo plea⯑ſure or pain, when they excite into action the ſenſorial power termed ſenſation; and deſire or averſion, when they excite into action the power of volition; and laſtly, the fibrous contractions which precede aſſociation; as is further explained in Sect. XII. 2. 1.

BEFORE the great variety of animal motions can be duly ar⯑ranged into natural claſſes and orders, it is neceſſary to ſmooth the way to this yet unconquered field of ſcience, by removing ſome ob⯑ſtacles which thwart our paſſage. I. To demonſtrate that the retina and other immediate organs of ſenſe poſſeſs a power of motion, and that theſe motions conſtitute our ideas, according to the fifth and ſeventh of the preceding aſſertions, claims our firſt attention.

Animal motions are diſtinguiſhed from the communicated motions, [15] mentioned in the firſt ſection, as they have no mechanical proportion to their cauſe; for the goad of a ſpur on the ſkin of a horſe ſhall induce him to move a load of hay. They differ from the gravitating mo⯑tions there mentioned as they are exerted with equal facility in all di⯑rections, and they differ from the chemical claſs of motions, becauſe no apparent decompoſitions or new combinations are produced in the moving materials.

Hence, when we ſay animal motion is excited by irritation, we do not mean that the motion bears any proportion to the mechanical im⯑pulſe of the ſtimulus; nor that it is affected by the general gravitation of the two bodies; nor by their chemical properties, but ſolely that certain animal fibres are excited into action by ſomething external to the moving organ.

In this ſenſe the ſtimulus of the blood produces the contractions of the heart; and the ſubſtances we take into our ſtomach and bowels irritate them to perform their neceſſary functions. The rays of light excite the retina into animal motion by their ſtimulus; at the ſame time that thoſe rays of light themſelves are phyſically converged to a focus by the inactive humours of the eye. The vibrations of the air irritate the auditory nerve into animal action; while it is probable that the tympanum of the ear at the ſame time undergoes a mecha⯑nical vibration.

To render this circumſtance more eaſy to be comprehended, mo⯑tion may be defined to be a variation of figure; for the whole univerſe may be conſidered as one thing poſſeſſing a certain figure; the mo⯑tions of any of its parts are a variation of this figure of the whole: this definition of motion will be further explained in Section XIV. 2. 2. on the production of ideas.

Now the motions of an organ of ſenſe are a ſucceſſion of configur⯑ations of that organ; theſe configurations ſucceed each other quicker or ſlower; and whatever configuration of this organ of ſenſe, that is, whatever portion of the motion of it is, or has uſually been, attended [16] to, conſtitutes an idea. Hence the configuration is not to be conſi⯑dered as an effect of the motion of the organ, but rather as a part or temporary termination of it; and that, whether a pauſe ſucceeds it, or a new configuration immediately takes place. Thus when a ſuc⯑ceſſion of moving objects are preſented to our view, the ideas of trumpets, horns, lords and ladies, trains and canopies, are confi⯑gurations, that is, parts or links of the ſucceſſive motions of the or⯑gan of viſion.

Theſe motions or configurations of the organs of ſenſe differ from the ſenſorial motions to be deſcribed hereafter, as they appear to be ſimply contractions of the fibrous extremities of thoſe organs, and in that reſpect exactly reſemble the motions or contractions of the larger muſcles, as appears from the following experiment. Place a circular piece of red ſilk about an inch in diameter on a ſheet of white paper in a ſtrong light, as in Plate I.—look for a minute on this area, or till the eye becomes ſomewhat fatigued, and then, gently cloſing your eyes, and ſhading them with your hand, a circular green area of the ſame apparent diameter becomes viſible in the cloſed eye. This green area is the colour reverſe to the red area, which had been pre⯑viouſly inſpected, as explained in the experiments on ocular ſpectra at the end of the work, and in Botanical Garden, P. I. additional note, No. I. Hence it appears, that a part of the retina, which had been fatigued by contraction in one direction, relieves itſelf by exert⯑ing the antagoniſt fibres, and producing a contraction in an oppoſite direction, as is common in the exertions of our muſcles. Thus when we are tired with long action of our arms in one direction, as in hold⯑ing a bridle on a journey, we occaſionally throw them into an oppo⯑ſite poſition to relieve the fatigued muſcles.

Mr. Locke has deſined an idea to be "whatever is preſent to the mind;" but this would include the exertions of volition, and the ſen⯑ſations of pleaſure and pain, as well as thoſe operations of our ſyſ⯑tem, which acquaint us with external objects; and is therefore too [17] unlimited for our purpoſe. Mr. Lock ſeems to have fallen into a further error, by conceiving, that the mind could form a general or abſtract idea by its own operation, which was the copy of no parti⯑cular perception; as of a triangle in general, that was neither acute, obtuſe, nor right angled. The ingenious Dr. Berkley and Mr. Hume have demonſtrated, that ſuch general ideas have no exiſtence in nature, not even in the mind of their celebrated inventor. We ſhall therefore take for granted at preſent, that our recollection or imagination of external objects conſiſts of a partial repetition of the perceptions, which were excited by thoſe external objects, at the time we became acquainted with them; and that our reflex ideas of the operations of our minds are partial repetitions of thoſe opera⯑tions.

II. The following article evinces that the organ of viſion conſiſts of a fibrous part as well as of the nervous medulla, like other white muſcles; and hence, as it reſembles the muſcular parts of the body in its ſtructure, we may conclude, that it muſt reſemble them in poſſeſſing a power of being excited into animal motion.—The ſub⯑ſequent experiments on the optic nerve, and on the colours remaining in the eye, are copied from a paper on ocular ſpectra publiſhed in the ſeventy-ſixth volume of the Philoſ. Tranſ. by Dr. R. Darwin of Shrewſbury; which, as I ſhall have frequent occaſion to refer to, is reprinted in this work, Sect. XL. The retina of an ox's eye was ſuſpended in a glaſs of warm water, and forcibly torn in a few places; the edges of theſe parts appeared jagged and hairy, and did not con⯑tract and become ſmooth like ſimple mucus, when it is diſtended till it breaks; which evinced that it conſiſted of fibres. This fibrous conſtruction became ſtill more diſtinct to the ſight by adding ſome cauſtic alcali to the water; as the adhering mucus was firſt eroded, and the hair-like fibres remained floating in the veſſel. Nor does the degree of tranſparency of the retina invalidate this evidence of its fi⯑brous [18] ſtructure, ſince Leeuwenhoek has ſhewn, that the cryſtalline humour itſelf conſiſts of fibres. Arc. Nat. V. I. 70.

Hence it appears, that as the muſcles conſiſt of larger fibres inter⯑mixed with a ſmaller quantity of nervous medulla, the organ of vi⯑ſion conſiſts of a greater quantity of nervous medulla intermixed with ſmaller fibres. It is probable that the locomotive muſcles of micro⯑ſcopic animals may have greater tenuity than theſe of the retina; and there is reaſon to conclude from analogy, that the other immediate organs of ſenſe, as the portio mollis of the auditory nerve, and the rete mucoſum of the ſkin, poſſeſs a ſimilarity of ſtructure with the retina, and a ſimilar power of being excited into animal motion.

III. The ſubſequent articles ſhew, that neither mechanical im⯑preſſions, nor chemical combinations of light, but that the animal activity of the retina conſtitutes viſion.

1. Much has been conjectured by philoſophers about the momen⯑tum of the rays of light; to ſubject this to experiment a very light horizontal balance was conſtructed by Mr. Michel, with about an inch ſquare of thin leaf-copper ſuſpended at each end of it, as de⯑ſcribed in Dr. Prieſtley's Hiſtory of Light and Colours. The focus of a very large convex mirror was thrown by Dr. Powel, in his lec⯑tures on experimental philoſophy, in my preſence, on one wing of this delicate balance, and it receded from the light; thrown on the other wing, it approached towards the light, and this repeatedly; ſo that no ſenſible impulſe could be obſerved, but what might well be aſcribed to the aſcent of heated air.

Whence it is reaſonable to conclude, that the light of the day muſt be much too weak in its dilute ſtate to make any mechanical impreſ⯑ſion on ſo tenacious a ſubſtance as the retina of the eye.—Add to this, that as the retina is nearly tranſparent, it could therefore make leſs reſiſtance to the mechanical impulſe of light; which, according to the obſervations related by Mr. Melvil in the Edinburgh Literary [19] Eſſays, only communicates heat, and ſhould therefore only commu⯑nicate momentum, where it is obſtructed, reflected, or refracted.—From whence alſo may be collected the final cauſe of this degree of tranſparency of the retina, viz. leaſt by the focus of ſtronger lights, heat and pain ſhould have been produced in the retina, inſtead of that ſtimulus which excites it into animal motion.

2. On looking long on an area of ſcarlet ſilk of about an inch in diameter laid on white paper, as in Plate I. the ſcarlet colour be⯑comes fainter, till at length it entirely vaniſhes, though the eye is kept uniformly and ſteadily upon it. Now if the change or motion of the retina was a mechanical impreſſion, or a chemical tinge of coloured light, the perception would every minute become ſtronger and ſtronger,—whereas in this experiment it becomes every inſtant weaker and weaker. The ſame circumſtance obtains in the conti⯑nued application of ſound, or of ſapid bodies, or of odorous ones, or of tangible ones, to their adapted organs of ſenſe.

Thus when a circular coin, as a ſhilling, is preſſed on the palm of the hand, the ſenſe of touch is mechanically compreſſed; but it is the ſtimulus of this preſſure that excites the organ of touch into ani⯑mal action, which conſtitutes the perception of hardneſs and of figure: for in ſome minutes the perception ceaſes, though the mechanical preſſure of the object remains.

3. Make with ink on white paper a very black ſpot about half an inch in diameter, with a tail about an inch in length, ſo as to reſem⯑ble a tadpole, as in Plate II.; look ſteadfaſtly for a minute on the center of this ſpot, and, on moving the eye a little, the figure of the tadpole will be ſeen on the white part of the paper; which figure of the tadpole will appear more luminous than the other part of the white paper; which can only be explained by ſuppoſing that a part of the retina, on which the tadpole was delineated, to have become more ſenſible to light than the other parts of it, which were expoſed to the [18] [...] [19] [...] [20] white paper; and not from any idea of mechanical impreſſion or che⯑mical combination of light with the retina.

4. When any one turns round rapidly, till he becomes dizzy, and falls upon the ground, the ſpectra of the ambient objects continue to preſent themſelves in rotation, and he ſeems to behold the objects ſtill in motion. Now if theſe ſpectra were impreſſions on a paſſive organ, they either muſt continue as they were received laſt, or not continue at all.

5. Place a piece of red ſilk about an inch in diameter on a ſheet of white paper in a ſtrong light, as in Plate I.; look ſteadily upon it from the diſtance of about half a yard for a minute; then cloſing your eye-lids, cover them with your hands and handkerchief, and a green ſpectrum will be ſeen in your eyes reſembling in form the piece of red ſilk. After ſome ſeconds of time the ſpectrum will diſ⯑appear, and in a few more ſeconds will reappear; and thus alter⯑nately three or four times, if the experiment be well made, till at length it vaniſhes entirely.

6. Place a circular piece of white paper, about four inches in di⯑ameter, in the ſunſhine, cover the center of this with a circular piece of black ſilk, about three inches in diameter; and the center of the black ſilk with a circle of pink ſilk, about two inches in dia⯑meter; and the center of the pink ſilk with a circle of yellow ſilk, about one inch in diameter; and the center of this with a circle of blue ſilk, about half an inch in diameter; make a ſmall ſpot with ink in the center of the blue ſilk, as in Plate III. look ſteadily for a mi⯑nute on this central ſpot, and then cloſing your eyes, and applying your hand at about an inch diſtance before them, ſo as to prevent too much or too little light from paſſing through the eye-lids, and you will ſee the moſt beautiful circles of colours that imagination can con⯑ceive; which are moſt reſembled by the colours occaſioned by pour⯑ing a drop or two of oil on a ſtill lake in a bright day. But theſe [21] circular iriſes of colours are not only different from the colours of the ſilks above mentioned, but are at the ſame time perpetually chang⯑ing as long as they exiſt.

From all theſe experiments it appears, that theſe ſpectra in the eye are not owing to the mechanical impulſe of light impreſſed on the re⯑tina; nor to its chemical combination with that organ; nor to the abſorption and emiſſion of light, as is ſuppoſed, perhaps erroneouſly, to take place in calcined ſhells and other phoſphoreſcent bodies, after having been expoſed to the light: for in all theſe caſes the ſpectra in the eye ſhould either remain of the ſame colour, or gradually decay, when the object is withdrawn; and neither their evaneſcence during the preſence of their object, as in the ſecond experiment, nor their change from dark to luminous, as in the third experiment, nor their rotation, as in the fourth experiment, nor the alternate preſence and evaneſcence of them, as in the fifth experiment, nor the perpetual change of colours of them, as in the laſt experiment, could exiſt.

IV. The ſubſequent articles ſhew, that theſe animal motions or configurations of our organs of ſenſe conſtitute our ideas.

1. If any one in the dark preſſes the ball of his eye, by applying his finger to the external corner of it, a luminous appearance is ob⯑ſerved; and by a ſmart ſtroke on the eye great flaſhes of fire are per⯑ceived. (Newton's Optics.) So that when the arteries, that are near the auditory nerve, make ſtronger pulſations than uſual, as in ſome fevers, an undulating ſound is excited in the ears. Hence it is not the preſence of the light and ſound, but the motions of the organ, that are immediately neceſſary to conſtitute the perception or idea of light and ſound.

2. During the time of ſleep, or in delirium, the ideas of imagina⯑tion are miſtaken for the perceptions of external objects; whence it appears, that theſe ideas of imagination are no other than a reiteration of thoſe motions of the organs of ſenſe, which were originally ex⯑cited by the ſtimulus of external objects: and in our waking hours [22] the ſimple ideas, that we call up by recollection or by imagination, as the colour of red, or the ſmell of a roſe, are exact reſemblances of the ſame ſimple ideas from perception; and in conſequence muſt be a repetition of thoſe very motions.

3. The diſagreeable ſenſation called the tooth-edge is originally excited by the painful jarring of the teeth in biting the edge of the glaſs, or porcelain cup, in which our food was given us in our in⯑fancy, as is further explained in the Section XVI. 10, on Inſtinct.—This diſagreeable ſenſation is afterwards excitable not only by a repe⯑tition of the ſound, that was then produced, but by imagination alone, as I have myſelf frequently experienced; in this caſe the idea of bit⯑ing a china cup, when I imagine it very diſtinctly, or when I ſee an⯑other perſon bite a cup or glaſs, excites an actual pain in the nerves of my teeth. So that this idea and pain ſeem to be nothing more than the reiterated motions of thoſe nerves, that were formerly ſo diſagreeably affected.

Other ideas that are excited by imagination or recollection in many inſtances produce ſimilar effects on the conſtitution, as our percep⯑tions had formerly produced, and are therefore undoubtedly a repe⯑tition of the ſame motions. A ſtory which the celebrated Baron Van Swieton relates of himſelf is to this purpoſe. He was preſent when the putrid carcaſe of a dead dog exploded with prodigious ſtench; and ſome years afterwards, accidentally riding along the ſame road, he was thrown into the ſame ſickneſs and vomiting by the idea of the ſtench, as he had before experienced from the perception of it.

4. Where the organ of ſenſe is totally deſtroyed, the ideas which were received by that organ ſeem to periſh along with it, as well as the power of perception. Of this a ſatisfactory inſtance has fallen under my obſervation. A gentleman about ſixty years of age had been totally deaf for near thirty years: he appeared to be a man of good underſtanding, and amuſed himſelf with reading, and by con⯑verſing either by the uſe of the pen, or by ſigns made with his fin⯑gers, [23] to repreſent letters. I obſerved that he had ſo far forgot the pronunciation of the language, that when he attempted to ſpeak, none of his words had diſtinct articulation, though his relations could ſometimes underſtand his meaning. But, which is much to the point, he aſſured me, that in his dreams he always imagined that people converſed with him by ſigns or writing, and never that he heard any one ſpeak to him. From hence it appears, that with the perceptions of ſounds he has alſo loſt the ideas of them; though the organs of ſpeech ſtill retain ſomewhat of their uſual habits of articu⯑lation.

This obſervation may throw ſome light on the medical treatment of deaf people; as it may be learnt from their dreams whether the auditory nerve be paralytic, or their deafneſs be owing to ſome de⯑fect of the external organ.

It rarely happens that the immediate organ of viſion is perfectly deſtroyed. The moſt frequent cauſes of blindneſs are occaſioned by defects of the external organ, as in cateracts and obfuſcations of the cornea. But I have had the opportunity of converſing with two men, who had been ſome years blind; one of them had a complete gutta ſerena, and the other had loſt the whole ſubſtance of his eyes. They both told me that they did not remember to have ever dreamt of viſible objects, ſince the total loſs of their ſight.

V. Another method of diſcovering that our ideas are animal mo⯑tions of the organs of ſenſe, is from conſidering the great analogy they bear to the motions of the larger muſcles of the body. In the following articles it will appear that they are originally excited into action by the irritation of external objects like our muſcles; are aſſo⯑ciated together like our muſcular motions; act in ſimilar time with them; are fatigued by continued exertion like them; and that the organs of ſenſe are ſubject to inflammation, numbneſs, palſy, con⯑vulſion, and the defects of old age, in the ſame manner as the muſ⯑cular fibres.

[24] 1. All our perceptions or ideas of external objects are univerſally allowed to have been originally excited by the ſtimulus of thoſe ex⯑ternal objects; and it will be ſhewn in a ſucceeding ſection, that it is probable that all our muſcular motions, as well thoſe that are become voluntary as thoſe of the heart and glandular ſyſtem, were originally in like manner excited by the ſtimulus of ſomething external to the organ of motion.

2. Our ideas are alſo aſſociated together after their production pre⯑ciſely in the ſame manner as our muſcular motions; which will likewiſe be fully explained in the ſucceeding ſection.

3. The time taken up in performing an idea is likewiſe much the ſame as that taken up in performing a muſcular motion. A muſi⯑cian can preſs the keys of an harpſichord with his fingers in the or⯑der of a tune he has been accuſtomed to play, in as little time as he can run over thoſe notes in his mind. So we many times in an hour cover our eye-balls with our eye-lids without perceiving that we are in the dark; hence the perception or idea of light is not changed for that of darkneſs in ſo ſmall a time as the twinkling of an eye; ſo that in this caſe the muſcular motion of the eye-lid is performed quicker than the perception of light can be changed for that of darkneſs.—So if a fire-ſtick be whirled round in the dark, a luminous circle appears to the obſerver; if it be whirled ſomewhat ſlower, this circle be⯑comes interrupted in one part; and then the time taken up in ſuch a revolution of the ſtick is the ſame that the obſerver uſes in changing his ideas: thus the [...] of Homer, the long ſhadow of the flying javelin, is elegantly deſigned to give us an idea of its velo⯑city, and not of its length.

4. The fatigue that follows a continued attention of the mind to one object is relieved by changing the ſubject of our thoughts; as the continued movement of one limb is relieved by moving another in its ſtead. Whereas a due exerciſe of the faculties of the mind ſtrengthens and improves thoſe faculties, whether of imagination or [25] recollection; as the exerciſe of our limbs in dancing or fencing in⯑creaſes the ſtrength and agility of the muſcles thus employed.

5. If the muſcles of any limb are inflamed, they do not move without pain; ſo when the retina is inflamed, its motions alſo are painful. Hence light is as intolerable in this kind of ophthalmia, as preſſure is to the finger in the paronychia. In this diſeaſe the patients frequently dream of having their eyes painfully dazzled; hence the idea of ſtrong light is painful as well as the reality. The firſt of theſe facts evinces that our perceptions are motions of the organs of ſenſe; and the latter, that our imaginations are alſo motions of the ſame organs.

6. The organs of ſenſe, like the moving muſcles, are liable to be⯑come benumbed, or leſs ſenſible, from compreſſion. Thus, if any perſon on a light day looks on a white wall, he may perceive the ra⯑mifications of the optic artery, at every pulſation of it, repreſented by darker branches on the white wall; which is evidently owing to its compreſſing the retina during the diaſtole of the artery. Savage Noſolog.

7. The organs of ſenſe and the moving muſcles are alike liable to be affected with palſy, as in the gutta ſerena, and in ſome caſes of deafneſs; and one ſide of the face has ſometimes loſt its power of ſen⯑ſation, but retained its power of motion; other parts of the body have loſt their motions but retained their ſenſation, as in the common he⯑miplagia; and in other inſtances both theſe powers have periſhed together.

8. In ſome convulſive diſeaſes a delirium or inſanity ſupervenes, and the convulſions ceaſe; and converſely the convulſions ſhall ſuper⯑vene, and the delirium ceaſe. Of this I have been a witneſs many times in a day in the paroxyſms of violent epilepſies; which evinces that one kind of delirium is a convulſion of the organs of ſenſe, and that our ideas are the motions of theſe organs: the ſubſequent caſes will illuſtrate this obſervation.

[26] Miſs G [...], a fair young lady, with light eyes and hair, was ſeized with moſt violent convulſions of her limbs, with outrageous hic⯑cough, and moſt vehement efforts to vomit: after near an hour was elapſed this tragedy ceaſed, and a calm talkative delirium ſupervened for about another hour; and theſe relieved each other at intervals during the greateſt part of three or four days. After having carefully conſidered this diſeaſe, I thought the convulſions of her ideas leſs dan⯑gerous than thoſe of her muſcles; and having in vain attempted to make any opiate continue in her ſtomach, an ounce of laudanum was rubbed along the ſpine of her back, and a dram of it was uſed as an enema; by this medicine a kind of drunken delirium was continued many hours; and when it ceaſed the convulſions did not return; and the lady continued well many years, except ſome ſlighter relapſes, which were relieved in the ſame manner.

Miſs H [...], an accompliſhed young lady, with light eyes and hair, was ſeized with convulſions of her limbs, with hiccough, and efforts to vomit, more violent than words can expreſs; theſe conti⯑nued near an hour, and were ſucceeded with a cataleptic ſpaſm of one arm, with the hand applied to her head; and after about twenty minutes theſe ſpaſms ceaſed, and a talkative reverie ſupervened for near another hour, from which no violence, which it was proper to uſe, could awaken her. Theſe periods of convulſions, firſt of the muſcles, and then of the ideas, returned twice a day for ſeveral weeks; and were at length removed by great doſes of opium, after a great va⯑riety of other medicines and applications had been in vain experienced. This lady was ſubject to frequent relapſes, once or twice a year for many years, and was as frequently relieved by the ſame method.

Miſs W [...], an elegant young lady, with black eyes and hair, had ſometimes a violent pain of her ſide, at other times a moſt painful ſtrangury, which were every day ſucceeded by delirium; which gave a temporary relief to the painful ſpaſms. After the vain exhibition [27] of variety of medicines and applications by different phyſicians, for more than a twelvemonth, ſhe was directed to take ſome doſes of opium, which were gradually increaſed, by which a drunken deliri⯑um was kept up for a day or two, and the pains prevented from re⯑turning. A fleſh diet, with a little wine or beer, inſtead of the low regimen ſhe had previouſly uſed, in a few weeks completely eſta⯑bliſhed her health; which, except a few relapſes, has continued for many years.

9. Laſtly, as we advance in life all the parts of the body become more rigid, and are rendered leſs ſuſceptible of new habits of motion, though they retain thoſe that were before eſtabliſhed. This is ſen⯑ſibly obſerved by thoſe who apply themſelves late in life to muſic, fencing, or any of the mechanic arts. In the ſame manner many elderly people retain the ideas they had learned early in life, but find great difficulty in acquiring new trains of memory; inſomuch that in extreme old age we frequently ſee a forgetfulneſs of the buſineſs of yeſterday, and at the ſame time a circumſtantial remembrance of the amuſements of their youth; till at length the ideas of recollec⯑tion and activity of the body gradually ceaſe together,—ſuch is the condition of humanity!—and nothing remains but the vital motions and ſenſations.

VI. 1. In oppoſition to this doctrine of the production of our ideas, it may be aſked, if ſome of our ideas, like other animal motions, are voluntary, why can we not invent new ones, that have not been re⯑ceived by perception? The anſwer will be better underſtood after having peruſed the ſucceeding ſection, where it will be explained, that the muſcular motions likewiſe are originally excited by the ſtimulus of bodies external to the moving organ; and that the will has only the power of repeating the motions thus excited.

2. Another objector may aſk, Can the motion of an organ of ſenſe reſemble an odour or a colour? To which I can only anſwer, that it has not been demonſtrated that any of our ideas reſemble the objects [28] that excite them; it has generally been believed that they do not; but this ſhall be diſcuſſed at large in Sect. XIV.

3. There is another objection that at firſt view would ſeem leſs eaſy to ſurmount. After the amputation of a foot or a finger, it has frequently happened, that an injury being offered to the ſtump of the amputated limb, whether from cold air, too great preſſure, or other accidents, the patient has complained of a ſenſation of pain in the foot or finger, that was cut off. Does not this evince that all our ideas are excited in the brain, and not in the organs of ſenſe? This objec⯑tion is anſwered, by obſerving that our ideas of the ſhape, place, and ſolidity of our limbs, are acquired by our organs of touch and of ſight, which are ſituated in our fingers and eyes, and not by any ſenſations in the limb itſelf.

In this caſe the pain or ſenſation, which formerly has ariſen in the foot or toes, and been propagated along the nerves to the central part of the ſenſorium, was at the ſame time accompanied with a viſible idea of the ſhape and place, and with a tangible idea of the ſolidity of the affected limb: now when theſe nerves are afterwards affected by any injury done to the remaining ſtump with a ſimilar degree or kind of pain, the ideas of the ſhape, place, or ſolidity of the loſt limb, return by aſſociation; as theſe ideas belong to the organs of ſight and touch, on which they were firſt excited.

4. If you wonder what organs of ſenſe can be excited into motion, when you call up the ideas of wiſdom or benevolence, which Mr. Locke has termed abſtracted ideas; I aſk you by what organs of ſenſe you firſt became acquainted with theſe ideas? And the anſwer will be reciprocal; for it is certain that all our ideas were originally ac⯑quired by our organs of ſenſe; for whatever excites our perception muſt be external to the organ that perceives it, and we have no other inlets to knowledge but by our perceptions: as will be further explained in Section XIV. and XV. on the Productions and Claſſes of Ideas.

VII. If our recollection or imagination be not a repetition of animal movements, I aſk, in my turn, What is it? You tell me it conſiſts of images or pictures of things. Where is this extenſive canvas hung up? or where are the numerous receptacles in which thoſe are depo⯑ſited? or to what elſe in the animal ſyſtem have they any ſimili⯑tude?

That pleaſing picture of objects, repreſented in miniature on the retina of the eye, ſeems to have given riſe to this illuſive oratory! It was forgot that this repreſentation belongs rather to the laws of light, than to thoſe of life; and may with equal elegance be ſeen in the camera obſcura as in the eye; and that the picture vaniſhes for ever, when the object is withdrawn.

I. ALL the fibrous contractions of animal bodies originate from the ſenſorium, and reſolve themſelves into four claſſes, correſpondent with the four powers or motions of the ſenſorium above deſcribed, and from which they have their cauſation.

1. Theſe fibrous contractions were originally cauſed by the irrita⯑tions excited by objects, which are external to the moving organ. As the pulſations of the heart are owing to the irritations excited by the ſtimulus of the blood; and the ideas of perception are owing to the irritations excited by external bodies.

2. But as painful or pleaſurable ſenſations frequently accompanied thoſe irritations, by habit theſe fibrous contractions became cauſeable by the ſenſations, and the irritations ceaſed to be neceſſary to their production. As the ſecretion of tears in grief is cauſed by the ſenſa⯑tion of pain; and the ideas of imagination, as in dreams or delirium, are excited by the pleaſure or pain, with which they were formerly accompanied.

3. But as the efforts of the will frequently accompanied theſe pain⯑ful or pleaſureable ſenſations, by habit the fibrous contractions became cauſable by volition; and both the irritations and ſenſations ceaſed to be neceſſary to their production. As the deliberate locomotions of [35] the body, and the ideas of recollection, as when we will to repeat the alphabet backwards.

4. But as many of theſe fibrous contractions frequently accom⯑panied other fibrous contractions, by habit they became cauſable by their aſſociations with them; and the irritations, ſenſations, and volition, ceaſed to be neceſſary to their production. As the actions of the muſcles of the lower limbs in fencing are aſſociated with thoſe of the arms; and the ideas of ſuggeſtion are aſſociated with other ideas, which precede or accompany them; as in repeating care⯑leſsly the alphabet in its uſual order after having began it.

II. We ſhall give the following names to theſe four claſſes of fibrous motions, and ſubjoin their definitions.

1. Irritative motions. That exertion or change of the ſen⯑ſorium, which is cauſed by the appulſes of external bodies, either ſimply ſubſides, or is ſucceeded by ſenſation, or it produces fibrous motions; it is termed irritation, and irritative motions are thoſe contractions of the muſcular fibres, or of the organs of ſenſe, that are immediately conſequent to this exertion or change of the ſenſorium.

2. Senſitive motions. That exertion or change of the ſenſorium, which conſtitutes pleaſure or pain, either ſimply ſubſides, or is ſucceeded by volition, or it produces fibrous motions; it is termed ſenſation, and the ſenſitive motions are thoſe contractions of the muſ⯑cular fibres, or of the organs of ſenſe, that are immediately conſe⯑quent to this exertion or change of the ſenſorium.

3. Voluntary motions. That exertion or change of the ſen⯑ſorium, which conſtitutes deſire or averſion, either ſimply ſubſides, or is ſucceeded by fibrous motions; it is then termed volition, and voluntary motions are thoſe contractions of the muſcular fibres, or of the organs of ſenſe, that are immediately conſequent to this exertion or change of the ſenſorium.

[36] 4. Aſſociate motions. That exertion or change of the ſenſo⯑rium, which accompanies fibrous motions, either ſimply ſubſides, or is ſucceeded by ſenſation or volition, or it produces other fibrous motions; it is then termed aſſociation, and the aſſociate motions are thoſe contractions of the muſcular fibres, or of the organs of ſenſe, that are immediately conſequent to this exertion or change of the ſenſorium.

I. 1. MANY of our muſcular motions are excited by perpetual ir⯑ritations, as thoſe of the heart and arterial ſyſtem by the circumfluent blood. Many other of them are excited by intermitted irritations, as thoſe of the ſtomach and bowels by the aliment we ſwallow; of the bile-ducts by the bile; of the kidneys, pancreas, and many other glands, by the peculiar fluids they ſeparate from the blood; and thoſe of the lacteal and other abſorbent veſſels by the chyle, lymph, and moiſture of the atmoſphere. Theſe motions are accelerated or re⯑tarded, as their correſpondent irritations are increaſed or diminiſhed, without our attention or conſciouſneſs, in the ſame manner as the various ſecretions of fruit, gum, reſin, wax, and honey, are pro⯑duced in the vegetable world, and as the juices of the earth and the moiſture of the atmoſphere are abſorbed by their roots and foliage.

[38] 2. Other muſcular motions, that are moſt frequently connected with our ſenſations, as thoſe of the ſphincters of the bladder and anus, and the muſculi erectores penis, were originally excited into motion by irritation, for young children make water, and have other evacua⯑tions without attention to theſe circumſtances; "et primis etiam ab incunabulis tenduntur ſaepius puerorum penes, amore nondum ex⯑pergefacto." So the nipples of young women are liable to become turgid by irritation, long before they are in a ſituation to be excited by the pleaſure of giving milk to the lips of a child.

3. The contractions of the larger muſcles of our bodies, that are moſt frequently connected with volition, were originally excited into action by internal irritations: as appears from the ſtretching or yawn⯑ing of all animals after long ſleep. In the beginning of ſome fevers this irritation of the muſcles produces perpetual ſtretching and yawn⯑ing; in other periods of fever an univerſal reſtleſſneſs ariſes from the ſame cauſe, the patient changing the attitude of his body every mi⯑nute. The repeated ſtruggles of the foetus in the uterus muſt be owing to this internal irritation: for the foetus can have no other in⯑ducement to move its limbs but the toedium or irkſomeneſs of a con⯑tinued poſture.

The following caſe evinces, that the motions of ſtretching the limbs after a continued attitude are not always owing to the power of the will. Mr. Dean, a maſon, of Auſtry in Leiceſterſhire had the ſpine of the third vertebra of the back enlarged; in ſome weeks his lower extremities became feeble, and at length quite paralytic: nei⯑ther the pain of bliſters, the heat of fomentations, nor the utmoſt ef⯑forts of the will could produce the leaſt motion in theſe limbs; yet twice or thrice a day for many months his feet, legs, and thighs, were affected for many minutes with forceable ſtretchings, attended with the ſenſation of fatigue; and he at length recovered the uſe of his limbs, though the ſpine continued protuberant. The ſame cir⯑cumſtance is frequently ſeen in a leſs degree in the common hemipla⯑gia; [39] and when this happens, I have believed repeated and ſtrong ſhocks of electricity to have been of great advantage.

4. In like manner the various organs of ſenſe are originally excited into motion by various external ſtimuli adapted to this purpoſe, which motions are termed perceptions or ideas; and many of theſe motions during our waking hours are excited by perpetual irritation, as thoſe of the organs of hearing and of touch. The former by the conſtant low indiſtinct noiſes that murmur around us, and the latter by the weight of our bodies on the parts which ſupport them; and by the unceaſing variations of the heat, moiſture, and preſſure of the atmoſ⯑phere; and theſe ſenſual motions, preciſely as the muſcular ones above mentioned, obey their correſpondent irritations without our at⯑tention or conſciouſneſs.

5. Other claſſes of our ideas are more frequently excited by our ſenſations of pleaſure or pain, and others by volition: but that theſe have all been originally excited by ſtimuli from external objects, and only vary in their combinations or ſeparations, has been fully evinced by Mr. Locke; and are by him termed the ideas of perception in con⯑tradiſtinction to thoſe, which he calls the ideas of reflection.

II. 1. Theſe muſcular motions, that are excited by perpetual irri⯑tation, are nevertheleſs occaſionally excitable by the ſenſations of pleaſure or pain, or by volition, as appears by the palpitation of the heart from fear, the increaſed ſecretion of ſaliva at the ſight of agree⯑able food, and the glow on the ſkin of thoſe who are aſhamed. There is an inſtance told in the Philoſophical Tranſactions of a man, who could for a time ſtop the motion of his heart when he pleaſed; and Mr. D. has often told me, he could ſo far increaſe the periſtaltic mo⯑tion of his bowels by voluntary efforts, as to produce an evacuation by ſtool at any time in half an hour.

2. In like manner the ſenſual motions, or ideas, that are excited by perpetual irritation, are nevertheleſs occaſionally excitable by ſen⯑ſation or volition; as in the night, when we liſten under the influ⯑ence [40] of fear, or from voluntary attention, the motions excited in the organ of hearing by the whiſpering of the air in our room, the pul⯑ſation of our own arteries, or the faint beating of a diſtant watch, become objects of perception.

III. Innumerable trains or tribes of other motions are aſſociated with theſe muſcular motions which are excited by irritation; as by the ſtimulus of the blood in the right chamber of the heart, the lungs are induced to expand themſelves; and the pectoral and intercoſtal muſ⯑cles, and the diaphragm, act at the ſame time by their aſſociations with them. And when the pharinx is irritated by agreeable food, the muſcles of deglutition are brought into action by aſſociation. Thus when a greater light falls on the eye, the iris is brought into action without our attention; and the ciliary proceſs, when the focus is formed before or behind the retina, by their aſſociations with the in⯑creaſed irritative motions of the organ of viſion. Many common ac⯑tions of life are produced in a ſimilar manner. If a fly ſettle on my forehead, whilſt I am intent on my preſent occupation, I diſlodge it with my finger, without exciting my attention or breaking the train of my ideas.

2. In like manner the irritative ideas ſuggeſt to us many other trains or tribes of ideas that are aſſociated with them. On this kind of connection, language, letters, hieroglyphics, and ever kind of ſym⯑bol, depend. The ſymbols themſelves produce irritative ideas, or ſenſual motions, which we do not attend to; and other ideas, that are ſucceeded by ſenſation, are excited by their aſſociation with them. And as theſe irritative ideas make up a part of the chain of our wak⯑ing thoughts, introducing other ideas that engage our attention, though themſelves are unattended to, we find it very difficult to in⯑veſtigate by what ſteps many of our hourly trains of ideas gain their admittance.

It may appear paradoxical, that ideas can exiſt, and not be attended to; but all our perceptions are ideas excited by irritation, and ſuc⯑ceeded [41] by ſenſation. Now when theſe ideas excited by irritation give us neither pleaſure nor pain, we ceaſe to attend to them. Thus whilſt I am walking through that grove before my window, I do not run againſt the trees or the benches, though my thoughts are strenuouſly exerted on ſome other object. This leads us to a diſtinct knowledge of irritative ideas, for the idea of the tree or bench, which I avoid, exiſts on my retina, and induces by aſſociation the action of certain locomotive muſcles; though neither itſelf nor the actions of thoſe muſcles engage my attention.

Thus whilſt we are converſing on this ſubject, the tone, note, and articulation of every individual word forms its correſpondent irri⯑tative idea on the organ of hearing; but we only attend to the aſſoci⯑ated ideas, that are attached by habit to theſe irritative ones, and are ſucceeded by ſenſation; thus when we read the words "PRINTING⯑PRESS" we do not attend to the ſhape, ſize, or exiſtence of the let⯑ters which compoſe theſe words, though each of them excites a cor⯑reſpondent irritative motion of our organ of viſion, but they intro⯑duce by aſſociation our idea of the moſt uſeful of modern inventions; the capacious reſervoir of human knowledge, whoſe branching ſtreams diffuſe ſciences, arts, and morality, through all nations and all ages.

I. 1. MANY of the motions of our muſcles, that are excited into action by irritation, are at the ſame time accompanied with painful or pleaſurable ſenſations; and at length become by habit cauſable by the ſenſations. Thus the motions of the ſpincters of the bladder and anus were originally excited into action by irritation; for young children give no attention to theſe evacuations; but as ſoon as they become ſenſible of the inconvenience of obeying theſe irritations, they ſuffer the water or excrement to accumulate, till it diſagreeably affects them; and the action of thoſe ſpincters is then in conſequence of this diſagreeable ſenſation. So the ſecretion of ſaliva, which in young children is copiouſly produced by irritation, and drops from their mouths, is frequently attended with the agreeable ſenſation produced by the maſtication of taſteful food; till at length the ſight of ſuch food to a hungry perſon excites into action theſe ſalival glands; as is ſeen in the ſlavering of hungry dogs.

The motions of thoſe muſcles, which are affected by laſcivious ideas, and thoſe which are exerted in ſmiling, weeping, ſtarting from fear, and winking at the approach of danger to the eye, and at times the actions of every large muſcle of the body become cauſable by our [43] ſenſations. And all theſe motions are performed with ſtrength and velocity in proportion to the energy of the ſenſation that excites them, and the quantity of ſenſorial power.

2. Many of the motions of our organs of ſenſe, or ideas, that were originally excited into action by irritation, become in like manner more frequently cauſable by our ſenſations of pleaſure or pain. Theſe motions are then termed the ideas of imagination, and make up all the ſcenery and tranſactions of our dreams. Thus when any painful or pleaſurable ſenſations poſſeſs us, as of love, anger, fear; whether in our ſleep or waking hours, the ideas, that have been formerly ex⯑cited by the objects of theſe ſenſations, now vividly recur before us by their connection with theſe ſenſations themſelves. So the fair ſmil⯑ing virgin, that excited your love by her preſence, whenever that ſenſation recurs, riſes before you in imagination; and that with all the pleaſing circumſtances, that had before engaged your attention. And in ſleep, when you dream under the influence of fear, all the robbers, fires, and precipices, that you formerly have ſeen or heard of, ariſe before you with terrible vivacity. All theſe ſenſual motions, like the muſcular ones above mentioned, are performed with ſtrength and velocity in proportion to the energy of the ſenſation of pleaſure or pain, which excites them, and the quantity of ſenſorial power.

II. 1. Many of theſe muſcular motions above deſcribed, that are moſt frequently excited by our ſenſations, are nevertheleſs occaſion⯑ally cauſable by volition; for we can ſmile or frown ſpontaneouſly, can make water before the quantity or acrimony of the urine pro⯑duces a diſagreeable ſenſation, and can voluntarily maſticate a nauſeous drug, or ſwallow a bitter draught, though our ſenſation would ſtrongly diſſuade us.

2. In like manner the ſenſual motions, or ideas, that are moſt fre⯑quently excited by our ſenſations, are nevertheleſs occaſionally cauſe⯑able by volition, as we can ſpontaneouſly call up our laſt night's dream before us, tracing it induſtriouſly ſtep by ſtep through all its [44] variety of ſcenery and tranſaction; or can voluntarily examine or re⯑peat the ideas, that have been excited by our diſguſt or admiration.

III. 1. Innumerable trains or tribes of motions are aſſociated with theſe ſenſitive muſcular motions above mentioned; as when a drop of water falling into the wind-pipe diſagreeably affects the air-veſſels of the lungs, they are excited into violent action; and with theſe ſenſitive motions are aſſociated the actions of the pectoral and inter⯑coſtal muſcles, and the diaphragm; till by their united and repeated ſuccuſſions the drop is returned through the larinx. The ſame occurs when any thing diſagreeably affects the noſtrils, or the ſtomach, or the uterus; variety of muſcles are excited by aſſociation into forcible action, not to be ſuppreſſed by the utmoſt efforts of the will; as in ſneezing, vomiting, and parturition.

2. In like manner with theſe ſenſitive ſenſual motions, or ideas of imagination, are aſſociated many other trains or tribes of ideas, which by ſome writers of metaphyſics have been claſſed under the terms of reſemblance, cauſation, and contiguity; and will be more fully treated of hereafter.

I. 1. Thoſe muſcles of the body that are attached to bones, have in general their principal connections with volition, as I move my pen or raiſe my body. Theſe motions were originally excited by ir⯑ritation, as was explained in the ſection on that ſubject, afterwards the ſenſations of pleaſure or pain, that accompanied the motions thus excited, induced a repetition of them; and at length many of them [46] were voluntarily practiſed in ſucceſſion or in combination for the common purpoſes of life, as in learning to walk, or to ſpeak; and are performed with ſtrength and velocity in proportion to the energy of the volition, that excites them, and the quantity of ſen⯑ſorial power.

2. Another great claſs of voluntary motions conſiſts of the ideas of recollection. We will to repeat a certain train of ideas, as of the alphabet backwards; and if any ideas, that do not belong to this intended train, intrude themſelves by other connections, we will to reject them, and voluntarily perſiſt in the determined train. So at my approach to a houſe which I have but once viſited, and that at the diſtance of many months, I will to recollect the names of the numerous family I expect to ſee there, and I do recollect them.

On this voluntary recollection of ideas our faculty of reaſon de⯑pends, as it enables us to acquire an idea of the diſſimilitude of any two ideas. Thus if you voluntarily produce the idea of a right-angled triangle, and then of a ſquare; and after having ex⯑cited theſe ideas repeatedly, you excite the idea of their difference, which is that of another right-angled triangle inverted over the former; you are ſaid to reaſon upon this ſubject, or to compare your ideas.

Theſe ideas of recollection, like the muſcular motions above mentioned, were originally excited by the irritation of external bodies, and were termed ideas of perception: afterwards the plea⯑ſure or pain, that accompanied theſe motions, induced a repetition of them in the abſence of the external body, by which they were firſt excited; and then they were termed ideas of imagination. At length they become voluntarily practiſed in ſucceſſion or in com⯑bination for the common purpoſes of life; as when we make our⯑ſelves maſters of the hiſtory of mankind, or of the ſciences they have inveſtigated; and are then called ideas of recollection; and [47] are performed with ſtrength and velocity in proportion to the energy of the volition that excites them, and the quantity of ſenſo⯑rial power.

II. 1. The muſcular motions above deſcribed, that are moſt fre⯑quently obedient to the will, are nevertheleſs occaſionally cauſable by painful or pleaſurable ſenſation, as in the ſtarting from fear, and the contraction of the calf of the leg in the cramp.

2. In like manner the ſenſual motions, or ideas, that are moſt frequently connected with volition, are nevertheleſs occaſionally cauſable by painful or pleaſurable ſenſation. As the hiſtories of men, or the deſcription of places, which we have voluntarily taken pains to remember, ſometimes occur to us in our dreams.

III. 1. The muſcular motions that are generally ſubſervient to volition, are alſo occaſionally cauſable by irritation, as in ſtretching the limbs after ſleep, and yawning. In this manner a contraction of the arm is produced by paſſing the electric fluid from the Leyden phial along its muſcles; and that even though the limb is para⯑lytic. The ſudden motion of the arm produces a diſagreeable ſenſa⯑tion in the joint, but the muſcles ſeem to be brought into action ſimply by irritation.

2. The ideas, that are generally ſubſervient to the will, are in like manner occaſionally excited by irritation; as when we view again an object, we have before well ſtudied, and often re⯑collected.

IV. 1. Innumerable trains or tribes of motions are aſſociated with theſe voluntary muſcular motions above mentioned; as when I will to extend my arm to a diſtant object, ſome other muſcles are brought into action, and preſerve the balance of my body. And when I wiſh to perform any ſteady exertion, as in threading a needle, or chop⯑ping with an ax, the pectoral muſcles are at the ſame time brought [48] into action to preſerve the trunk of the body motionleſs, and we ceaſe to reſpire for a time.

2. In like manner the voluntary ſenſual motions, or ideas of recollection, are aſſociated with many other trains or tribes of ideas. As when I voluntarily recollect a gothic window, that I ſaw ſome time ago, the whole front of the cathedral occurs to me at the ſame time.

I. 1. Many of our muſcular motions were originally excited in ſucceſſive trains, as the contractions of the auricles and of the ven⯑tricles of the heart; and others in combined tribes, as the various diviſions of the muſcles which compoſe the calf of the leg, which were originally irritated into ſynchronous action by the taedium or irkſomeneſs of a continued poſture. By frequent repetitions theſe motions acquire aſſociations, which continue during our lives, and even after the deſtruction of the greateſt part of the ſenſorium; for the heart of a viper or frog will continue to pulſate long after it is taken from the body; and when it has entirely ceaſed to move, if any part of it is goaded with a pin, the whole heart will again re⯑new its pulſations. This kind of connection we ſhall term irri⯑tative [50] aſſociation, to diſtinguiſh it from ſenſitive and voluntary aſſociations.

2. In like manner many of our ideas are originally excited in tribes; as all the objects of ſight, after we become ſo well ac⯑quainted with the laws of viſion, as to diſtinguiſh figure and diſ⯑tance as well as colour; or in trains, as while we paſs along the objects that ſurround us. The tribes thus received by irritation be⯑come aſſociated by habit, and have been termed complex ideas by the writers of metaphyſics, as this book, or that orange. The trains have received no particular name, but theſe are alike aſſoci⯑ations of ideas, and frequently continue during our lives. So the taſte of a pine-apple, though we eat it blindfold, recalls the colour and ſhape of it; and we can ſcarcely think on ſolidity without figure.

II. 1. By the various efforts of our ſenſations to acquire or avoid their objects, many muſcles are daily brought into ſucceſſive or ſynchronous actions; theſe become aſſociated by habit, and are then excited together with great facility, and in many inſtances gain in⯑diſſoluble connections. So the play of puppies and kittens is a re⯑preſentation of their mode of fighting or of taking their prey; and the motions of the muſcles neceſſary for thoſe purpoſes become aſſociated by habit, and gain a great adroitneſs of action by theſe early repetitions: ſo the motions of the abdominal muſcles, which were originally brought into concurrent action with the protruſive motion of the rectum or bladder by ſenſation, become ſo conjoined with them by habit, that they not only eaſily obey theſe ſenſa⯑tions occaſioned by the ſtimulus of the excrement and urine, but are brought into violent and unreſtrainable action in the ſtrangury and teneſmus. This kind of connection we ſhall term ſenſitive aſ⯑ſociation.

2. So many of our ideas, that have been excited together or in ſucceſſion by our ſenſations, gain ſynchronous or ſucceſſive aſſoci⯑ations, [51] that are ſometimes indiſſoluble but with life. Hence the idea of an inhuman or diſhonourable action perpetually calls up before us the idea of the wretch that was guilty of it. And hence thoſe unconquerable antipathies are formed, which ſome people have to the ſight of peculiar kinds of food, of which in their infancy they have eaten to exceſs or by conſtraint.

III. 1. In learning any mechanic art, as muſic, dancing, or the uſe of the ſword, we teach many of our muſcles to act together or in ſucceſſion by repeated voluntary efforts; which by habit become formed into tribes or trains of aſſociation, and ſerve all our pur⯑poſes with great facility, and in ſome inſtances acquire an indiſ⯑ſoluble union. Theſe motions are gradually formed into a habit of acting together by a multitude of repetitions, whilſt they are yet ſeparately cauſable by the will, as is evident from the long time that is taken up by children in learning to walk and to ſpeak; and is experienced by every one, when he firſt attempts to ſkate upon the ice or to ſwim: theſe we ſhall term voluntary aſſociations.

2. All theſe muſcular movements, when they are thus aſſociated into tribes or trains, become afterwards not only obedient to volition, but to the ſenſations and irritations; and the ſame movement com⯑poſes a part of many different tribes or trains of motion. Thus a ſingle muſcle, when it acts in conſort with its neighbours on one ſide, aſſiſts to move the limb in one direction; and in an⯑other, when it acts with thoſe in its neighbourhood on the other ſide; and in other directions, when it acts ſeparately or joint⯑ly with thoſe that lie immediately under or above it; and all theſe with equal facility after their aſſociations have been well eſtabliſhed.

The facility, with which each muſcle changes from one aſſociated tribe to another, and that either backwards or forwards, is well obſervable in the muſcles of the arm in moving the windlaſs of an [52] air-pump; and the ſlowneſs of thoſe muſcular movements, that have not been aſſociated by habit, may be experienced by any one, who ſhall attempt to ſaw the air quick perpendicularly with one hand, and horizontally with the other at the ſame time.

3. In learning every kind of ſcience we voluntarily aſſociate many tribes and trains of ideas, which afterwards are ready for all the purpoſes either of volition, ſenſation, or irritation; and in ſome in⯑ſtances acquire indiſſoluble habits of acting together, ſo as to affect our reaſoning, and influence our actions. Hence the neceſſity of a good education.

Theſe aſſociate ideas are gradually formed into habits of acting together by frequent repetition, while they are yet ſeparately obe⯑dient to the will; as is evident from the difficulty we experience in gaining ſo exact an idea of the front of St. Paul's church, as to be able to delineate it with accuracy, or in recollecting a poem of a few pages.

And theſe ideas, thus aſſociated into tribes, not only make up the parts of the trains of volition, ſenſation, and irritation; but the ſame idea compoſes a part of many different tribes and trains of ideas. So the ſimple idea of whiteneſs compoſes a part of the com⯑plex idea of ſnow, milk, ivory; and the complex idea of the letter A compoſes a part of the ſeveral aſſociated trains of ideas that make up the variety of words, in which this letter enters.

The numerous trains of theſe aſſociated ideas are divided by Mr. Hume into three claſſes, which he has termed contiguity, cauſa⯑tion, and reſemblance. Nor ſhould we wonder to find them thus connected together, ſince it is the buſineſs of our lives to diſpoſe them into theſe three claſſes; and we become valuable to ourſelves and our friends, as we ſucceed in it. Thoſe who have combined an extenſive claſs of ideas by the contiguity of time or place, are men learned in the hiſtory of mankind, and of the ſciences they have cultivated. Thoſe who have connected a great claſs of ideas [53] of reſemblances, poſſeſs the ſource of the ornaments of poetry and oratory, and of all rational analogy. While thoſe who have connected great claſſes of ideas of cauſation, are furniſhed with the powers of producing effects. Theſe are the men of active wiſdom, who lead armies to victory, and kingdoms to proſperity; or diſcover and improve the ſciences, which meliorate and adorn the condition of humanity.

Irritation.

I. THE various organs of ſenſe require various kinds of ſtimula⯑tion to excite them into action; the particles of light penetrate the cornea and humours of the eye, and then irritate the naked retina; ſapid particles, diſſolved or diffuſed in water or ſaliva, and odorous ones, mixed or combined with the air, irritate the extremities of the nerves of taſte and ſmell; which either penetrate, or are expanded on the membranes of the tongue and noſtrils; the auditory nerves are ſtimulated by the vibrations of the atmoſphere communicated by means of the tympanum and of the fluid, whether of air or of water, behind it; and the nerves of touch by the hardneſs of ſurrounding bodies, though the cuticle is interpoſed between theſe bodies and the medulla of the nerve.

As the nerves of the ſenſes have each their appropriated objects, which ſtimulate them into activity; ſo the muſcular fibres, which are the terminations of other ſets of nerves, have their peculiar ob⯑jects, [55] which excite them into action; the longitudinal muſcles are ſtimulated into contraction by extenſion, whence the ſtretching or pandiculation after a long continued poſture, during which they have been kept in a ſtate of extenſion; and the hollow muſcles are excited into action by diſtention, as thoſe of the rectum and bladder are in⯑duced to protrude their contents from their ſenſe of the diſtention rather than of the acrimony of thoſe contents.

There are other objects adapted to ſtimulate the nerves, which terminate in variety of membranes, and thoſe eſpecially which form the terminations of canals; thus the preparations of mercury particu⯑larly affect the ſalivary glands, ipecacuhana affects the ſphincter of the anus, cantharides that of the bladder, and laſtly every gland of the body appears to be indued with a kind of taſte, by which it ſe⯑lects or forms each its peculiar fluid from the blood; and by which it is irritated into activity.

Many of theſe external properties of bodies, which ſtimulate our organs of ſenſe, do not ſeem to effect this by a ſingle impulſe, but by repeated impulſes; as the nerve of the ear is probably not excit⯑able by a ſingle vibration of air, nor the optic nerve by a ſingle par⯑ticle of light; which circumſtance produces ſome analogy between thoſe two ſenſes, at the ſame time the ſolidity of bodies is perceived by a ſingle application of a ſolid body to the nerves of touch, and that even through the cuticle; and we are probably poſſeſſed of a pe⯑culiar ſenſe to diſtinguiſh the nice degrees of heat and cold.

The ſenſes of touch and of hearing acquaint us with the mechani⯑cal impact and vibration of bodies, thoſe of ſmell and taſte ſeem to acquaint us with ſome of their chemical properties, while the ſenſe of viſion and of heat acquaint us with the exiſtence of their peculiar fluids.

Senſation and Volition.

II. Many motions are produced by pleaſure or pain, and that even in contradiction to the power of volition, as in laughing, or in the ſtrangury; but as no name has been given to pleaſure or pain, at the time it is exerted ſo as to cauſe fibrous motions, we have uſed the term ſenſation for this purpoſe; and mean it to bear the ſame analogy to pleaſure and pain, that the word volition does to deſire and averſion.

1. It was mentioned in the fifth Section, that what we have termed ſenſation is a motion of the central parts, or of the whole ſenſorium, beginning at ſome of the extremities of it. This appears firſt, becauſe our pains and pleaſures are always cauſed by our ideas or muſcular motions, which are the motions of the extremities of the ſenſorium. And, ſecondly, becauſe the ſenſation of pleaſure or pain frequently continues ſome time after the ideas or muſcular motions which excited it have ceaſed: for we often feel a glow of pleaſure from an agreeable reverie, for many minutes after the ideas, that were the ſubject of it, have eſcaped our memory; and frequently experi⯑ence a dejection of ſpirits without being able to aſſign the cauſe of it but by much recollection.

When the ſenſorial faculty of deſire or averſion is exerted ſo as to cauſe fibrous motions, it is termed volition; which is ſaid in Sect. V. to be a motion of the central parts, or of the whole ſenſorium, terminating in ſome of the extremities of it. This appears, firſt, becauſe our deſires and averſions always terminate in recollecting and comparing our ideas, or in exerting our muſcles; which are the mo⯑tions of the extremities of the ſenſorium. And, ſecondly, becauſe deſire or averſion begins, and frequently continues for a time in the central parts of the ſenſorium, before it is peculiarly exerted at the [57] extremities of it: for we ſometimes feel deſire or averſion without immediately knowing their objects, and in conſequence without im⯑mediately exerting any of our muſcular or ſenſual motions to attain them: as in the beginning of the paſſion of love, and perhaps of hun⯑ger, or in the ennui of indolent people.

Though ſenſation and volition begin or terminate at the extremities or central parts of the ſenſorium, yet the whole of it is frequently in⯑fluenced by the exertion of theſe faculties, as appears from their ef⯑fects on the external habit: for the whole ſkin is reddened by ſhame, and an univerſal trembling is produced by fear: and every muſcle of the body is agitated in angry people by the deſire of revenge.

There is another very curious circumſtance, which ſhews that ſen⯑ſation and volition are movements of the ſenſorium in contrary di⯑rections; that is, that volition begins at the central parts of it, and proceeds to the extremities; and that ſenſation begins at the extre⯑mities, and proceeds to the central parts: I mean that theſe two ſen⯑ſorial faculties cannot be ſtrongly exerted at the ſame time; for when we exert our volition ſtrongly, we do not attend to pleaſure or pain; and converſely, when we are ſtrongly affected with the ſenſation of pleaſure or pain, we uſe no volition. As will be further explained in Section XVIII. on ſleep, and Section XXXIV. on volition.

2. All our emotions and paſſions ſeem to ariſe out of the exertions of theſe two faculties of the animal ſenſorium. Pride, hope, joy, are the names of particular pleaſures: ſhame, deſpair, ſorrow, are the names of peculiar pains: and love, ambition, avarice, of particular deſires: hatred, diſguſt, fear, anxiety, of particular averſions. Whilſt the paſſion of anger includes the pain from a recent injury, and the averſion to the adverſary that occaſioned it. And compaſſion is the pain we experience at the ſight of miſery, and the deſire of re⯑lieving it.

There is another tribe of deſires, which are commonly termed ap⯑petites, and are the immediate conſequences of the abſence of ſome [58] irritative motions. Thoſe, which ariſe from defect of internal irrita⯑tions, have proper names conferred upon them, as hunger, thirſt, luſt, and the deſire of air, when our reſpiration is impaired by noxi⯑ous vapours; and of warmth, when we are expoſed to too great a degree of cold. But thoſe, whoſe ſtimuli are external to the body, are named from the objects, which are by nature conſtituted to ex⯑cite them; theſe deſires originate from our paſt experience of the pleaſurable ſenſations they occaſion, as the ſmell of an hyacinth, or the taſte of a pine-apple.

Whence it appears, that our pleaſures and pains are at leaſt as vari⯑ous and as numerous as our irritations; and that our deſires and aver⯑ſions muſt be as numerous as our pleaſures and pains. And that as ſenſation is here uſed as a general term for our numerous pleaſures and pains, when they produce the contractions of our fibres; ſo volition is the general name for our deſires and averſions, when they produce fibrous contractions. Thus when a motion of the central parts, or of the whole ſenſorium, terminates in the exertion of our muſcles, it is generally called voluntary action; when it terminates in the exertion of our ideas, it is termed recollection, reaſoning, determining.

3. As the ſenſations of pleaſure and pain are originally introduced by the irritations of external objects: ſo our deſires and averſions are ori⯑ginally introduced by thoſe ſenſations; for when the objects of our pleaſures or pains are at a diſtance, and we cannot inſtantaneouſly poſſeſs the one, or avoid the other, then deſire or averſion is pro⯑duced, and a voluntary exertion of our ideas or muſcles ſucceeds.

The pain of hunger excites you to look out for food, the tree, that ſhades you, preſents its odoriferous fruit before your eyes, you ap⯑proach, pluck, and eat.

The various movements of walking to the tree, gathering the fruit, and maſticating it, are aſſociated motions introduced by their connection with ſenſation; but if from the uncommon height of the tree, the fruit be inacceſſible, and you are prevented from quickly [59] poſſeſſing the intended pleaſure, deſire is produced. The conſequence of this deſire is, firſt, a deliberation about the means to gain the ob⯑ject of pleaſure in proceſs of time, as it cannot be procured immedi⯑ately; and, ſecondly, the muſcular action neceſſary for this purpoſe.

You voluntarily call up all your ideas of cauſation, that are related to the effect you deſire, and voluntarily examine and compare them, and at length determine whether to aſcend the tree, or to gather ſtones from the neighbouring brook, is eaſier to practiſe, or more promiſing of ſucceſs; and, finally, you gather the ſtones, and re⯑peatedly ſling them to diſlodge the fruit.

Hence then we gain a criterion to diſtinguiſh voluntary acts or thoughts from thoſe cauſed by ſenſation. As the former are always employed about the means to acquire pleaſurable objects, or the means to avoid painful ones; while the latter are employed in the poſſeſſion of thoſe, which are already in our power.

Hence the activity of this power of volition produces the great dif⯑ference between the human and the brute creation. The ideas and the actions of brutes are almoſt perpetually employed about their pre⯑ſent pleaſures, or their preſent pains; and, except in the few inſtances which are mentioned in Section XVI. on inſtinct, they ſeldom buſy themſelves about the means of procuring future bliſs, or of avoiding future miſery; ſo that the acquiring of languages, the making of tools, and labouring for money, which are all only the means to pro⯑cure pleaſures; and the praying to the Deity, as another means to procure happineſs, are characteriſtic of human nature.

4. As there are many diſeaſes produced by the quantity of the ſen⯑ſation of pain or pleaſure being too great or too little; ſo are there diſeaſes produced by the ſuſceptibility of the conſtitution to motions cauſable by theſe ſenſations being too dull or too vivid. This ſuſcepti⯑bility of the ſyſtem to ſenſitive motions is termed ſenſibility, to diſ⯑tinguiſh it from ſenſation, which is the actual exiſtence or exertion of pain or pleaſure.

[60] Other claſſes of diſeaſes are owing to the exceſſive promptitude, or ſluggiſhneſs of the conſtitution to voluntary exertions, as well as to the quantity of deſire or of averſion. This ſuſceptibility of the ſyſtem to voluntary motions is termed voluntarity, to diſtinguiſh it from volition, which is the exertion of deſire or averſion: theſe diſeaſes will be treated of at length in the progreſs of the work.

Aſſociation.

III. 1. It is not eaſy to aſſign a cauſe, why thoſe animal move⯑ments, that have once occurred in ſucceſſion, or in combination, ſhould afterwards have a tendency to ſucceed or accompany each other. It is a property of animation, and diſtinguiſhes this order of being from the other productions of nature.

When a child firſt wrote the word man, it was diſtinguiſhed in his mind into three letters, and thoſe letters into many parts of letters; but by repeated uſe the word man becomes to his hand in writing it, as to his organs of ſpeech in pronouncing it, but one movement without any deliberation, or ſenſation, or irritation, interpoſed be⯑tween the parts of it. And as many ſeparate motions of our muſcles thus become united, and form, as it were, one motion; ſo each ſe⯑parate motion before ſuch union may be conceived to conſiſt of many parts or ſpaces moved through; and perhaps even the individual fibres of our muſcles have thus gradually been brought to act in concert, which habits began to be acquired as early as the very formation of the moving organs, long before the nativity of the animal; as ex⯑plained in the Section XVI. 2. on inſtinct.

2. There are many motions of the body, belonging to the irritative claſs, which might by a haſty obſerver be miſtaken for aſſociated ones; as the periſtaltic motion of the ſtomach and inteſtines, and the contractions of the heart and arteries; might be ſuppoſed to be aſſoci⯑ated [61] with the irritative motions of their nerves of ſenſe, rather than to be excited by the irritation of their muſcular fibres by the diſten⯑tion, acrimony, or momentum of the blood. So the diſtention or elongation of muſcles by objects external to them irritates them into contraction, though the cuticle or other parts may intervene between the ſtimulating body and the contracting muſcle. Thus a horſe voids his excrement when its weight or bulk irritates the rectum or ſphincter ani. The motion of theſe muſcles act from the irritation of diſten⯑tion, when he excludes his excrement, but the muſcles of the abdo⯑men and diaphragm are brought into motion by aſſociation with thoſe of the ſphincter and rectum.

I. Of fibrous contraction.

1. IF two particles of iron lie near each other without motion, and afterwards approach each other; it is reaſonable to conclude that ſomething beſides the iron particles is the cauſe of their approxima⯑tion; this inviſible ſomething is termed magnetiſm. In the ſame manner, if the particles, which compoſe an animal muſcle, do not touch each other in the relaxed ſtate of the muſcle, and are brought into contact during the contraction of the muſcle; it is reaſonable to conclude, that ſome other agent is the cauſe of this new approxima⯑tion. For nothing can act, where it does not exiſt; for to act in⯑cludes to exiſt; and therefore the particles of the muſcular fibre (which in its ſtate of relaxation are ſuppoſed not to touch) cannot af⯑fect each other without the influence of ſome intermediate agent; this agent is here termed the ſpirit of animation, or ſenſorial power, but may with equal propriety be termed the power, which cauſes contraction; or may be called by any other name, which the reader may chooſe to affix to it.

The contraction of a muſcular fibre may be compared to the follow⯑ing electric experiment, which is here mentioned not as a philoſophi⯑cal analogy, but as an illuſtration or ſimile to facilitate the conception of a difficult ſubject. Let twenty very ſmall Leyden phials properly [65] coated be hung in a row by fine ſilk threads at a ſmall diſtance from each other; let the internal charge of one phial be poſitive, and of the other negative alternately, if a communication be made from the in⯑ternal ſurface of the firſt to the external ſurface of the laſt in the row, they will all of them inſtantly approach each other, and thus ſhorten a line that might connect them like a muſcular fibre. See Botanic Garden, p. 1. Canto I. l. 202, note on Gymnotus.

The attractions of electricity or of magnetiſm do not apply philo⯑ſophically to the illuſtration of the contraction of animal fibres, ſince the force of thoſe attractions increaſes in ſome proportion inverſely as the diſtance, but in muſcular motion there appears no difference in velocity or ſtrength during the beginning or end of the contraction, but what may be clearly aſcribed to the varying mechanic advantage in the approximation of one bone to another. Nor can muſcular mo⯑tion be aſſimilated with greater plauſibility to the attraction of coheſion or elaſticity; for in bending a ſteel ſpring, as a ſmall ſword, a leſs force is required to bend it the firſt inch than the ſecond; and the ſecond than the third; the particles of ſteel on the convex ſide of the bent ſpring endeavouring to reſtore themſelves more powerfully the further they are drawn from each other. See Botanic Garden, P. 1. addit. Note XVIII.

I am aware that this may be explained another way, by ſuppoſing the elaſticity of the ſpring to depend more on the compreſſion of the particles on the concave ſide than on the extenſion of them on the con⯑vex ſide; and by ſuppoſing the elaſticity of the elaſtic gum to depend more on the reſiſtance to the lateral compreſſion of its particles than to the longitudinal extenſion of them. Nevertheleſs in muſcular contraction, as above obſerved, there appears no difference in the ve⯑locity or force of it at its commencement or at its termination; from whence we muſt conclude that animal contraction is governed by laws of its own, and not by thoſe of mechanics, chemiſtry, magne⯑tiſm, or electricity.

[66] On theſe accounts I do not think the experiments concluſive, which were lately publiſhed by Galvani, Volta, and others, to ſhew a ſimilitude between the ſpirit of animation, which contracts the muſcular fibres, and the electric fluid. Since the electric fluid may act only as a more potent ſtimulus exciting the muſcular fibres into action, and not by ſupplying them with a new quantity of the ſpirit of life. Thus in a recent hemiplegia I have frequently obſerved, when the patient yawned and ſtretched himſelf, that the paralytic limbs moved alſo, though they were totally diſobedient to the will. And when he was electriſied by paſſing ſhocks from the affected hand to the affected foot, a motion of the paralytic limbs was alſo produced. Now as in the act of yawning the muſcles of the paralytic limbs were excited into action by the ſtimulus of the irkſomeneſs of a continued poſture, and not by any additional quantity of the ſpirit of life; ſo we may conclude, that the paſſage of the electric fluid, which pro⯑duced a ſimilar effect, acted only as a ſtimulus, and not by ſupplying any addition of ſenſorial power.

If nevertheleſs this theory ſhould ever become eſtabliſhed, a ſtimu⯑lus muſt be called an eductor of vital ether; which ſtimulus may conſiſt of ſenſation or volition, as in the electric eel, as well as in the appulſes of external bodies; and by drawing off the charges of vital fluid may occaſion the contraction or motions of the muſcular ſibres, and organs of ſenſe.

2. The immediate effect of the action of the ſpirit of animation or ſenſorial power on the fibrous parts of the body, whether it acts in the mode of irritation, ſenſation, volition, or aſſociation, is a con⯑traction of the animal fibre, according to the ſecond law of animal cauſation. Sect. IV. Thus the ſtimulus of the blood induces the contraction of the heart; the agreeable taſte of a ſtrawberry produces the contraction of the muſcles of deglutition; the effort of the will contracts the muſcles, which move the limbs in walking; and by aſſociation other muſcles of the trunk are brought into contraction to [67] preſerve the balance of the body. The fibrous extremities of the or⯑gans of ſenſe have been ſhewn, by the ocular ſpectra in Sect. III. to ſuffer ſimilar contraction by each of the above modes of excitation; and by their configurations to conſtitute our ideas.

3. After animal fibres have for ſome time been excited into contrac⯑tion, a relaxation ſucceeds, even though the exciting cauſe continues to act. In reſpect to the irritative motions this is exemplified in the periſtaltic contractions of the bowels; which ceaſe and are renewed alternately, though the ſtimulus of the aliment continues to be uni⯑formly applied; in the ſenſitive motions, as in ſtrangury, teneſmus, and parturition, the alternate contractions and relaxations of the muſ⯑cles exiſt, though the ſtimulus is perpetual. In our voluntary ex⯑ertions it is experienced, as no one can hang long by the hands, how⯑ever vehemently he wills ſo to do; and in the aſſociate motions the conſtant change of our attitudes evinces the neceſſity of relaxation to thoſe muſcles, which have been long in action.

This relaxation of a muſcle after its contraction, even though the ſtimulus continues to be applied, appears to ariſe from the expenditure or diminution of the ſpirit of animation previouſly reſident in the muſ⯑cle, according to the ſecond law of animal cauſation in Sect. IV. In thoſe conſtitutions, which are termed weak, the ſpirit of anima⯑tion becomes ſooner exhauſted, and tremulous motions are produced, as in the hands of infirm people, when they lift a cup to their mouths. This quicker exhauſtion of the ſpirit of animation is probably owing to a leſs quantity of it reſiding in the acting fibres, which therefore more frequently require a ſupply from the nerves, which belong to them.

4. If the ſenſorial power continues to act, whether it acts in the mode of irritation, ſenſation, volition, or aſſociation, a new con⯑traction of the animal fibre ſucceeds after a certain interval; which interval is of ſhorter continuance in weak people than in ſtrong ones. This is exemplified in the ſhaking of the hands of weak people, when [68] they attempt to write. In a manuſcript epiſtle of one of my cor⯑reſpondents, which is written in a ſmall hand, I obſerved from four to ſix zigzags in the perpendicular ſtroke of every letter, which ſhews that both the contractions of the fingers, and intervals be⯑tween them, muſt have been performed in very ſhort periods of time.

The times of contraction of the muſcles of enfeebled people being leſs, and the intervals between thoſe contractions being leſs alſo, ac⯑counts for the quick pulſe in fevers with debility, and in dying ani⯑mals. The ſhortneſs of the intervals between one contraction and another in weak conſtitutions, is probably owing to the general defi⯑ciency of the quantity of the ſpirit of animation, and that therefore there is a leſs quantity of it to be received at each interval of the ac⯑tivity of the fibres. Hence in repeated motions, as of the fingers in performing on the harpſichord, it would at firſt ſight appear, that ſwiftneſs and ſtrength were incompatible; nevertheleſs the ſingle contraction of a muſcle is performed with greater velocity as well as with greater force by vigorous conſtitutions, as in throwing a javelin.

There is however another circumſtance, which may often contri⯑bute to cauſe the quickneſs of the pulſe in nervous fevers, as in ani⯑mals bleeding to death in the ſlaughter-houſe; which is the deficient quantity of blood; whence the heart is but half diſtended, and in con⯑ſequence ſooner contracts. See Sect. XXXII. 2. 1.

For we muſt not confound frequency of repetition with quickneſs of motion, or the number of pulſations with the velocity, with which the fibres, which conſtitute the coats of the arteries, contract them⯑ſelves. For where the frequency of the pulſations is but ſeventy-five in a minute, as in health; the contracting fibres, which conſtitute the ſides of the arteries, may move through a greater ſpace in a given time, than where the frequency of pulſation is one hundred and fifty in a minute, as in ſome fevers with great debility. For if in thoſe [69] fevers the arteries do not expand themſelves in their diaſtole to more than half the uſual diameter of their diaſtole in health, the fibres which conſtitute their coats, will move through a leſs ſpace in a mi⯑nute than in health, though they make two pulſations for one.

Suppoſe the diameter of the artery during its ſyſtole to be one line, and that the diameter of the ſame artery during its diaſtole is in health is four lines, and in a fever with great debility only two lines. It fol⯑lows, that the arterial fibres contract in health from a circle of twelve lines in circumference to a circle of three lines in circumference, that is they move through a ſpace of nine lines in length. While the ar⯑terial fibres in the fever with debility would twice contract from a circle of ſix lines to a circle of three lines; that is while they move through a ſpace equal to ſix lines. Hence though the frequency of pulſation in fever be greater as two to one, yet the velocity of con⯑traction in health is greater as nine to ſix, or as three to two.

On the contrary in inflammatory diſeaſes with ſtrength, as in the pleuriſy, the velocity of the contracting ſides of the arteries is much greater than in health, for if we ſuppoſe the number of pulſations in a pleuriſy to be half as much more than in health, that is as one hun⯑dred and twenty to eighty, (which is about what generally happens in inflammatory diſeaſes) and if the diameter of the artery in diaſtole be one third greater than in health, which I believe is near the truth, the reſult will be, that the velocity of the contractile ſides of the ar⯑teries will be in a pleuriſy as two and a half to one, compared to the velocity of their contraction in a ſtate of health, for if the circum⯑ference of the ſyſtole of the artery be three lines, and the diaſtole in health be twelve lines in circumference, and in a pleuriſy eighteen lines; and ſecondly, if the artery pulſates thrice in the diſeaſed ſtate for twice in the healthy one, it follows, that the velocity of contrac⯑tion in the diſeaſed ſtate to that in the healthy ſtate will be forty-five to eighteen, or as two and a half to one.

[70] From hence it would appear, that if we had a criterion to deter⯑mine the velocity of the arterial contractions, it would at the ſame time give us their ſtrength, and thus be of more ſervice in diſtinguiſh⯑ing diſeaſes, than the knowledge of their frequency. As ſuch a cri⯑terion cannot be had, the frequency of pulſation, the age of the pa⯑tient being allowed for, will in ſome meaſure aſſiſt us to diſtinguiſh arterial ſtrength from arterial debility, ſince in inflammatory diſeaſes with ſtrength the frequency ſeldom exceeds one hundred and eighteen or one hundred and twenty pulſations in a minute; unleſs under pe⯑culiar circumſtance, as the great additional ſtimuli of wine or of ex⯑ternal heat.

5. After a muſcle or organ of ſenſe has been excited into contrac⯑tion, and the ſenſorial power ceaſes to act, the laſt ſituation or con⯑figuration of it continues; unleſs it be diſturbed by the action of ſome antagoniſt fibres, or other extraneous power. Thus in weak or lan⯑guid people, wherever they throw their limbs on their bed or ſofa, there they lie, till another exertion changes their attitude; hence one kind of ocular ſpectra ſeems to be produced after looking at bright objects; thus when a fire-ſtick is whirled round in the night, there appears in the eye a complete circle of fire; the action or configuration of one part of the retina not ceaſing before the return of the whirling fire.

Thus if any one looks at the ſetting ſun for a ſhort time, and then covers his cloſed eyes with his hand, he will for many ſeconds of time perceive the image of the ſun on his retina. A ſimilar image of all other bodies would remain ſome time in the eye, but is effaced by the eternal change of the motions of the extremity of this nerve in our attention to other objects. See Sect. XVII. 1. 3. on Sleep. Hence the dark ſpots, and other ocular ſpectra, are more frequently attended to, and remain longer in the eyes of weak people, as after violent ex⯑erciſe, intoxication, or want of ſleep.

[71] 6. A contraction of the fibres ſomewhat greater than uſual intro⯑duces pleaſurable ſenſation into the ſyſtem, according to the fourth law of animal cauſation. Hence the pleaſure in the beginning of drunkenneſs is owing to the increaſed action of the ſyſtem from the ſtimulus of vinous ſpirit or of opium. If the contractions be ſtill greater in energy or duration, painful ſenſations are introduced, as in conſequence of great heat, or cauſtic applications, or fatigue.

If any part of the ſyſtem, which is uſed to perpetual activity, as the ſtomach, or heart, or the fine veſſels of the ſkin, acts for a time with leſs energy, another kind of painful ſenſation enſues, which is called hunger, or faintneſs, or cold. This occurs in a leſs degree in the locomotive muſcles, and is called wearyſomeneſs. In the two former kinds of ſenſation there is an expenditure of ſenſorial power, in theſe latter there is an accumulation of it.

7. We have uſed the words exertion of ſenſorial power as a general term to expreſs either irritation, ſenſation, volition, or aſſociation; that is, to expreſs the activity or motion of the ſpirit of animation, at the time it produces the contractions of the fibrous parts of the ſyſtem. It may be ſuppoſed that there may exiſt a greater or leſs mobility of the fibrous parts of our ſyſtem, or a propenſity to be ſtimulated into contraction by the greater or leſs quantity or energy of the ſpirit of animation; and that hence if the exertion of the ſenſorial power be in its natural ſtate, and the mobility of the fibres be increaſed, the ſame quantity of fibrous contraction will be cauſed, as if the mobility of the fibres continues in its natural ſtate, and the ſenſorial exertion be increaſed.

Thus it may be conceived, that in diſeaſes accompanied with ſtrength, as in inflammatory fevers with arterial ſtrength, that the cauſe of greater fibrous contraction may exiſt in the increaſed mobi⯑lity of the ſibres, whoſe contractions are thence both more forceable and more frequent. And that in diſeaſes attended with debility, as in nervous fevers, where the fibrous contractions are weaker, and [72] more frequent, it may be conceived that the cauſe conſiſts in a de⯑creaſe of mobility of the fibres; and that thoſe weak conſtitutions, which are attended with cold extremities and large pupils of the eyes, may poſſeſs leſs mobility of the contractile fibres, as well as leſs quan⯑tity of exertion of the ſpirit of animation.

In anſwer to this mode of reaſoning it may be ſufficient to obſerve, that the contractile fibres conſiſt of inert matter, and when the ſen⯑ſorial power is withdrawn, as in death, they poſſeſs no power of mo⯑tion at all, but remain in their laſt ſtate, whether of contraction or relaxation, and muſt thence derive the whole of this property from the ſpirit of animation. At the ſame time it is not improbable, that the moving fibres of ſtrong people may poſſeſs a capability of receiving or containing a greater quantity of the ſpirit of animation than thoſe of weak people.

In every contraction of a fibre there is an expenditure of the ſenſo⯑rial power, or ſpirit of animation; and where the exertion of this ſenſorial power has been for ſome time increaſed, and the muſcles or organs of ſenſe have in conſequence acted with greater energy, its propenſity to activity is proportionally leſſened; which is to be aſcrib⯑ed to the exhauſtion or diminution of its quantity. On the contrary, where there has been leſs fibrous contraction than uſual for a certain time, the ſenſorial power or ſpirit of animation becomes accumulated in the inactive part of the ſyſtem. Hence vigour ſucceeds reſt, and hence the propenſity to action of all our organs of ſenſe and muſcles is in a ſtate of perpetual fluctuation. The irritability for inſtance of the retina, that is, its quantity of ſenſorial power, varies every mo⯑ment according to the brightneſs or obſcurity of the object laſt beheld compared with the preſent one. The ſame occurs to our ſenſe of heat, and to every part of our ſyſtem, which is capable of being ex⯑cited into action.

When this variation of the exertion of the ſenſorial power becomes much and permanently above or beneath the natural quantity, it be⯑comes [73] a diſeaſe. If the irritative motions be too great or too little, it ſhews that the ſtimulus of external things affect this ſenſorial power too violently or too inertly. If the ſenſitive motions be too great or too little, the cauſe ariſes from the deficient or exuberant quantity of ſenſation produced in conſequence of the motions of the muſcular fibres or organs of ſenſe; if the voluntary actions are diſeaſed the cauſe is to be looked for in the quantity of volition produced in con⯑ſequence of the deſire or averſion occaſioned by the painful or plea⯑ſurable ſenſations above mentioned. And the diſeaſes of aſſociations probably depend on the greater or leſs quantity of the other three ſen⯑ſorial powers by which they were formed.

From whence it appears that the propenſity to action, whether it be called irritability, ſenſibility, voluntarity, or aſſociability, is only another mode of expreſſion for the quantity of ſenſorial power reſiding in the organ to be excited. And that on the contrary the words in⯑irritability and inſenſibility, together with inaptitude to voluntary and aſſociate motions, are ſynonymous with deficiency of the quantity of ſenſorial power, or of the ſpirit of animation, reſiding in the organs to be excited.

II. Of ſenſorial Exertion.

1. There are three circumſtances to be attended to in the produc⯑tion of animal motions. 1ſt. The ſtimulus. 2d. The ſenſorial power. 3d. The contractile fibre. 1ſt. A ſtimulus, external to the organ, originally induces into action the ſenſorial faculty termed irritation; this produces the contraction of the fibres, which, if it be perceived at all, introduces pleaſure or pain; which in their active ſtate are termed ſenſation; which is another ſenſorial faculty, and occaſionally pro⯑duces contraction of the fibres; this pleaſure or pain is therefore to be conſidered as another ſtimulus, which may either act alone or in con⯑junction with the former faculty of the ſenſorium termed irritation. [74] This new ſtimulus of pleaſure or pain either induces into action the ſenſorial faculty termed ſenſation, which then produces the contrac⯑tion of the fibres; or it introduces deſire or averſion, which excite into action another ſenſorial faculty, termed volition, and may there⯑fore be conſidered as another ſtimulus, which either alone or in con⯑junction with one or both of the two former faculties of the ſenſorium produces the contraction of animal fibres. There is another ſenſorial power, that of aſſociation, which perpetually, in conjunction with one or more of the above, and frequently ſingly, produces the con⯑traction of animal fibres, and which is itſelf excited into action by the previous motions of contracting fibres.

Now as the ſenſorial power, termed irritation, reſiding in any par⯑ticular fibres, is excited into exertion by the ſtimulus of external bodies acting on thoſe fibres; the ſenſorial power, termed ſenſation, reſiding in any particular fibres is excited into exertion by the ſtimulus of pleaſure or pain acting on thoſe fibres; the ſenſorial power, termed volition, reſiding in any particular fibres is excited into exertion by the ſtimulus of deſire or averſion; and the ſenſorial power, termed aſſociation, reſiding in any particular fibres, is excited into action by the ſtimulus of other fibrous motions, which had frequently preceded them. The word ſtimulus may therefore be uſed without impro⯑priety of language, for any of theſe four cauſes, which excite the four ſenſorial powers into exertion. For though the immediate cauſe of volition has generally been termed a motive; and that of irritation only has generally obtained the name of ſtimulus; yet as the imme⯑diate cauſe, which excites the ſenſorial powers of ſenſation, or of aſ⯑ſociation into exertion, have obtained no general name, we ſhall uſe the word ſtimulus for them all.

Hence the quantity of motion produced in any particular part of the animal ſyſtem will be as the quantity of ſtimulus and the quantity of ſenſorial power, or ſpirit of animation, reſiding in the contracting fibres. Where both theſe quantities are great, ſtrength is produced, [75] when that word is applied to the motions of animal bodies. Where either of them is deficient, weakneſs is produced, as applied to the motions of animal bodies.

Now as the ſenſorial power, or ſpirit of animation, is perpetually exhauſted by the expenditure of it in fibrous contractions, and is per⯑petually renewed by the ſecretion or production of it in the brain and ſpinal marrow, the quantity of animal ſtrength muſt be in a perpetual ſtate of fluctuation on this account; and if to this be added the un⯑ceaſing variation of all the four kinds of ſtimulus above deſcribed, which produce the exertions of the ſenſorial powers, the ceaſeleſs vi⯑ciſſitude of animal ſtrength becomes eaſily comprehended.

If the quantity of ſenſorial power remains the ſame, and the quan⯑tity of ſtimulus be leſſened, a weakneſs of the fibrous contractions enſues, which may be denominated debility from defect of ſtimulus. If the quantity of ſtimulus remains the ſame, and the quantity of ſen⯑ſorial power be leſſened, another kind of weakneſs enſues, which may be termed debility from defect of ſenſorial power; the former of theſe is called by Dr. Brown, in his Elements of Medicine, direct debility, and the latter indirect debility. The coincidence of ſome parts of this work with correſpondent deductions in the Brunonian Elementa Medicina, a work (with ſome exceptions) of great genius, muſt be conſidered as confirmations of the truth of the theory, as they were probably arrived at by different trains of reaſoning.

Thus in thoſe who have been expoſed to cold and hunger there is a deficiency of ſtimulus. While in nervous fever there is a deficiency of ſenſorial power. And in habitual drunkards, in a morning before their uſual potation, there is a deficiency both of ſtimulus and of ſen⯑ſorial power. While, on the other hand, in the beginning of intoxica⯑tion there is an exceſs of ſtimulus; in the hot-ach, after the hands have been immerſed in ſnow, there is a redundancy of ſenſorial power; and in inflammatory diſeaſes with arterial ſtrength, there is an exceſs of both.

[76] Hence if the ſenſorial power be leſſened, while the quantity of ſti⯑mulus remains the ſame as in nervous fever, the frequency of repetition of the arterial contractions may continue, but their force in reſpect to removing obſtacles, as in promoting the circulation of the blood, or the velocity of each contraction, will be diminiſhed, that is, the animal ſtrength will be leſſened. And ſecondly, if the quantity of ſenſorial power be leſſened, and the ſtimulus be increaſed to a certain degree, as in giving opium in nervous fevers, the arterial contractions may be performed more frequently than natural, yet with leſs ſtrength.

And thirdly, if the ſenſorial power continues the ſame in reſpect to quantity, and the ſtimulus be ſomewhat diminiſhed, as in going into a darkiſh room, or into a coldiſh bath, ſuppoſe of about eighty degrees of heat, as Buxton-bath, a temporary weakneſs of the affected fibres is induced, till an accumulation of ſenſorial power gradually ſucceeds, and counterbalances the deficiency of ſtimulus, and then the bath ceaſes to feel cold, and the room ceaſes to appear dark; becauſe the fibres of the ſubcutaneous veſſels, or of the organs of ſenſe, act with their uſual energy.

A ſet of muſcular fibres may thus be ſtimulated into violent ex⯑ertion, that is, they may act frequently, and with their whole ſen⯑ſorial power, but may nevertheleſs not act ſtrongly; becauſe the quantity of their ſenſorial power was originally ſmall, or was previ⯑ouſly exhauſted. Hence a ſtimulus may be great, and the irritation in conſequence act with its full force, as in the hot paroxiſms of nervous fever; but if the ſenſorial power, termed irritation, be ſmall in quantity, the force of the fibrous contractions, and the times of their continuance in their contracted ſtate, will be proportionally ſmall.

In the ſame manner in the hot paroxiſm of putrid fevers, which are ſhewn in Sect. XXXIII. to be inflammatory fevers with arterial debility, the ſenſorial power termed ſenſation is exerted with great activity, yet the fibrous contractions, which produce the circulation [77] of the blood, are performed without ſtrength, becauſe the quantity of ſenſorial power then reſiding in that part of the ſyſtem is ſmall.

Thus in irritative fever with arterial ſtrength, that is, with ex⯑ceſs of ſpirit of animation, the quantity of exertion during the hot part of the paroxiſm is to be eſtimated from the quantity of ſtimulus, and the quantity of ſenſorial power. While in ſenſitive (or inflam⯑matory) fever with arterial ſtrength, that is, with exceſs of ſpirit of animation, the violent and forcible actions of the vaſcular ſyſtem during the hot part of the paroxiſm are induced by the exertions of two ſenſorial powers, which are excited by two kinds of ſtimulus. Theſe are the ſenſorial power of irritation excited by the ſtimulus of bodies external to the moving fibres, and the ſenſorial power of ſen⯑ſation excited by the pain in conſequence of the increaſed contractions of thoſe moving fibres.

And in inſane people in ſome caſes the force of their muſcular ac⯑tions will be in proportion to the quantity of ſenſorial power, which they poſſeſs, and the quantity of the ſtimulus of deſire or averſion, which excites their volition into action. At the ſame time in other caſes the ſtimulus of pain or pleaſure, and the ſtimulus of external bodies, may excite into action the ſenſorial powers of ſenſation and irritation, and thus add greater force to their muſcular actions.

2. The application of the ſtimulus, whether that ſtimulus be ſome quality of external bodies, or pleaſure or pain, or deſire or averſion, or a link of aſſociation, excites the correſpondent ſenſorial power into action, and this cauſes the contraction of the fibre. On the contrac⯑tion of the fibre a part of the ſpirit of animation becomes expended, and the fibre ceaſes to contract, though the ſtimulus continues to be applied; till in a certain time the fibre having received a ſupply of ſenſorial power is ready to contract again, if the ſtimulus continues to be applied. If the ſtimulus on the contrary be withdrawn, the ſame quantity of quieſcent ſenſorial power becomes reſident in the fibre as before its contraction; as appears from the readineſs for ac⯑tion [78] of the large locomotive muſcles of the body in a ſhort time after common exertion.

But in thoſe muſcular fibres, which are ſubject to conſtant ſtimu⯑lus, as the arteries, glands, and capillary veſſels, another phenomenon occurs, if their accuſtomed ſtimulus be withdrawn; which is, that the ſenſorial power becomes accumulated in the contractile fibres, owing to the want of its being perpetually expended, or carried away, by their uſual unremitted contractions. And on this account thoſe muſcular fibres become afterwards excitable into their natural actions by a much weaker ſtimulus; or into unnatural violence of action by their accuſtomed ſtimulus, as is ſeen in the hot fits of intermittent fevers, which are in conſequence of the previous cold ones. Thus the minute veſſels of the ſkin are conſtantly ſtimulated by the fluid matter of heat; if the quantity of this ſtimulus of heat be a while diminiſhed, as in covering the hands with ſnow, the veſſels ceaſe to act, as appears from the paleneſs of the ſkin; if this cold application of ſnow be continued but a ſhort time, the ſenſorial power, which had habitually been ſupplied to the fibres, becomes now accumulated in them, owing to the want of its being expended by their accuſtomed contractions. And thence a leſs ſtimulus of heat will now excite them into violent contractions.

If the quieſcence of fibres, which had previouſly been ſubject to perpetual ſtimulus, continues a longer time; or their accuſtomed ſtimulus be more completely withdrawn; the accumulation of ſenſo⯑rial power becomes ſtill greater, as in thoſe expoſed to cold and hun⯑ger; pain is produced, and the organ gradually dies from the chemical changes, which take place in it; or it is at a great diſtance of time reſtored to action by ſtimulus applied with great caution in ſmall quantity, as happens to ſome larger animals and to many inſects, which during the winter months lie benumbed with cold, and are ſaid to ſleep, and to perſons apparently drowned, or apparently frozen to death. Snails have been ſaid to revive by throwing them [79] into water after having been many years ſhut up in the cabinets of the curious; and eggs and ſeeds in general are reſtored to life after many months of torpor by the ſtimulus of warmth and moiſture.

The inflammation of ſchirrous tumours, which have long exiſted in a ſtate of inaction, is a proceſs of this kind; as well as the ſenſibi⯑lity acquired by inflamed tendons and bones, which had at their formation a ſimilar ſenſibility, which had ſo long lain dormant in their uninflamed ſtate.

3. If after long quieſcence from defect of ſtimulus the fibres, which had previouſly been habituated to perpetual ſtimulus, are again expoſed to but their uſual quantity of it; as in thoſe who have ſuf⯑fered the extremes of cold or hunger; a violent exertion of the af⯑fected organ commences, owing, as above explained, to the great accumulation of ſenſorial power. This violent exertion not only di⯑miniſhes the accumulated ſpirit of animation, but at the ſame time induces pleaſure or pain into the ſyſtem, which, whether it be ſuc⯑ceeded by inflammation or not, becomes an additional ſtimulus, and acting along with the former one, produces ſtill greater exertions; and thus reduces the ſenſorial power in the contracting fibres beneath its natural quantity.

When the ſpirit of animation is thus exhauſted by uſeleſs exer⯑tions, the organ becomes torpid or unexcitable into action, and a ſecond fit of quieſcence ſucceeds that of abundant activity. During this ſecond fit of quieſcence the ſenſorial power becomes again accu⯑mulated, and another fit of exertion follows in train. Theſe viciſſi⯑tudes of exertion and inertion of the arterial ſyſtem conſtitute the paroxiſms of remittent fevers; or intermittent ones, when there is an interval of the natural action of the arteries between the exacerba⯑tions.

In theſe paroxiſms of fevers, which conſiſt of the libration of the arterial ſyſtem between the extremes of exertion and quieſcence, either the fits become leſs and leſs violent from the contractile fibres be⯑coming [80] leſs excitable to the ſtimulus by habit, that is, by becoming accuſtomed to it, as explained below XII. 3. 1. or the whole ſenſo⯑rial power becomes exhauſted, and the arteries ceaſe to beat, and the patient dies in the cold part of the paroxiſm. Or ſecondly, ſo much pain is introduced into the ſyſtem by the violent contractions of the fibres, that inflammation ariſes, which prevents future cold fits by expending a part of the ſenſorial power in the extenſion of old veſſels or the production of new ones; and thus preventing the too great ac⯑cumulation or exertion of it in other parts of the ſyſtem; or which by the great increaſe of ſtimulus excites into great action the whole glandular ſyſtem as well as the arterial, and thence a greater quantity of ſenſorial power is produced in the brain, and thus its exhauſtion in any peculiar part of the ſyſtem ceaſes to be affected.

4. Or thirdly, in conſequence of the painful or pleaſurable ſenſa⯑tion above mentioned, deſire and averſion are introduced, and inordinate volition ſucceeds; which by its own exertions expends ſo much of the ſpirit of animation, that the two other ſenſorial faculties, or irritation and ſenſation, act ſo much feebler; that the paroxiſms of fever, or that libration between the extremes of exertion and inactivity of the arterial ſyſtem, gradually ſubſides. On this account a temporary in⯑ſanity is a favourable ſign in fevers, as I have had ſome opportunities of obſerving.

III. Of repeated Stimulus.

1. When a ſtimulus is repeated more frequently than the expendi⯑ture of ſenſorial power can be renewed in the acting organ, the effect of the ſtimulus becomes gradually diminiſhed. Thus if two grains of opium be ſwallowed by a perſon unuſed to ſo ſtrong a ſtimulus, all the vaſcular ſyſtems in the body act with greater energy, all the ſe⯑cretions and the abſorption from thoſe ſecreted fluids are increaſed in [81] quantity; and pleaſure or pain are introduced into the ſyſtem, which adds an additional ſtimulus to that already too great. After ſome hours the ſenſorial power becomes diminiſhed in quantity, expended by the great activity of the ſyſtem; and thence, when the ſtimulus of the opium is withdrawn, the fibres will not obey their uſual de⯑gree of natural ſtimulus, and a conſequent torpor or quieſcence ſuc⯑ceeds, as is experienced by drunkards, who on the day after a great exceſs of ſpirituous potation feel indigeſtion, head-ach, and general debility.

In this fit of torpor or quieſcence of a part or of the whole of the ſyſtem, an accumulation of the ſenſorial power in the affected fibres is formed, and occaſions a ſecond paroxyſm of exertion by the applica⯑tion only of the natural ſtimulus, and thus a libration of the ſenſorial exertion between one exceſs and the other continues for two or three days, where the ſtimulus was violent in degree; and for weeks in ſome fevers, from the ſtimulus of contagious matter.

But if a ſecond doſe of opium be exhibited before the fibres have regained their natural quantity of ſenſorial power, its effect will be much leſs than the former, becauſe the ſpirit of animation or ſen⯑ſorial power is in part exhauſted by the previous exceſs of exertion. Hence all medicines repeated too frequently gradually loſe their effect, as opium and wine. Many things of diſagreeable taſte at firſt ceaſe to be diſagreeable by frequent repetition, as tobacco; grief and pain gradually diminiſh, and at length ceaſe altogether, and hence life itſelf becomes tolerable.

Beſides the temporary diminution of the ſpirit of animation or ſen⯑ſorial power, which is naturally ſtationary or reſident in every living fibre, by a ſingle exhibition of a powerful ſtimulus, the contractile fibres themſelves, by the perpetual application of a new quantity of ſtimulus, before they have regained their natural quantity of ſſnſorial power, appear to ſuffer in their capability of receiving ſo much as the natural quantity of ſenſorial power; and hence a permanent defici⯑ency [82] of ſpirit of animation takes place, however long the ſtimulus may have been withdrawn. On this cauſe depends the permanent debility of thoſe, who have been addicted to intoxication, the general weakneſs of old age, and the natural debility or inirritability of thoſe, who have pale ſkins and large pupils of their eyes.

There is a curious phenomenon belongs to this place, which has always appeared difficult of ſolution; and that is, that opium or aloes may be exhibited in ſmall doſes at firſt, and gradually increaſed to very large ones without producing ſtupor or diarrhoea. In this caſe, though the opium and aloes are given in ſuch ſmall doſes as not to produce intoxication or catharſis, yet they are exhibited in quantities ſufficient in ſome degree to exhauſt the ſenſorial power, and hence a ſtronger and a ſtronger doſe is required; otherwiſe the medicine would ſoon ceaſe to act at all.

On the contrary, if the opium or aloes be exhibited in a large doſe at firſt, ſo as to produce intoxication or diarrhoea; after a few repe⯑titions the quantity of either of them may be diminiſhed, and they will ſtill produce this effect. For the more powerful ſtimulus diſ⯑ſevers the progreſſive catenations of animal motions, deſcribed in Sect. XVII. and introduces a new link between them; whence every repetition ſtrengthens this new aſſociation or catenation, and the ſti⯑mulus may be gradually decreaſed, or be nearly withdrawn, and yet the effect ſhall continue; becauſe the ſenſorial power of aſſociation or catenation being united with the ſtimulus, increaſes in energy with every repetition of the catenated circle; and it is by theſe means that all the irritative aſſociations of motions are originally produced.

2. When a ſtimulus is repeated at ſuch diſtant intervals of time, that the natural quantity of ſenſorial power becomes completely re⯑ſtored in the acting fibres, it will act with the ſame energy as when firſt applied. Hence thoſe who have lately accuſtomed themſelves to large doſes of opium by beginning with ſmall ones, and gradually in⯑creaſing them, and repeating them frequently, as mentioned in the [83] preceding paragraph; if they intermit the uſe of it for a few days only, muſt begin again with as ſmall doſes as they took at firſt, other⯑wiſe they will experience the inconveniences of intoxication.

On this circumſtance depend the conſtant unfailing effects of the various kinds of ſtimulus, which excite into action all the vaſcular ſyſtems in the body; the arterial, venous, abſorbent, and glandular veſſels, are brought into perpetual unwearied action by the fluids, which are adapted to ſtimulate them; but theſe have the ſenſorial power of aſſociation added to that of irritation, and even in ſome de⯑gree that of ſenſation, and even of volition, as will be ſpoken of in their places; and life itſelf is thus carried on by the production of ſen⯑ſorial power being equal to its waſte or expenditure in the perpetual movement of the vaſcular organization.

3. When a ſtimulus is repeated at uniform intervals of time with ſuch diſtances between them, that the expenditure of ſenſorial power in the acting fibres becomes completely renewed, the effect is pro⯑duced with greater facility or energy. For the ſenſorial power of aſſociation is combined with the ſenſorial power of irritation, or, in common language, the acquired habit aſſiſts the power of the ſtimulus.

This circumſtance not only obtains in the annual and diurnal cate⯑nations of animal motions explained in Sect. XXXVI. but in every leſs circle of actions or ideas, as in the burthen of a ſong, or the ite⯑rations of a dance; and conſtitutes the pleaſure we receive from re⯑petition and imitation; as treated of in Sect. XXII. 2.

4. When a ſtimulus has been many times repeated at uniform in⯑tervals, ſo as to produce the complete action of the organ, it may then be gradually diminiſhed, or totally withdrawn, and the action of the organ will continue. For the ſenſorial power of aſſociation becomes united with that of irritation, and by frequent repetition be⯑comes at length of ſufficient energy to carry on the new link in [84] the circle of actions, without the irritation which at firſt intro⯑duced it.

Hence, when the bark is given at ſtated intervals for the cure of intermittent fevers, if ſixty grains of it be given every three hours for the twenty-four hours preceding the expected paroxyſm, ſo as to ſti⯑mulate the defective part of the ſyſtem into action, and by that means to prevent the torpor or quieſcence of the fibres, which conſtitutes the cold fit; much leſs than half the quantity, given before the time at which another paroxyſm of quieſcence would have taken place, will be ſufficient to prevent it; becauſe now the ſenſorial power, termed aſſociation, acts in a twofold manner. Firſt, in reſpect to the period of the catenation in which the cold fit was produced, which is now diſſevered by the ſtronger ſtimulus of the firſt doſes of the bark; and, ſecondly, becauſe each doſe of bark being repeated at periodical times, has its effect increaſed by the ſenſorial faculty of aſſociation being com⯑bined with that of irritation.

Now, when ſixty grains of Peruvian bark are taken twice a day, ſuppoſe at ten o'clock and at ſix, for a fortnight, the irritation ex⯑cited by this additional ſtimulus becomes a part of the diurnal circle of actions, and will at length carry on the increaſed action of the ſyſtem without the aſſiſtance of the ſtimulus of the bark. On this theory the bitter medicines, chalybeates, and opiates in appropri⯑ated doſes, exhibited for a fortnight, give permanent ſtrength to pale feeble children, and other weak conſtitutions.

5. When a defect of ſtimulus, as of heat, recurs at certain diurnal intervals, which induces ſome torpor or quieſcence of a part of the ſyſtem, the diurnal catenation of actions becomes diſordered, and a new aſſociation with this link of torpid action is formed; on the next period the quantity of quieſcence will be increaſed, ſuppoſe the ſame defect of ſtimulus to recur, becauſe now the new aſſociation con⯑ſpires [85] with the defective irritation in introducing the torpid action of this part of the diurnal catenation. In this manner many fever⯑fits commence, where the patient is for ſome days indiſpoſed at cer⯑tain hours, before the cold paroxiſm of fever is completely formed. See Sect. XVII. 3. 3. on Catenation of Animal Motions.

6. If a ſtimulus, which at firſt excited the affected organ into ſo great exertion as to produce ſenſation, be continued for a certain time, it will ceaſe to produce ſenſation both then and when repeated, though the irritative motions in conſequence of it may continue or be re⯑excited.

Many catenations of irritative motions were at firſt ſucceeded by ſenſation, as the apparent motions of objects when we walk paſt them, and probably the vital motions themſelves in the early ſtate of our ex⯑iſtence. But as thoſe ſenſations were followed by no movements of the ſyſtem in conſequence of them, they gradually ceaſed to be pro⯑duced, not being joined to any ſucceeding link of catenation. Hence contagious matter, which has for ſome weeks ſtimulated the ſyſtem into great and permanent ſenſation, ceaſes afterwards to produce gene⯑ral ſenſation, or inflammation, though it may ſtill induce topical irri⯑tations. See Sect. XXXIII. 2. 8. XIX. 10.

Our abſorbent ſyſtem then ſeems to receive thoſe contagious mat⯑ters, which it has before experienced, in the ſame manner as it im⯑bibes common moiſture or other fluids; that is, without being thrown into ſo violent action as to produce ſenſation; the conſe⯑quence of which is an increaſe of daily energy or activity, till inflam⯑mation and its conſequences ſucceed.

7. If a ſtimulus excites an organ into ſuch violent contractions as to produce ſenſation, the motions of which organ had not uſually pro⯑duced ſenſation, this new ſenſorial power, added to the irritation oc⯑caſioned by the ſtimulus, increaſes the activity of the organ. And if this activity be catenated with the diurnal circle of actions, and in⯑creaſing inflammation is produced; as in the evening paroxyſms of [86] ſmall-pox, and other fevers with inflammation. And hence ſchirrous tumours, tendons and membranes, and probably the arteries them⯑ſelves become inflamed, when they are ſtrongly ſtimulated.

IV. Of Stimulus greater than natural.

1. A quantity of ſtimulus greater than natural, producing an in⯑creaſed exertion of ſenſorial power, whether that exertion be in the mode of irritation, ſenſation, volition, or aſſociation, diminiſhes the general quantity of it. This fact is obſervable in the progreſs of in⯑toxication, as the increaſed quantity or energy of the irritative mo⯑tions, owing to the ſtimulus of vinous ſpirit, introduces much plea⯑ſurable ſenſation into the ſyſtem, and much exertion of muſcular or ſenſual motions in conſequence of this increaſed ſenſation; the volun⯑tary motions, and even the aſſociate ones, become much impaired or diminiſhed; and delirium and ſtaggering ſucceed. See Sect. XXI. on Drunkenneſs. And hence the great proſtration of the ſtrength of the locomotive muſcles in ſome fevers, is owing to the exhauſtion of ſenſorial power by the increaſed action of the arterial ſyſtem.

In like manner a ſtimulus greater than natural, applied to a part of the ſyſtem, increaſes the exertion of ſenſorial power in that part, and diminiſhes it in ſome other part. As in the commencement of ſcarlet fever, it is uſual to ſee great redneſs and heat on the faces and breaſts of children, while at the ſame time their ſeet are colder than natural; partial heats are obſervable in other fevers with debility, and are ge⯑nerally attended with torpor or quieſcence of ſome other part of the ſyſtem. But theſe partial exertions of ſenſorial power are ſometimes attended with increaſed partial exertions in other parts of the ſyſtem, which ſympathize with them, as the fluſhing of the face after a full meal. Both theſe therefore are to be aſcribed to ſympathetic aſſoci⯑ations, [87] explained in Sect. XXXV. and not to general exhauſtion or accumulation of ſenſorial power.

2. A quantity of ſtimulus greater than natural, producing an in⯑creaſed exertion of ſenſorial power in any particular organ, diminiſhes the quantity of it in that organ. This appears from the contractions of animal fibres being not ſo eaſily excited by a leſs ſtimulus after the organ has been ſubjected to a greater. Thus after looking at any lu⯑minous object of a ſmall ſize, as at the ſetting ſun, for a ſhort time, ſo as not much to fatigue the eye, this part of the retina becomes leſs ſenſible to ſmaller quantities of light; hence when the eyes are turned on other leſs luminous parts of the ſky, a dark ſpot is ſeen reſembling the ſhape of the ſun, or other luminous object which we laſt behold. See Sect. XL. No. 2.

Thus we are ſome time before we can diſtinguiſh objects in an obſcure room after coming from bright day-light, though the iris preſently contracts itſelf. We are not able to hear weak ſounds after loud ones. And the ſtomachs of thoſe who have been much habituated to the ſtronger ſtimulus of fermented or ſpirituous liquors, are not excited into due action by weaker ones.

3. A quantity of ſtimulus ſomething greater than the laſt men⯑tioned, or longer continued, induces the organ into ſpaſmodic action, which ceaſes and recurs alternately. Thus on looking for a time on the ſetting ſun, ſo as not greatly to ſatigue the ſight, a yellow ſpec⯑trum is ſeen when the eyes are cloſed and covered, which continues for a time, and then diſappears and recurs repeatedly before it entirely vaniſhes. See Sect. XL. No. 5. Thus the action of vomiting ceaſes and is renewed by intervals, although the emetic drug is thrown up with the firſt effort. A teneſmus continues by intervals ſome time after the excluſion of acrid excrement; and the pulſations of the heart of a viper are ſaid to continue ſome time after it is cleared from its blood.

[88] In theſe caſes the violent contractions of the fibres produce pain according to law 4; and this pain conſtitutes an additional kind or quantity of excitement, which again induces the fibres into contrac⯑tion, and which painful excitement is again renewed, and again in⯑duces contractions of the fibres with gradually diminiſhing effect.

4. A quantity of ſtimulus greater than that laſt mentioned, or longer continued, induces the antagoniſt muſcles into ſpaſmodic ac⯑tion. This is beautifully illuſtrated by the ocular ſpectra deſcribed in Sect. XL. No. 6. to which the reader is referred. From thoſe experiments there is reaſon to conclude that the fatigued part of the retina throws itſelf into a contrary mode of action like oſcitation or pandiculation, as ſoon as the ſtimulus, which has fatigued it, is withdrawn; but that it ſtill remains liable to be excited into action by any other colours except the colour with which it has been fa⯑tigued. Thus the yawning and ſtretching the limbs after a con⯑tinued action or attitude ſeems occaſioned by the antagoniſt muſcles being ſtimulated by their extenſion during the contractions of thoſe in action, or in the ſituation in which that action laſt left them.

5. A quantity of ſtimulus greater than the laſt, or longer conti⯑nued, induces variety of convulſions or fixed ſpaſms either of the af⯑fected organ or of the moving fibres in the other parts of the body. In reſpect to the ſpectra in the eye, this is well illuſtrated in No. 7 and 8, of Sect. XL. Epilectic convulſions, as the emproſthotonos and opiſthotonos, with the cramp of the calf of the leg, locked jaw, and other cataleptic fits, appear to originate from pain, as ſome of theſe patients ſcream aloud before the convulſion takes place; which ſeems at firſt to be an effort to relieve painful ſenſation, and after⯑wards an effort to prevent it.

In theſe caſes the violent contractions of the fibres produce ſo much pain, as to conſtitute a perpetual excitement; and that in ſo great a degree as to allow but ſmall intervals of relaxation of the con⯑tracting [89] fibres as in convulſions, or no intervals at all as in fixed ſpaſms.

6. A quantity of ſtimulus greater than the laſt, or longer con⯑tinued, produces a paralyſis of the organ. In many caſes this para⯑lyſis is only a temporary effect, as on looking long on a ſmall area of bright red ſilk placed on a ſheet of white paper on the floor in a ſtrong light, the red ſilk gradually becomes paler, and at length diſappears; which evinces that a part of the retina, by being violently excited, becomes for a time unaffected by the ſtimulus of that colour. Thus cathartic medicines, opiates, poiſons, contageous matter, ceaſe to in⯑fluence our ſyſtem after it has been habituated to the uſe of them, except by the exhibition of increaſed quantities of them; our fibres not only become unaffected by ſtimuli, by which they have previ⯑ouſly been violently irritated, as by the matter of the ſmall-pox or meaſles; but they alſo become unaffected by ſenſation, where the violent exertions, which diſabled them, were in conſequence of too great quantity of ſenſation. And laſtly the fibres, which become diſobedient to volition, are probably diſabled by their too violent exertions in conſequence of too great a quantity of vo⯑lition.

After every exertion of our fibres a temporary paralyſis ſucceeds, whence the intervals of all muſcular contractions, as mentioned in No. 3 and 4 of this Section; the immediate cauſe of theſe more per⯑manent kinds of paralyſis is probably owing in the ſame manner to the too great exhauſtion of the ſpirit of animation in the affected part; ſo that a ſtronger ſtimulus is required, or one of a different kind from that, which occaſioned thoſe too violent contractions, to again excite the affected organ into activity; and if a ſtronger ſtimulus could be applied, it muſt again induce paralyſis.

For theſe powerful ſtimuli excite pain at the ſame time, that they produce irritation; and this pain not only excites fibrous motions by its ſtimulus, but it alſo produces volition; and thus all theſe ſtimuli [90] acting at the ſame time, and ſometimes with the addition of their aſ⯑ſociations, produce ſo great exertion as to expend the whole of the ſenſorial power in the affected fibres.

V. Of Stimulus leſs than natural.

1. A quantity of ſtimulus leſs than natural, producing a decreaſed exertion of ſenſorial power, occaſions an accumulation of the general quantity of it. This circumſtance is obſervable in the haemiplagia, in which the patients are perpetually moving the muſcles, which are unaffected. On this account we awake with greater vigour after ſleep, becauſe during ſo many hours, the great uſual expenditure of ſenſorial power in the performance of voluntary actions, and in the exertions of our organs of ſenſe, in conſequence of the irritations oc⯑caſioned by external objects had been ſuſpended, and a conſequent ac⯑cumulation had taken place.

In like manner the exertion of the ſenſorial power leſs than natural in one part of the ſyſtem, is liable to produce an increaſe of the ex⯑ertion of it in ſome other part. Thus by the action of vomiting, in which the natural exertion of the motions of the ſtomach are de⯑ſtroyed or diminiſhed, an increaſed abſorption of the pulmonary and cellular lymphatics is produced, as is known by the increaſed abſorp⯑tion of the fluid depoſited in them in dropſical caſes. But theſe par⯑tial quieſcences of ſenſorial power are alſo ſometimes attended with other partial quieſcences, which ſympathize with them, as cold and pale extremities from hunger. Theſe therefore are to be aſcribed to the aſſociations of ſympathy explained in Sect. XXXV. and not to the general accumulation of ſenſorial power.

2. A quantity of ſtimulus leſs than natural, applied to fibres pre⯑viouſly accuſtomed to perpetual ſtimulus, is ſucceeded by accumula⯑tion [91] of ſenſorial power in the affected organ. The truth of this pro⯑poſition is evinced, becauſe a ſtimulus leſs than natural, if it be ſomewhat greater than that above mentioned, will excite the organ ſo circumſtanced into violent activity. Thus on a froſty day with wind, the face of a perſon expoſed to the wind is at firſt pale and ſhrunk; but on turning the face from the wind, it becomes ſoon of a glow with warmth and fluſhing. The glow of the Ikin in emerg⯑ing from the cold-bath is owing to the ſame cauſe.

It does not appear, that an accumulation of ſenſorial power above the natural quantity is acquired by thoſe muſcles, which are not ſub⯑ject to perpetual ſtimulus, as the locomotive muſcles: theſe, after the greateſt fatigue, only acquire by reſt their uſual aptitude to motion; whereas the vaſcular ſyſtem, as the heart and arteries, after a ſhort quieſcence, are thrown into violent action by their natural quantity of ſtimulus.

Nevertheleſs by this accumulation of ſenſorial power during the application of decreaſed ſtimulus, and by the exhauſtion of it during the action of increaſed ſtimulus, it is wiſely provided, that the ac⯑tions of the vaſcular muſcles and organs of ſenſe are not much de⯑ranged by ſmall variations of ſtimulus; as the quantity of ſenſorial power becomes in ſome meaſure inverſely as the quantity of ſti⯑mulus.

3. A quantity of ſtimulus leſs than that mentioned above, and continued for ſome time, induces pain in the affected organ, as the pain of cold in the hands, when they are immerſed in ſnow, is owing to a deficiency of the ſtimulation of heat. Hunger is a pain from the deficiency of the ſtimulation of food. Pain in the back at the com⯑mencement of ague-fits, and the head-achs which attend feeble peo⯑ple, are pains from defect of ſtimulus, and are hence relieved by opi⯑um, eſſential oils, ſpirit of wine.

As the pains, which originate from defect of ſtimulus, only occur in thoſe parts of the ſyſtem, which have been previouſy ſubjected to [92] perpetual ſtimulus; and as an accumulation of ſenſorial power is pro⯑duced in the quieſcent organ along with the pain, as in cold or hunger, there is reaſon to believe, that the pain is owing to the ac⯑cumulation of ſenſorial power. For, in the locomotive muſcles, in the retina of the eye, and other organs of ſenſes, no pain occurs from the abſence of ſtimulus, nor any great accumulation of ſenſorial power beyond their natural quantity, ſince theſe organs have not been uſed to a perpetual ſupply of it. There is indeed a greater ac⯑cumulation occurs in the organ of viſion after its quieſcence, becauſe it is ſubject to more conſtant ſtimulus.

4. A certain quantity of ſtimulus leſs than natural induces the moving organ into feebler and more frequent contractions, as men⯑tioned in No. I. 4. of this Section. For each contraction moving through a leſs ſpace, or with leſs force, that is, with leſs expendi⯑ture of the ſpirit of animation, is ſooner relaxed, and the ſpirit of ani⯑mation derived at each interval into the acting fibres being leſs, theſe intervals likewiſe become ſhorter. Hence the tremours of the hands of people accuſtomed to vinous ſpirit, till they take their uſual ſtimu⯑lus; hence the quick pulſe in fevers attended with debility, which is greater than in fevers attended with ſtrength; in the latter the pulſe ſeldom beats above 120 times in a minute, in the former it fre⯑quently exceeds 140.

It muſt be obſerved, that in this and the two following articles the decreaſed action of the ſyſtem is probably more frequently occaſioned by deficiency in the quantity of ſenſorial power, than in the quantity of ſtimulus. Thus thoſe feeble conſtitutions which have large pupils of their eyes, and all who labour under nervous fevers, ſeem to owe their want of natural quantity of activity in the ſyſtem to the defici⯑ency of ſenſorial power; ſince, as far as can be ſeen, they frequently poſſeſs the natural quantity of ſtimulus.

5. A certain quantity of ſtimulus, leſs than that above mentioned, inverts the order of ſucceſſive fibrous contractions; as in vomiting [93] the vermicular motions of the ſtomach and duodenum are inverted, and their contents ejected, which is probably owing to the exhauſtion of the ſpirit of animation in the acting muſcles by a previous exceſſive ſtimulus, as by the root of ipecacuanha, and the conſequent defect of ſenſorial power. The ſame retrograde motions affect the whole inteſtinal canal in ileus; and the oeſophagus in globus hyſtericus. See this further explained in Sect. XXIX. No. 11. on Retrograde Motions.

I muſt obſerve, alſo, that ſomething ſimilar happens in the pro⯑duction of our ideas, or ſenſual motions, when they are too weakly excited; when any one is thinking intenſely about one thing, and careleſsly converſing about another, he is liable to uſe the word of a contrary meaning to that which he deſigned, as cold weather for hot weather, ſummer for winter.

6. A certain quantity of ſtimulus, leſs than that above mentioned, is ſucceeded by paralyſis, firſt of the voluntary and ſenſitive motions, and afterwards of thoſe of irritation and of aſſociation, which conſti⯑tutes death.

VI. Cure of increaſed Exertion.

1. The cure, which nature has provided for the increaſed exertion of any part of the ſyſtem, conſiſts in the conſequent expenditure of the ſenſorial power. But as a greater torpor follows this exhauſtion of ſenſorial power, as explained in the next paragraph, and a greater ex⯑ertion ſucceeds this torpor, the conſtitution frequently ſinks under theſe increaſing librations between exertion and quieſcence; till at length complete quieſcence, that is, death, cloſes the ſcene.

For, during the great exertion of the ſyſtem in the hot fit of fever, an increaſe of ſtimulus is produced from the greater momentum of [94] the blood, the greater diſtention of the heart and arteries, and the in⯑creaſed production of heat, by the violent actions of the ſyſtem oc⯑caſioned by this augmentation of ſtimulus, the ſenſorial power be⯑comes diminiſhed in a few hours much beneath its natural quantity, the veſſels at length ceaſe to obey even theſe great degrees of ſtimulus, as ſhewn in Sect. XL. 9. 1. and a torpor of the whole or of a part of the ſyſtem enſues.

Now as this ſecond cold fit commences with a greater deficiency of ſenſorial power, it is alſo attended with a greater deficiency of ſtimu⯑lus than in the preceding cold fit, that is, with leſs momentum of blood, leſs diſtention of the heart. On this account the ſecond cold fit becomes more violent and of longer duration than the firſt; and as a greater accumulation of ſenſorial power muſt be produced before the ſyſtem of veſſels will again obey the diminiſhed ſtimulus, it follows, that the ſecond hot fit of fever will be more violent than the former one. And that unleſs ſome other cauſes counteract either the violent exertions in the hot fit, or the great torpor in the cold fit, life will at length be extinguiſhed by the expenditure of the whole of the ſenſorial power. And from hence it appears, that the true means of curing fevers muſt be ſuch as decreaſe the action of the ſyſtem in the hot fit, and increaſe it in the cold fit; that is, ſuch as prevent the too great diminution of ſenſorial power in the hot fit, and the too great accumulation of it in the cold one.

2. Where the exertion of the ſenſorial powers is much increaſed, as in the hot-fits of fever or inflammation, the following are the uſual means of relieving it. Decreaſe the irritations by blood-letting, and other evacuations; by cold water taken into the ſtomach, or injected as an enema, or uſed externally; by cold air breathed into the lungs, and diffuſed over the ſkin; with food of leſs ſtimulus than the patient has been accuſtomed to.

3. As a cold fit, or paroxyſm of inactivity of ſome parts of the ſyſtem, generally precedes the hot fit, or paroxyſm of exertion, by [95] which the ſenſorial power becomes accumulated, this cold paroxyſm ſhould be prevented by ſtimulant medicines and diet, as wine, opium, bark, warmth, cheerfulneſs, anger, ſurpriſe.

4. Excite into greater action ſome other part of the ſyſtem, by which means the ſpirit of animation may be in part expended, and thence the inordinate actions of the diſeaſed part may be leſſened. Hence when a part of the ſkin acts violently, as of the face in the eruption of the ſmall-pox, if the feet be cold they ſhould be covered. Hence the uſe of a bliſter applied near a topical inflammation. Hence opium and warm bath relieve pains both from exceſs and defect of ſtimulus.

5. Firſt increaſe the general ſtimulation above its natural quantity, which may in ſome degree exhauſt the ſpirit of animation, and then decreaſe the ſtimulation beneath its natural quantity. Hence after ſudorific medicines and warm air, the application of refrigerants may have greater effect, if they could be adminiſtered without danger of producing too great torpor of ſome part of the ſyſtem; as frequently happens to people in health from coming out of a warm room into the cold air, by which a topical inflammation in conſequence of torpor of the mucous membrane of the noſtril is produced, and is termed a cold in the head.

VII. Cure of decreaſed Exertion.

1. WHERE the exertion of the ſenſorial powers is much decreaſed, as in the cold fits of fever, as gradual accumulation of the ſpirit of animation takes place; as occurs in all caſes where inactivity or tor⯑por of a part of the ſyſtem exiſts; this accumulation of ſenſorial power increaſes, till ſtimuli leſs than natural are ſufficient to throw it [96] into action, then the cold fit ceaſes; and from the action of the na⯑tural ſtimuli a hot one ſucceeds with increaſed activity of the whole ſyſtem.

So in fainting fits, or ſyncope, there is a temporary deficiency of ſenſorial exertion, and a conſequent quieſcence of a great part of the ſyſtem. This quieſcence continues, till the ſenſorial power becomes again accumulated in the torpid organs; and then the uſual diurnal ſtimuli excite the reviveſcent parts again into action; but as this kind of quieſcence continues but a ſhort time compared to the cold parox⯑yſm of an ague, and leſs affects the circulatory ſyſtem, a leſs ſuper⯑abundancy of exertion ſucceeds in the organs previouſly torpid, and a leſs exceſs of arterial activity. See Sect. XXXIV. 1. 6.

2. In the diſeaſes occaſioned by a defect of ſenſorial exertion, as in cold fits of ague, hyſteric complaint, and nervous fever, the follow⯑ing means are thoſe commonly uſed. 1. Increaſe the ſtimulation above its natural quantity for ſome weeks, till a new habit of more energetic contraction of the fibres is eſtabliſhed. This is to be done by wine, opium, bark, ſteel, given at exact periods, and in appro⯑priate quantities; for if theſe medicines be given in ſuch quantity, as to induce the leaſt degree of intoxication, a debility ſucceeds from the uſeleſs exhauſtion of ſpirit of animation in conſequence of too great exertion of the muſcles or organs of ſenſe. To theſe irritative ſtimuli ſhould be added the ſenſitive ones of cheerful ideas, hope, affection.

3. Change the kinds of ſtimulus. The habits acquired by the conſtitution depend on ſuch nice circumſtances, that when one kind of ſtimulus ceaſes to excite the ſenſorial power into the quantity of exertion neceſſary to health, it is often ſufficient to change the ſti⯑mulus for another apparently ſimilar in quantity and quality. Thus when wine ceaſes to ſtimulate the conſtitution, opium in appropriate doſes ſupplies the defect; and the contrary. This is alſo obſerved in [97] the effects of cathartic medicines, when one loſes its power, another, apparently leſs efficacious, will ſucceed. Hence a change of diet, drink, and ſtimulating medicines, is often advantageous in diſeaſes of debility.

4. Stimulate the organs, whoſe motions are aſſociated with the torpid parts of the ſyſtem. The actions of the minute veſſels of the various parts of the external ſkin are not only aſſociated with each other, but are ſtrongly aſſociated with thoſe of ſome of the internal membranes, and particularly of the ſtomach. Hence when the ex⯑ertion of the ſtomach is leſs than natural, and indigeſtion and heart⯑burn ſucceed, nothing ſo certainly removes theſe ſymptoms as the ſtimulus of a bliſter on the back. The coldneſs of the extremities, as of the noſe, ears, or fingers, are hence the beſt indication for the ſucceſsful application of bliſters.

5. Decreaſe the ſtimulus for a time. By leſſening the quantity of heat for a minute or two by going into the cold bath, a great accumulation of ſenſorial power is produced; for not only the minute veſſels of the whole external ſkin for a time become inactive, as appears by their paleneſs; but the minute veſſels of the lungs loſe much of their ac⯑tivity alſo by concert with thoſe of the ſkin, as appears from the dif⯑ficulty of breathing at firſt going into cold water. On emerging from the bath the ſenſorial power is thrown into great exertion by the ſtimulus of the common degree of the warmth of the atmo⯑ſphere, and a great production of animal heat is the conſequence. The longer a perſon continues in the cold bath the greater muſt be the preſent inertion of a great part of the ſyſtem, and in conſequence a greater accumulation of ſenſorial power. Whence M. Pome rè⯑commends ſome melancholy patients to be kept from two to ſix hours in ſpring-water, and in baths ſtill colder.

6 Decreaſe the ſtimulus for a time below the natural, and then increaſe it above natural. The effect of this proceſs, improperly uſed, is ſeen in giving much food, or applying much warmth, to thoſe [98] who have been previouſly expoſed to great hunger, or to great cold. The accumulated ſenſorial power is thrown into ſo violent exertion, that inflammations and mortifications ſupervene, and death cloſes the cataſtrophe. In many diſeaſes this method is the moſt ſucceſsful; hence the bark in agues produces more certain effect after the previ⯑ous exhibition of emetics. In diſeaſes attended with violent pain, opium has double the effect, if veneſection and a cathartic have been previouſly uſed. On this ſeems to have been founded the ſucceſsful practice of Sydenham, who uſed veneſection and a cathartic in chlo⯑roſis before the exhibition of the bark, ſteel, and opiates.

7. Prevent any unneceſſary expenditure of ſenſorial power. Hence in fevers with debility, a decumbent poſture is preferred, with ſilence, little light, and ſuch a quantity of heat as may prevent any chill ſen⯑ſation, or any coldneſs of the extremities. The pulſe of patients in fevers with debility increaſes in frequency above ten pulſations in a minute on their riſing out of bed. For the expenditure of ſenſorial power to preſerve an erect poſture of the body adds to the general de⯑ficiency of it, and thus affects the circulation.

8. The longer in time and the greater in degree the quieſcence or inertion of an organ has been, ſo that it ſtill retains life or excitability, the leſs ſtimulus ſhould at firſt be applied to it. The quantity of ſti⯑mulation is a matter of great nicety to determine, where the torpor or quieſcence of the fibres has been experienced in a great degree, or for a conſiderable time, as in cold fits of the ague, in continued fevers with great debility, or in people famiſhed at ſea, or periſhing with cold. In the two laſt caſes, very minute quantities of food ſhould be firſt ſupplied, and very few additional degrees of heat. In the two former caſes, but little ſtimulus of wine or medicine, above what they had been lately accuſtomed to, ſhould be exhibited, and this at frequent and ſtated intervals, ſo that the effect of one quantity may be obſerved before the exhibition of another.

If theſe circumſtances are not attended to, as the ſenſorial power [99] becomes accumulated in the quieſcent fibres, an inordinate exertion takes place by the increaſe of ſtimulus acting on the accumulated quantity of ſenſorial power, and either the paralyſis, or death of the contractile fibres enſues, from the total expenditure of the ſenſorial power in the affected organ, owing to this increaſe of exertion, like the debility after intoxication. Or, ſecondly, the violent exertions above mentioned produce painful ſenſation, which becomes a new ſtimulus, and by thus producing inflammation, and increaſing the activity of the fibres already too great, ſooner exhauſts the whole of the ſenſorial power in the acting organ, and mortification, that is, the death of the part, ſupervenes.

Hence there have been many inſtances of people, whoſe limbs have been long benumbed by expoſure to cold, who have loſt them by mortification on their being too haſtily brought to the fire; and of others, who were nearly famiſhed at ſea, who have died ſoon after having taken not more than an uſual meal of food. I have heard of two well-atteſted inſtances of patients in the cold fit of ague, who have died from the exhibition of gin and vinegar, by the inflammation which enſued. And in many fevers attended with debility, the un⯑limited uſe of wine, and the wanton application of bliſters, I believe, has deſtroyed numbers by the debility conſequent to too great ſtimu⯑lation, that is, by the exhauſtion of the ſenſorial power by its in⯑ordinate exertion.

Wherever the leaſt degree of intoxication exiſts, a proportional de⯑bility is the conſequence; but there is a golden rule by which the neceſſary and uſeful quantity of ſtimulus in fevers with debility may be aſcertained. When wine or beer are exhibited either alone or di⯑luted with water, if the pulſe becomes ſlower the ſtimulus is of a proper quantity; and ſhould be repeated every two or three hours, or when the pulſe again becomes quicker.

In the chronical debility brought on by drinking ſpirituous or fer⯑mented liquors, there is another golden rule by which I have ſucceſs⯑fully [100] directed the quantity of ſpirit which they may ſafely leſſen, for there is no other means by which they can recover their health. It ſhould be premiſed, that where the power of digeſtion in theſe pa⯑tients is totally deſtroyed, there is not much reaſon to expect a return to healthful vigour.

I have directed ſeveral of theſe patients to omit one fourth part of the quantity of vinous ſpirit they have been lately accuſtomed to, and if in a fortnight their appetite increaſes, they are adviſed to omit another fourth part; but if they perceive that their digeſtion becomes impaired from the want of this quantity of ſpirituous potation, they are adviſed to continue as they are, and rather bear the ills they have, than riſk the encounter of greater. At the ſame time fleſh-meat with or without ſpice is recommended, with Peruvian bark and ſteel in ſmall quantities between their meals, and half a grain of opium or a grain, with five or eight grains of rhubarb at night.

I. 1. THE fibres of the vegetable world, as well as thoſe of the animal, are excitable into a variety of motion by the irritations of ex⯑ternal objects. This appears particularly in the mimoſa or ſenſitive plant, whoſe leaves contract on the ſlighteſt injury; the dionaea muſ⯑cipula, which was lately brought over from the marſhes of America, preſents us with another curious inſtance of vegetable irritability; its leaves are armed with ſpines on their upper edge, and are ſpread on the ground around the ſtem; when an inſect creeps on any of them in its paſſage to the flower or ſeed, the leaf ſhuts up like a ſteel rat⯑trap, and deſtroys its enemy. See Botanic Garden, Part II. note on Silene.

The various ſecretions of vegetables, as of odour, fruit, gum, reſin, wax, honey, ſeem brought about in the ſame manner as in the glands of animals: the taſteleſs moiſture of the earth is converted by the hop-plant into a bitter juice; as by the caterpillar in the nut⯑ſhell [102] the ſweet kernel is converted into a bitter powder. While the power of abſorption in the roots and barks of vegetables is excited into action by the fluids applied to their mouths like the lacteals and lymphatics of animals.

2. The individuals of the vegetable world may be conſidered as in⯑ferior or leſs perfect animals; a tree is a congeries of many living buds, and in this reſpect reſembles the branches of coralline, which are a congeries of a multitude of animals. Each of theſe buds of a tree has its proper leaves or petals for lungs, produces its viviparous or its oviparous offspring in buds or ſeeds; has its own roots, which extending down the ſtem of the tree are interwoven with the roots of the other buds, and form the bark, which is the only living part of the ſtem, is annually renewed, and is ſuperinduced upon the former bark, which then dies, and with its ſtagnated juices gradually hardening into wood forms the concentric circles, which we ſee in blocks of timber.

The following circumſtances evince the individuality of the buds of trees. Firſt, there are many trees, whoſe whole internal wood is periſhed, and yet the branches are vegete and healthy. Secondly, the fibres of the barks of trees are chiefly longitudinal, reſembling roots, as is beautifully ſeen in thoſe prepared barks, that were lately brought from Otaheita. Thirdly, in horizontal wounds of the bark of trees, the fibres of the upper lip are always elongated downwards like roots, but thoſe of the lower lip do not approach to meet them. Fourthly, if you wrap wet moſs round any joint of a vine, or cover it with moiſt earth, roots will ſhoot out from it. Fifthly, by the inoculation or engraſting of trees many fruits are produced from one ſtem. Sixthly, a new tree is produced from a branch plucked from an old one, and ſet in the ground. Whence it appears that the buds of deciduous trees are ſo many annual plants, that the bark is a con⯑texture of the roots of each individual bud; and that the internal wood [103] is of no other uſe but to ſupport them in the air, and that thus they reſemble the animal world in their individuality.

The irritability of plants, like that of animals, appears liable to be increaſed or decreaſed by habit; for thoſe trees or ſhrubs, which are brought from a colder climate to a warmer, put out their leaves and bloſſoms a fortnight ſooner than the indigenous ones.

Profeſſor Kalm, in his Travels in New York, obſerves that the apple-trees brought from England bloſſom a fortnight ſooner than the native ones. In our country the ſhrubs, that are brought a degree or two from the north, are obſerved to flouriſh better than thoſe, which come from the ſouth. The Siberian barley and cabbage are ſaid to grow larger in this climate than the ſimilar more ſouthern ve⯑getables. And our hoards of roots, as of potatoes and onions, ge⯑minate with leſs heat in ſpring, after they have been accuſtomed to the winter's cold, than in autumn after the ſummer's heat.

II. The ſtamens and piſtils of flowers ſhew evident marks of ſen⯑ſibility, not only from many of the ſtamens and ſome piſtils approach⯑ing towards each other at the ſeaſon of impregnation, but from many of them cloſing their petals and calyxes during the cold parts of the day. For this cannot be aſcribed to irritation, becauſe cold means a defect of the ſtimulus of heat; but as the want of accuſtomed ſtimuli produces pain, as in coldneſs, hunger, and thirſt of animals, theſe motions of vegetables in cloſing up their flowers muſt be aſcribed to the diſagreeable ſenſation, and not to the irritation of cold. Others cloſe up their leaves during darkneſs, which, like the former, cannot be owing to irritation, as the irritating material is withdrawn.

The approach of the anthers in many flowers to the ſtigmas, and of the piſtils of ſome flowers to the anthers, muſt be aſcribed to the paſſion of love, and hence belongs to ſenſation, not to irritation.

III. That the vegetable world poſſeſſes ſome degree of voluntary powers, appears from their neceſſity to ſleep, which we have ſhewn in Sect. XVIII. to conſiſt in the temporary abolition of voluntary [104] power. This voluntary power ſeems to be exerted in the circular movement of the tendrils of vines, and other climbing vegetables; or in the efforts to turn the upper ſurface of their leaves, or their flowers to the light.

IV. The aſſociations of fibrous motions are obſervable in the ve⯑getable world, as well as in the animal. The diviſions of the leaves of the ſenſitive plant have been accuſtomed to contract at the ſame time from the abſence of light; hence if by any other circumſtance, as a ſlight ſtroke or injury, one diviſion is irritated into contraction, the neighbouring ones contract alſo, from their motions being aſſoci⯑ated with thoſe of the irritated part. So the various ſtamina of the claſs of ſyngeneſia have been accuſtomed to contract together in the evening, and thence if you ſtimulate one of them with a pin, accord⯑ing to the experiment of M. Colvolo, they all contract from their ac⯑quired aſſociations.

To evince that the collapſing of the ſenſitive plant is not owing to any mechanical vibrations propagated along the whole branch, when a ſingle leaf is ſtruck with the finger, a leaf of it was ſlit with ſharp ſciſſors, and ſome ſeconds of time paſſed before the plant ſeemed ſen⯑ſible of the injury; and then the whole branch collapſed as far as the principal ſtem: this experiment was repeated ſeveral times with the leaſt poſſible impulſe to the plant.

V. 1. For the numerous circumſtances in which vegetable buds are analogous to animals, the reader is referred to the additional notes at the end of the Botanic Garden, Part 1. It is there ſhewn, that the roots of vegetables reſemble the lacteal ſyſtem of animals; the ſap-veſſels in the early ſpring, before their leaves expand, are analo⯑gous to the placental veſſels of the foetus; that the leaves of land⯑plants reſemble lungs, and thoſe of aquatic plants the gills of ſiſh; that there are other ſyſtems of veſſels reſembling the vena portarum of quadrupeds, or the aorta of fiſh; that the digeſtive power of ve⯑getables is ſimiilar to that of animals converting the fluids, which they [105] abſorb, into ſugar; that their ſeeds reſemble the eggs of animals, and their buds and bulbs their viviparous offspring. And, laſtly, that the anthers and ſtigmas are real animals, attached indeed to their parent tree like polypi or coral inſects, but capable of ſpontaneous motion; that they are affected with the paſſion of love, and furniſhed with powers of reproducing their ſpecies, and are fed with honey like the moths and butterflies, which plunder their nectaries. See Bota⯑nic Garden, Part I. add. note XXXIX.

The male flowers of valliſneria approach ſtill nearer to apparent animality, as they detach themſelves from the parent plant, and float on the ſurface of the water to the female ones. Botanic Garden, Part II. Art. Valliſneria. Other flowers of the claſſes of monecia and diecia, and polygamia, diſcharge the fecundating farina, which float⯑ing in the air is carried to the ſtigma of the female flowers, and that at conſiderable diſtances. Can this be affected by any ſpecific attrac⯑tion? or, like the diffuſion of the odorous particles of flowers, is it left to the currents of winds, and the accidental miſcarriages of it counteracted by the quantity of its production?

2. This leads us to a curious enquiry, whether vegetables have ideas of external things? As all our ideas are originally received by our ſenſes, the queſtion may be changed to, whether vegetables poſſeſs any organs of ſenſe? Certain it is, that they poſſeſs a ſenſe of heat and cold, another of moiſture and dryneſs, and another of light and darkneſs; for they cloſe their petals occaſionally from the pre⯑ſence of cold, moiſture, or darkneſs. And it has been already ſhewn, that theſe actions cannot be performed ſimply from irritation, becauſe cold and darkneſs are negative quantities, and on that ac⯑count ſenſation or volition are implied, and in conſequence a ſenſo⯑rium or union of their nerves. So when we go into the light, we contract the iris; not from any ſtimulus of the light on the fine muſcles of the iris, but from its motions being aſſociated with the ſenſation of too much light on the retina: which could not take [106] place without a ſenſorium or center of union of the nerves of the iris with thoſe of viſion. See Botanic Garden, Part I. Canto 3. l. 440. note.

Beſides theſe organs of ſenſe, which diſtinguiſh cold, moiſture, and darkneſs, the leaves of mimoſa, and of dionaea, and of droſera, and the ſtamens of many flowers, as of the berbery, and the numerous claſs of ſyngeneſia, are ſenſible to mechanic impact, that is, they poſſeſs a ſenſe of touch, as well as a common ſenſorium; by the medium of which their muſcles are excited into action. Laſtly, in many flowers the anthers, when mature, approach the ſtigma, in others the female organ approaches to the male. In a plant of collin⯑ſonia, a branch of which is now before me, the two yellow ſtamens are about three eights of an inch high, and diverge from each other, at an angle of about fifteen degrees, the purple ſtyle is half an inch, high, and in ſome flowers is now applied to the ſtamen on the right hand, and in others to that of the left; and will, I ſuppoſe, change place to-morrow in thoſe, where the anthers have not yet effuſed their powder.

I aſk, by what means are the anthers in many flowers, and ſtigmas in other flowers, directed to find their paramours? How do either of them know, that the other exiſts in their vicinity? Is this curious kind of ſtorge produced by mechanic attraction, or by the ſenſation of love? The latter opinion is ſupported by the ſtrongeſt analogy, becauſe a reproduction of the ſpecies is the conſequence; and then another organ of ſenſe muſt be wanted to direct theſe vegetable amourettes to find each other, one probably analogous to our ſenſe of ſmell, which in the animal world directs the new-born infant to its ſource of nou⯑riſhment, and they may thus poſſeſs a faculty of perceiving as well as of producing odours.

Thus, beſides a kind of taſte at the extremities of their roots, ſimi⯑lar to that of the extremities of our lacteal veſſels, for the purpoſe of ſelecting their proper food; and beſides different kinds of irritability [107] reſiding in the various glands, which ſeparate honey, wax, reſin, and other juices from their blood; vegetable life ſeems to poſſeſs an organ of ſenſe to diſtinguiſh the variations of heat, another to diſtinguiſh the varying degrees of moiſture, another of light, another of touch, and probably another analogous to our ſenſe of ſmell. To theſe muſt be added the indubitable evidence of their paſſion of love, and I think we may truly conclude, that they are furniſhed with a common ſen⯑ſorium belonging to each bud, and that they muſt occaſionally repeat thoſe perceptions either in their dreams or waking hours, and conſe⯑quently poſſeſs ideas of ſo many of the properties of the external world, and of their own exiſtence.

I. PHILOSOPHERS have been much perplexed to underſtand, in what manner we become acquainted with the external world; inſo⯑much that Dr. Berkly even doubted its exiſtence, from having ob⯑ſerved (as he thought) that none of our ideas reſemble their cor⯑reſpondent objects. Mr. Hume aſſerts, that our belief depends on the greater diſtinctneſs or energy of our ideas from perception; and Mr. Reid has lately contended, that our belief of external objects is an in⯑nate principle neceſſarily joined with our perceptions.

So true is the obſervation of the famous Malbranch, "that our ſenſes are not given us to diſcover the eſſences of things, but to ac⯑quaint us with the means of preſerving our exiſtence," (L. I. ch. v.) a melancholy reflection to philoſophers!

Some philoſophers have divided all created beings into material and [109] immaterial: the former including all that part of being, which obeys the mechanic laws of action and reaction, but which can begin no motion of itſelf; the other is the cauſe of all motion, and is either termed the power of gravity, or of ſpecific attraction, or the ſpirit of animation. This immaterial agent is ſuppoſed to exiſt in or with matter, but to be quite diſtinct from it, and to be equally capable of exiſtence, after the matter, which now poſſeſſes it, is decompoſed.

Nor is this theory ill ſupported by analogy, ſince heat, electricity, and magnetiſm, can be given to or taken from a piece of iron; and muſt therefore exiſt, whether ſeparated from the metal, or combined with it. From a parity of reaſoning, the ſpirit of animation would appear to be capable of exiſting as well ſeparately from the body as with it.

I beg to be underſtood, that I do not wiſh to diſpute about words, and am ready to allow, that the powers of gravity, ſpecific attraction, electricity, magnetiſm, and even the ſpirit of animation, may conſiſt of matter of a finer kind; and to believe, with St. Paul and Mal⯑branch, that the ultimate cauſe only of all motion is immaterial, that is God. St. Paul ſays, "in him we live and move, and have our being;" and, in the 15th chapter to the Corinthians, diſtinguiſhes between the pſyche or living ſpirit, and the pneuma or reviving ſpirit. By the words ſpirit of animation or ſenſorial power, I mean only that animal life, which mankind poſſeſſes in common with brutes, and in ſome degree even with vegetables, and leave the conſideration of the immortal part of us, which is the object of religion, to thoſe who treat of revelation.

III. Of Viſion.

OUR eyes obſerve a difference of colour, or of ſhade, in the pro⯑minences and depreſſions of objects, and that thoſe ſhades uniformly vary, when the ſenſe of touch obſerves any variation. Hence when the retina becomes ſtimulated by colours or ſhades of light in a certain form, as in a circular ſpot; we know by experience, that this is a ſign, that a tangible body is before us; and that its figure is reſembled by the miniature figure of the part of the organ of viſion, that is thus ſtimulated.

Here whilſt the ſtimulated part of the retina reſembles exactly the viſible figure of the whole in miniature, the various kinds of ſtimuli from different colours mark the viſible figures of the minuter parts; and by habit we inſtantly recall the tangible figures.

Thus when a tree is the object of ſight, a part of the retina re⯑ſembling a flat branching figure is ſtimulated by various ſhades of co⯑lours; but it is by ſuggeſtion, that the gibboſity of the tree, and the moſs, that fringes its trunk, appear before us. Theſe are ideas of ſug⯑geſtion, which we feel or attend to, aſſociated with the motions of the retina, or irritative ideas, which we do not attend to.

So that though our viſible ideas reſemble in miniature the outline of the figure of coloured bodies, in other reſpects they ſerve only as [118] a language, which by acquired aſſociations introduce the tangible ideas of bodies. Hence it is, that this ſenſe is ſo readily deceived by the art of the painter to our amuſement and inſtruction. The reader will find much very curious knowledge on this ſubject in Biſhop Berk⯑ley's Eſſay on Viſion, a work of great ingenuity.

The immediate object however of the ſenſe of viſion is light; this fluid, though its velocity is ſo great, appears to have no perceptible mechanical impulſe, as was mentioned in the third Section, but ſeems to ſtimulate the retina into animal motion by its tranſmiſſion, through this part of the ſenſorium: for though the eyes of cats or other animals appear luminous in obſcure places; yet it is probable, that none of the light, which falls on the retina, is reflected from it, but adheres to or enters into combination with the choroide coat be⯑hind it.

The combination of the particles of light with opake bodies, and therefore with the choroide coat of the eye, is evinced from the heat, which is given out, as in other chemical combinations. For the ſun⯑beams communicate no heat in their paſſage through tranſparent bodies, with which they do not combine, as the air continues cool even in the focus of the largeſt burning-glaſſes, which in a moment vitrifies a particle of opaque matter.

IV. Of the Organ of Hearing.

IT is generally believed, that the tympanum of the ear vibrates mechanically, when expoſed to audible ſounds, like the ſtrings of one muſical inſtrument, when the ſame notes are ſtruck upon another. Nor is this opinion improbable, as the muſcles and cartilages of the larynx are employed in producing variety of tones by mechanical vi⯑bration: ſo the muſcles and bones of the ear ſeem adapted to increaſe [119] or diminiſh the tenſion of the tympanum for the purpoſes of ſimilar mechanical vibrations.

But it appears from diſſection, that the tympanum is not the im⯑mediate organ of hearing, but that like the humours and cornea of the eye, it is only of uſe to prepare the object for the immediate organ. For the portio mollis of the auditory nerve is not ſpread upon the tympanum, but upon the veſtibulum, and cochlea, and ſemicircular canals of the ear; while between the tympanum and the expanſion of the auditory nerve the cavity is ſaid by Dr. Cotunnus and Dr. Meckel to be filled with water; as they had frequently obſerved by freezing the heads of dead animals before they diſſected them; and water being a more denſe fluid than air is much better adapted to the pro⯑pagation of vibrations. We may add, that even the external opening of the ear is not abſolutely neceſſary for the perception of ſound: for ſome people, who from theſe defects would have been completely deaf, have diſtinguiſhed acute or grave ſounds by the tremours of a ſtick held between their teeth propagated along the bones of the head, (Haller. Phyſ. T. V. p. 295.)

Hence it appears, that the immediate organ of hearing is not affected by the particles of the air themſelves, but is ſtimulated into animal motion by the vibrations of them. And it is probable from the looſe bones, which are found in the heads of ſome fiſhes, that the vibra⯑tions of water are ſenſible to the inhabitants of that element by a ſimilar organ.

The motions of the atmoſphere, which we become acquainted with by the ſenſe of touch, are combined with its ſolidity, weight, or vis inertiae; whereas thoſe, that are perceived by this organ, depend alone on its elaſticity. But though the vibration of the air is the immediate object of the ſenſe of hearing, yet the ideas, we receive by this ſenſe, like thoſe received from light, are only as a language, which by ac⯑quired aſſociations acquaints us with thoſe motions of tangible bodies, [120] which depend on their elaſticity; and which we had before learned by our ſenſe of touch.

V. Of Smell and of Taſte.

THE objects of ſmell are diſſolved in the fluid atmoſphere, and thoſe of taſte in the ſaliva, or other aqueous fluid, for the better dif⯑fuſing them on their reſpective organs, which ſeem to be ſtimulated into animal motion perhaps by the chemical affinities of theſe particles, which conſtitute the ſapidity and odoroſity of bodies with the nerves of ſenſe, which perceive them.

Mr. Volta has lately obſerved a curious circumſtance relative to our ſenſe of taſte. If a bit of clean lead and a bit of clean ſilver be ſepa⯑rately applied to the tongue and palate no taſte is perceived; but by applying them in contact in reſpect to the parts out of the mouth, and nearly ſo in reſpect to the parts, which are immediately applied to the tongue and palate, a ſaline or acidulous taſte is perceived, as of a fluid like a ſtream of electricity paſſing from one of them to the other. This new application of the ſenſe of taſte deſerves further inveſtiga⯑tion, as it may acquaint us with new properties of matter.

VI. Of the Senſe of Heat.

THERE are many experiments in chemical writers, that evince the exiſtence of heat as a fluid element, which covers and pervades all bodies, and is attracted by the ſolutions of ſome of them, and is de⯑truded from the combination of others. Thus from the combinations of metals with acids, and from thoſe combinations of animal fluids, [121] which are termed ſecretions, this fluid matter of heat is given out amongſt the neighbouring bodies; and in the ſolutions of ſalts in wa⯑ter, or of water in air, it is abſorbed from the bodies, that ſurround them; whilſt in its facility in paſſing through metallic bodies, and its difficulty in pervading reſins and glaſs, it reſembles the properties of the electric aura; and is like that excited by friction, and ſeems like that to gravitate amongſt other bodies in its uncombined ſtate, and to find its equilibrium.

There is no circumſtance of more conſequence in the animal eco⯑nomy than a due proportion of this fluid of heat; for the digeſtion of our nutriment in the ſtomach and bowels, and the proper qualities of all our ſecreted fluids, as they are produced or prepared partly by animal and partly by chemical proceſſes, depend much on the quantity of heat; the exceſs of which, or its deficiency, alike gives us pain, and induces us to avoid the circumſtances that occaſion them. And in this the perception of heat eſſentially differs from the perceptions of the ſenſe of touch, as we receive pain from too great preſſure of ſolid bodies, but none from the abſence of it. It is hence probable, that nature has provided us with a ſet of nerves for the perception of this fluid, which anatomiſts have not yet attended to.

There may be ſome difficulty in the proof of this aſſertion; if we look at a hot fire, we experience no pain of the optic nerve, though the heat along with the light muſt be concentrated upon it. Nor does warm water or warm oil poured into the ear give pain to the organ of hearing; and hence as theſe organs of ſenſe do not perceive ſmall exceſſes or deficiences of heat; and as heat has no greater ana⯑logy to the ſolidity or to the figures of bodies, than it has to their colours or vibrations; there ſeems no ſufficient reaſon for our aſcrib⯑ing the perception of heat and cold to the ſenſe of touch; to which it has generally been attributed, either becauſe it is diffuſed beneath the whole ſkin like the ſenſe of touch, or owing to the inaccuracy of our obſervations, or the deſect of our languages.

[122] There is another circumſtance would induce us to believe, that the perceptions of heat and cold do not belong to the organ of touch; ſince the teeth, which are the leaſt adapted for the perceptions of ſo⯑lidity or figure, are the moſt ſenſible to heat or cold; whence we are forewarned from ſwallowing thoſe materials, whoſe degree of coldneſs or of heat would injure our ſtomachs.

The following is an extract from a letter of Dr. R. W. Darwin, of Shrewſbury, when he was a ſtudent at Edinburgh. "I made an experiment yeſterday in our hoſpital, which much favours your opi⯑nion, that the ſenſation of heat and of touch depend on different ſets of nerves. A man who had lately recovered from a fever, and was ſtill weak, was ſeized with violent cramps in his legs and feet; which were removed by opiates, except that one of his feet remained inſen⯑ſible. Mr. Ewart pricked him with a pin in five or ſix places, and the patient declared he did not feel it in the leaſt, nor was he ſenſible of a very ſmart pinch. I then held a red-hot poker at ſome diſtance, and brought it gradually nearer till it came within three inches, when he aſſerted that he felt it quite diſtinctly. I ſuppoſe ſome violent ir⯑ritation on the nerves of touch had cauſed the cramps, and had left them paralytic; while the nerves of heat, having ſuffered no increaſed ſtimulus, retained their irritability."

VII. Of the Senſe of Extenſion.

THE organ of touch is properly the ſenſe of preſſure, but the muſ⯑cular fibres themſelves conſtitute the organ of ſenſe, that feels ex⯑tenſion. The ſenſe of preſſure is always attended with the ideas of the figure and ſolidity of the object, neither of which accompany our perception of extenſion. The whole ſet of muſcles, whether they are hollow ones, as the heart, arteries, and inteſtines, or longitudinal [123] ones attached to bones, contract themſelves, whenever they are ſti⯑mulated by forcible elongation; and it is obſervable, that the white muſcles, which conſtitute the arterial ſyſtem, ſeem to be excited into contraction from no other kinds of ſtimulus, according to the experiments of Haller. And hence the violent pain in ſome inflam⯑mations, as in the paronychia, obtains immediate relief by cutting the membrane, that was ſtretched by the tumour of the ſubjacent parts.

Hence the whole muſcular ſyſtem may be conſidered as one organ of ſenſe, and the various attitudes of the body, as ideas belonging to this organ, of many of which we are hourly conſcious, while many others, like the irritative ideas of the other ſenſes, are performed with⯑out our attention.

When the muſcles of the heart ceaſe to act, the refluent blood again diſtends or elongates them; and thus irritated they contract as before. The ſame happens to the arterial ſyſtem, and I ſuppoſe to the capillaries, inteſtines, and various glands of the body.

When the quantity of urine, or of excrement, diſtends the bladder, or rectum, thoſe parts contract, and exclude their contents, and many other muſcles by aſſociation act along with them; but if theſe evacuations are not ſoon complied with, pain is produced by a little further extenſion of the muſcular fibres: a ſimilar pain is cauſed in the muſcles, when a limb is much extended for the reduction of diſ⯑located bones; and in the puniſhment of the rack: and in the pain⯑ful cramps of the calf of the leg, or of other muſcles, for a greater degree of contraction of a muſcle, than the movement of the two bones, to which its ends are affixed, will admit of, muſt give ſimi⯑lar pain to that, which is produced by extending it beyond its due length. And the pain from punctures or inciſions ariſes from the diſ⯑tention of the fibres, as the knife paſſes through them; for it nearly ceaſes as ſoon as the diviſion is completed.

[124] All theſe motions of the muſcles, that are thus naturally excited by the ſtimulus of diſtending bodies, are alſo liable to be called into ſtrong action by their catenation, with the irritations or ſenſations produced by the momentum of the progreſſive particles of blood in the arteries, as in inflammatory fevers, or by acrid ſubſtances on other ſenſible organs, as in the ſtrangury, or teneſmus, or cholera.

We ſhall conclude this account of the ſenſe of extenſion by obſerv⯑ing, that the want of its object is attended with a diſagreeable ſenſa⯑tion, as well as the exceſs of it. In thoſe hollow muſcles, which have been accuſtomed to it, this diſagreeable ſenſation is called faint⯑neſs, emptineſs, and ſinking; and, when it ariſes to a certain degree, is attended with ſyncope, or a total quieſcence of all motions, but the internal irritative ones, as happens from ſudden loſs of blood, or in the operation of tapping in the dropſy.

VIII. Of the Appetites of Hunger, Thirſt, Heat, Extenſion, the want of freſh Air, animal Love, and the Suckling of Children.

HUNGER is moſt probably perceived by thoſe numerous ramifica⯑tions of nerves that are ſeen about the upper opening of the ſtomach; and thirſt by the nerves about the fauces, and the top of the gula. The ideas of theſe ſenſes are few in the generality of mankind, but are more numerous in thoſe, who by diſeaſe, or indulgence, deſire particular kinds of foods or liquids.

A ſenſe of heat has already been ſpoken of, which may with pro⯑priety be called an appetite, as we painfully deſire it, when it is de⯑ficient in quantity.

The ſenſe of extenſion may be ranked amongſt theſe appetites, ſince the deficiency of its object gives diſagrecable ſenſation; when this happens in the arterial ſyſtem, it is called faintneſs, and ſeems to [125] bear ſome analogy to hunger and to cold; which like it are attended with emptineſs of a part of the vaſcular ſyſtem.

The ſenſe of want of freſh air has not been attended to, but is as diſtinct as the others, and the firſt perhaps that we experience after our nativity; from the want of the object of this ſenſe many diſeaſes are produced, as the jail-fever, plague, and other epidemic maladies. Animal love is another appetite, which occurs later in life, and the females of lactiferous animals have another natural inlet of pleaſure or pain from the ſuckling their offspring. The want of which either owing to the death of their progeny, or to the faſhion of their country, has been fatal to many of the ſex. The males have alſo pectoral glands, which are frequently turgid with a thin milk at their nativity, and are furniſhed with nipples, which erect on titilla⯑tion like thoſe of the female; but which ſeem now to be of no fur⯑ther uſe, owing perhaps to ſome change which theſe animals have undergone in the gradual progreſſion of the formation of the earth, and of all that it inhabit.

Theſe ſeven laſt mentioned ſenſes may properly be termed appetites, as they differ from thoſe of touch, ſight, hearing, taſte, and ſmell, in this reſpect; that they are affected with pain as well by the defect of their objects as by the exceſs of them, which is not ſo in the latter. Thus cold and hunger give us pain, as well as an exceſs of heat or ſatiety; but it is not ſo with darkneſs and ſilence.

IX. Before we conclude this Section on the organs of ſenſe, we muſt obſerve, that, as far as we know, there are many more ſenſes, than have been here mentioned, as every gland ſeems to be influenced to ſeparate from the blood, or to abſorb from the cavities of the body, or from the atmoſphere, its appropriated fluid, by the ſtimulus of that fluid on the living gland; and not by mechanical capillary ab⯑ſorption, nor by chemical affinity. Hence it appears, that each of [126] theſe glands muſt have a peculiar organ to perceive theſe irritations, but as theſe irritations are not ſucceeded by ſenſation, they have not acquired the names of ſenſes.

However when theſe glands are excited into motions ſtronger than uſual, either by the acrimony of their fluids, or by their own irrita⯑bility being much increaſed, then the ſenſation of pain is produced in them as in all the other ſenſes of the body; and theſe pains are all of different kinds, and hence the glands at this time really become each a different organ of ſenſe, though theſe different kinds of pain have acquired no names.

Thus a great exceſs of light does not give the idea of light but of pain; as in forcibly opening the eye when it is much inflamed. The great exceſs of preſſure or diſtention, as when the point of a pin is preſſed upon our ſkin, produces pain, (and when this pain of the ſenſe of touch is ſlighter, it is termed itching, or tickling,) without any idea of ſolidity or of figure: an exceſs of heat produces ſmarting, of cold another kind of pain; it is probable by this ſenſe of heat the pain produced by cauſtic bodies is perceived, and of electricity, as all theſe are fluids, that permeate, diſtend, or decompoſe the parts that feel them.

I. AS the conſtituent elements of the material world are only per⯑ceptible to our organs of ſenſe in a ſtate of combination; it follows, that the ideas or ſenſual motions excited by them, are never received ſingly, but ever with a greater or leſs degree of combination. So the colours of bodies or their hardneſſes occur with their figures: every ſmell and taſte has its degree of pungency as well as its peculiar fla⯑vour: and each note in muſic is combined with the tone of ſome in⯑ſtrument. It appears from hence, that we can be ſenſible of a num⯑ber of ideas at the ſame time, ſuch as the whiteneſs, hardneſs, and coldneſs, of a ſnow-ball, and can experience at the ſame time many irritative ideas of ſurrounding bodies, which we do not attend to, as mentioned in Section VII. 3. 2. But thoſe ideas which belong to the ſame ſenſe, ſeem to be more eaſily combined into ſynchronous tribes, than thoſe which were not received by the ſame ſenſe, as we can [128] more eaſily think of the whiteneſs and figure of a lump of ſugar at the ſame time, than the whiteneſs and ſweetneſs of it.

2. As theſe ideas, or ſenſual motions, are thus excited with greater or leſs degrees of combination; ſo we have a power, when we repeat them either by our volition or ſenſation, to increaſe or diminiſh this degree of combination, that is, to form compounded ideas from thoſe, which were more ſimple; and abſtract ones from thoſe, which were more complex, when they were firſt excited; that is, we can repeat a part or the whole of thoſe ſenſual motions, which did conſtitute our ideas of perception; and the repetition of which now conſtitutes our ideas of recollection, or of imagination.

3. Thoſe ideas, which we repeat without change of the quantity of that combination, with which we firſt received them, are called complex ideas, as when you recollect Weſtminſter Abbey, or the planet Saturn: but it muſt be obſerved, that theſe complex ideas, thus re-excited by volition, ſenſation, or aſſociation, are ſeldom per⯑fect copies of their correſpondent perceptions, except in our dreams, where other external objects do not detract our attention.

4. Thoſe ideas, which are more complex than the natural objects that firſt excited them, have been called compounded ideas, as when we think of a ſphinx, or griffin.

5. And thoſe that are leſs complex than the correſpondent natural objects, have been termed abſtracted ideas: thus ſweetneſs, and white⯑neſs, and ſolidity, are received at the ſame time from a lump of ſugar, yet I can recollect any of theſe qualities without thinking of the others, that were excited along with them.

When ideas are ſo far abſtracted as in the above example, they have been termed ſimple by the writers of metaphyſics, and ſeem indeed to be more complete repetitions of the ideas or ſenſual motions, originally excited by external objects.

Other claſſes of theſe ideas, where the abſtraction has not been ſo great, have been termed, by Mr. Locke, modes, ſubſtances, and re⯑lations, [129] but they ſeem only to differ in their degree of abſtraction from the complex ideas that were at firſt excited; for as theſe complex or natural ideas are themſelves imperfect copies of their correſpondent perceptions, ſo theſe abſtract or general ideas are only ſtill more im⯑perfect copies of the ſame perceptions. Thus when I have ſeen an object but once, as a rhinoceros, my abſtract idea of this animal is the fame as my complex one. I may think more or leſs diſtinctly of a rhinoceros, but it is the very rhinoceros that I ſaw, or ſome part or property of him, which recurs to my mind.

But when any claſs of complex objects becomes the ſubject of con⯑verſation, of which I have ſeen many individuals, as a caſtle or an army, ſome property or circumſtance belonging to it is peculiarly al⯑luded to; and then I feel in my own mind, that my abſtract idea of this complex object is only an idea of that part, property, or attitude of it, that employs the preſent converſation, and varies with every ſentence that is ſpoken concerning it. So if any one ſhould ſay, "one may ſit upon a horſe ſafer than on a camel," my abſtract idea of the two animals includes only an outline of the level back of the one, and the gibboſity on the back of the other. What noiſe is that in the ſtreet?—Some horſes trotting over the pavement. Here my idea of the horſes includes principally the ſhape and motion of their legs. So alſo the abſtract ideas of goodneſs and courage are ſtill more imperfect repreſentations of the objects they were received from; for here we abſtract the material parts, and recollect only the qualities.

Thus we abſtract ſo much from ſome of our complex ideas, that at length it becomes difficult to determine of what perception they partake; and in many inſtances our idea ſeems to be no other than of the ſound or letters of the word, that ſtands for the collective tribe, of which we are ſaid to have an abſtracted idea, as noun, verb, chi⯑maera, apparition.

[130] 6. Ideas have been divided into thoſe of perception and thoſe of reflection, but as whatever is perceived muſt be external to the organ that perceives it, all our ideas muſt originally be ideas of per⯑ception.

7. Others have divided our ideas into thoſe of memory, and thoſe of imagination; they have ſaid that a recollection of ideas in the order they were received conſtitutes memory, and without that order ima⯑gination; but all the ideas of imagination, excepting the few that are termed ſimple ideas, are parts of trains or tribes in the order they were received: as if I think of a ſphinx, or a griffin, the fair face, boſom, wings, claws, tail, are all complex ideas in the order they were received: and it behoves the writers, who adhere to this de⯑finition, to determine, how ſmall the trains muſt be, that ſhall be called imagination; and how great thoſe, that ſhall be called memory.

Others have thought that the ideas of memory have a greater viva⯑city than thoſe of imagination: but the ideas of a perſon in ſleep, or in a waking reverie, where the trains connected with ſenſation are un⯑interrupted, are more vivid and diſtinct than thoſe of memory, ſo that they cannot be diſtinguiſhed by this criterion.

The very ingenious author of the Elements of Criticiſm has de⯑ſcribed what he conceives to be a ſpecies of memory, and calls it ideal preſence; but the inſtances he produces are the reveries of ſenſation, and are therefore in truth connections of the imagination, though they are recalled in the order they were received.

The ideas connected by aſſociation are in common diſcourſe attri⯑buted to memory, as we talk of memorandum-rings, and tie a knot on our handkerchiefs to bring ſomething into our minds at a diſtance of time. And a ſchool-boy, who can repeat a thouſand unmeaning lines in Lilly's Grammar, is ſaid to have a good memory. But theſe have been already ſhewn to belong to the claſs of aſſociation; and are termed ideas of ſuggeſtion.

II. Laſtly, the method already explained of claſſing ideas into thoſe excited by irritation, ſenſation, volition, or aſſociation, we hope will be found more convenient both for explaining the operations of the mind, and for comparing them with thoſe of the body; and for the illuſtration and the cure of the diſeaſes of both, and which we ſhall here recapitulate.

1. Irritative ideas are thoſe, which are preceded by irritation, which is excited by objects external to the organs of ſenſe: as the idea of that tree, which either I attend to, or which I ſhun in walking near it without attention. In the former caſe it is termed perception, in the latter it is termed ſimply an irritative idea.

2. Senſitive ideas are thoſe, which are preceded by the ſenſation of pleaſure or pain; as the ideas, which conſtitute our dreams or reve⯑ries, this is called imagination.

3. Voluntary ideas are thoſe, which are preceded by voluntary exertion, as when I repeat the alphabet backwards: this is called re⯑collection.

4. Aſſociate ideas are thoſe, which are preceded by other ideas or muſcular motions, as when we think over or repeat the alphabet by rote in its uſual order; or ſing a tune we are accuſtomed to; this is called ſuggeſtion.

III. 1. Perceptions ſignify thoſe ideas, which are preceded by ir⯑ritation and ſucceeded by the ſenſation of pleaſure or pain, for what⯑ever excites our attention intereſts us; that is, it is accompanied with pleaſure or pain; however ſlight may be the degree or quantity of either of them.

The word memory includes two claſſes of ideas, either thoſe which are preceded by voluntary exertion, or thoſe which are ſuggeſted by their aſſociations with other ideas.

2. Reaſoning is that operation of the ſenſorium, by which we ex⯑cite two or many tribes of ideas; and then re-excite the ideas, in which they differ, or correſpond. If we determine this difference, it [132] is called judgment; if we in vain endeavour to determine it, it is called doubting.

If we re-excited the ideas, in which they differ, it is called diſ⯑tinguiſhing. If we re-excite thoſe in which they correſpond, it is called comparing.

3. Invention is an operation of the ſenſorium, by which we volun⯑tarily continue to excite one train of ideas, ſuppoſe the deſign of raiſing water by a machine; and at the ſame time attend to all other ideas, which are connected with this by every kind of catenation; and combine or ſeparate them voluntarily for the purpoſe of obtaining ſome end.

For we can create nothing new, we can only combine or ſeparate the ideas, which we have already received by our perceptions: thus if I wiſh to repreſent a monſter, I call to my mind the ideas of every thing diſagreeable and horrible, and combine the naſtineſs and glut⯑tony of a hog, the ſtupidity and obſtimacy of an aſs, with the fur and awkwardneſs of a bear, and call the new combination Caliban. Yet ſuch a monſter may exiſt in nature, as all his attributes are parts of nature. So when I wiſh to repreſent every thing, that is excellent, and amiable; when I combine benevolence with cheerfulneſs, wiſ⯑dom, knowledge, taſte, wit, beauty of perſon, and elegance of manners, and aſſociate them in one lady as a pattern to the world, it is called invention; yet ſuch a perſon may exiſt,—ſuch a perſon does exiſt!—It is [...], who is as much a monſter as Caliban.

4. In reſpect to conſciouſneſs, we are only conſcious of our ex⯑iſtence, when we think about it; as we only perceive the lapſe of time, when we attend to it; when we are buſied about other objects, neither the lapſe of time nor the conſciouſneſs of our own exiſtence can occupy our attention. Hence, when we think of our own ex⯑iſtence, we only excite abſtracted or reflex ideas (as they are termed), of our principal pleaſures or pains, of our deſires or averſions, or of the [133] figure, ſolidity, colour, or other properties of our bodies, and call that act of the ſenſorium a conſciouſneſs of our exiſtence. Some philoſo⯑phers, I believe it is Des Cartes, has ſaid, "I think, therefore I exiſt." But this is not right reaſoning, becauſe thinking is a mode of exiſtence; and it is thence only ſaying, "I exiſt, therefore I exiſt." For there are three modes of exiſtence, or in the language of grammarians three kinds of verbs. Firſt, ſimply I am, or exiſt. Secondly, I am acting, or exiſt in a ſtate of activity, as I move. Thirdly, I am ſuffering, or exiſt in a ſtate of being acted upon, as I am moved. The when, and the where, as applicable to this exiſtence, depends on the ſucceſſive motions of our own or of other bodies; and on their reſpective ſitua⯑tions, as ſpoken of Sect. XIV. 2. 5.

5. Our identity is known by our acquired habits or catenated trains of ideas and muſcular motions; and perhaps, when we compare in⯑fancy with old age, in thoſe alone can our identity be ſuppoſed to exiſt. For what elſe is there of ſimilitude between the firſt ſpeck of living entity and the mature man?—every deduction of reaſoning, every ſen⯑timent or paſſion, with every fibre of the corporeal part of our ſyſtem, has been ſubject almoſt to annual mutation; while ſome catenations alone of our ideas and muſcular actions have continued in part un⯑changed.

By the facility, with which we can in our waking hours voluntarily produce certain ſucceſſive trains of ideas, we know by experience, that we have before reproduced them; that is, we are conſcious of a time of our exiſtence previous to the preſent time; that is, of our iden⯑tity now and heretofore. It is theſe habits of action, theſe catenations of ideas and muſcular motions, which begin with life, and only ter⯑minate with it; and which we can in ſome meaſure deliver to our poſterity; as explained in Sect. XXXIX.

6. When the progreſſive motions of external bodies make a part of our preſent catenation of ideas, we attend to the lapſe of time; which appears the longer, the more frequently we thus attend to it; as when [134] we expect ſomething at a certain hour, which much intereſts us, whether it be an agreeable or diſagreeable event; or when we count the paſſing ſeconds on a ſtop-watch.

When an idea of our own perſon, or a reflex idea of our pleaſures and pains, deſires and averſions, makes a part of this catenation, it is termed conſciouſneſs; and if this idea of conſciouſneſs makes a part of a catenation, which we excite by recollection, and know by the facility with which we excite it, that we have before experienced it, it is called identity, as explained above.

7. In reſpect to freewill, it is certain, that we cannot will to think of a new train of ideas, without previouſly thinking of the firſt link of it; as I cannot will to think of a black ſwan, without previouſly think⯑ing of a black ſwan. But if I now think of a tail, I can voluntarily recollect all animals, which have tails; my will is ſo far free, that I can purſue the ideas linked to this idea of tail, as far as my knowledge of the ſubject extends; but to will without motive is to will without deſire or averſion; which is as abſurd as to feel without pleaſure or pain; they are both ſoleciſms in the terms. So far are we governed by the catenations of motions, which affect both the body and the mind of man, and which begin with our irritability, and end with it.

I. ALL thoſe internal motions of animal bodies, which contribute to digeſt their aliment, produce their ſecretions, repair their injuries, or increaſe their growth, are performed without our attention or con⯑ſciouſneſs. [136] They exiſt as well in our ſleep, as in our waking hours, as well in the foetus during the time of geſtation, as in the infant after nativity, and proceed with equal regularity in the vegetable as in the animal ſyſtem. Theſe motions have been ſhewn in a former part of this work to depend on the irritations of peculiar fluids, and as they have never been claſſed amongſt the inſtinctive actions of animals, are precluded from our preſent diſquiſition.

But all thoſe actions of men or animals, that are attended with con⯑ſciouſneſs, and ſeem neither to have been directed by their appetites, taught by their experience, nor deduced from obſervation or tradition, have been referred to the power of inſtinct. And this power has been explained to be a divine ſomething, a kind of inſpiration; whilſt the poor animal, that poſſeſſes it, has been thought little better than a machine!

The irkſomeneſs, that attends a continued attitude of the body, or the pains, that we receive from heat, cold, hunger, or other injuri⯑ous circumſtances, excite us to general locomotion: and our ſenſes are ſo formed and conſtituted by the hand of nature, that certain objects preſent us with pleaſure, others with pain, and we are induced to ap⯑proach and embrace theſe, to avoid and abhor thoſe, as ſuch ſenſa⯑tions direct us.

Thus the palates of ſome animals are gratefully affected by the maſtication of fruits, others of grains, and others of fleſh; and they are thence inſtigated to attain, and to conſume thoſe materials; and are furniſhed with powers of muſcular motion, and of digeſtion pro⯑per for ſuch purpoſes.

Theſe ſenſations and deſires conſtitute a part of our ſyſtem, as our muſcles and bones conſtitute another part: and hence they may alike be termed natural or connate; but neither of them can properly be termed inſtinctive: as the word inſtinct in its uſual acceptation refers only to the actions of animals, as above explained: the origin of theſe actions is the ſubject of our preſent enquiry.

[137] The reader is intreated carefully to attend to this definition of in⯑ſtinctive actions, leſt by uſing the word inſtinct without adjoining any accurate idea to it, he may not only include the natural deſires of love and hunger, and the natural ſenſations of pain or pleaſure, but the figure and contexture of the body, and the faculty of reaſon itſelf un⯑der this general term.

II. We experience ſome ſenſations, and perform ſome actions before our nativity; the ſenſations of cold and warmth, agitation and reſt, fulneſs and inanition, are inſtances of the former; and the repeated ſtruggles of the limbs of the foetus, which begin about the middle of geſtation, and thoſe motions by which it frequently wraps the um⯑bilical chord around its neck or body, and even ſometimes ties it on a knot; are inſtances of the latter. Smellie's Midwifery, (Vol. I. p. 182).

By a due attention to theſe circumſtances many of the actions of young animals, which at firſt ſight ſeemed only referable to an inex⯑plicable inſtinct, will appear to have been acquired like all other ani⯑mal actions, that are attended with conſciouſneſs, by the repeated efforts of our muſcles under the conduct of our ſenſations or deſires.

The chick in the ſhell begins to move its feet and legs on the ſixth day of incubation (Mattreican, p. 138); or on the ſeventh day, (Langley); afterwards they are ſeen to move themſelves gently in the liquid that ſurrounds them, and to open and ſhut their mouths, (Harvei, de Generat. p. 62, and 197. Form de Poulet. ii. p. 129). Puppies before the membranes are broken, that involve them, are ſeen to move themſelves, to put out their tongues, and to open and ſhut their mouths, (Harvey, Gipſon, Riolan, Haller). And calves lick themſelves and ſwallow many of their hairs before their nativity: which however puppies do not, (Swammerden, p 319. Flemyng Phil. Tranſ. Ann. 1755. 42). And towards the end of geſtation, the foetus of all animals are proved to drink part of the liquid in which they ſwim, (Haller. Phyſiol. T. 8. 204). The white of egg is found [138] in the mouth and gizzard of the chick, and is nearly or quite con⯑ſumed before it is hatched, (Harvei de Generat. 58). And the liquor amnii is found in the mouth and ſtomach of the human foetus, and of calves; and how elſe ſhould that excrement be produced in the in⯑teſtines of all animals, which is voided in great quantity ſoon after their birth; Gipſon, Med. Eſſays, Edinb. V. i. 13. Halleri Phyſiolog. T. 3. p. 318. and T. 8). In the ſtomach of a calf the quantity of this liquid amounted to about three pints, and the hairs amongſt it were of the ſame colour with thoſe on its ſkin, (Blaſii Anat. Animal, p. m. 122). Theſe facts are atteſted by many other writers of credit, beſides thoſe above mentioned.

III. It has been deemed a ſurpriſing inſtance of inſtinct, that calves and chickens ſhould be able to walk by a few efforts almoſt immedi⯑ately after their nativity: whilſt the human infant in thoſe countries where he is not incumbered with clothes, as in India, is five or ſix months, and in our climate almoſt a twelvemonth, before he can ſafely ſtand upon his feet.

The ſtruggles of all animals in the womb muſt reſemble their mode of ſwimming, as by this kind of motion they can beſt change their attitude in water. But the ſwimming of the calf and chicken re⯑ſembles their manner of walking, which they have thus in part ac⯑quired before their nativity, and hence accompliſh it afterwards with very few efforts, whilſt the ſwimming of the human creature re⯑ſembles that of the frog, and totally differs from his mode of walking.

There is another circumſtance to be attended to in this affair, that not only the growth of thoſe peculiar parts of animals, which are firſt wanted to ſecure their ſubſiſtence, are in general furtheſt advanced before their nativity: but ſome animals come into the world more completely formed throughout their whole ſyſtem than others: and are thence much forwarder in all their habits of motion. Thus the colt, and the lamb, are much more perfect animals than the blind [139] puppy, and the naked rabbit; and the chick of the pheaſant, and the partridge, has more perfect plumage, and more perfect eyes, as well as greater aptitude to locomotion, than the callow neſtlings of the dove, and of the wren. The parents of the former only find it ne⯑ceſſary to ſhew them their food, and to teach them to take it up; whilſt thoſe of the latter are obliged for many days to obtrude it into their gaping mouths.

IV. From the facts mentioned in No. 2. of this Section, it is evinced that the foetus learns to ſwallow before its nativity; for it is ſeen to open its mouth, and its ſtomach is found filled with the liquid that ſur⯑rounds it. It opens its mouth, either inſtigated by hunger, or by the irkſomeneſs of a continued attitude of the muſcles of its face; the liquor amnii, in which it ſwims, is agreeable to its palate, as it con⯑ſiſts of a nouriſhing material, (Haller Phyſ. T. 8. p. 204). It is tempted to experience its taſte further in the mouth, and by a few efforts learns to ſwallow, in the ſame manner as we learn all other animal actions, which are attended with conſciouſneſs, by the repeated efforts of our muſcles under the conduct of our ſenſations or volitions.

The inſpiration of air into the lungs is ſo totally different from that of ſwallowing a fluid in which we are immerſed, that it cannot be acquired before our nativity. But at this time, when the circula⯑tion of the blood is no longer continued through the placenta, that ſuffocating ſenſation, which we feel about the precordia, when we are in want of freſh air, diſagreeably affects the infant: and all the muſcles of the body are excited into action to relieve this oppreſſion; thoſe of the breaſt, ribs, and diaphragm are found to anſwer this pur⯑poſe, and thus reſpiration is diſcovered, and is continued throughout our lives, as often as the oppreſſion begins to recur. Many infants, both of the human creature, and of quadrupeds, ſtruggle for a minute after they are born before they begin to breathe, (Haller Phyſ. T. 8. p. 400. ib. pt. 2. p. 1). Mr. Buſfon thinks the action of the dry air upon the nerves of ſmell of new-born animals, by producing an en⯑deavour [140] to ſneeze, may contribute to induce this firſt inſpiration, and that the rarefaction of the air by the warmth of the lungs contributes to induce expiration, Hiſt. Nat. Tom. 4. p. 174. Which latter it may effect by producing a diſagreeable ſenſation by its delay, and a conſequent effort to relieve it. Many children ſneeze before they reſpire, but not all, as far as I have obſerved, or can learn from others.

At length, by the direction of its ſenſe of ſmell, or by the officious care of its mother, the young animal approaches the odoriferous rill of its future nouriſhment, already experienced to ſwallow. But in the act of ſwallowing, it is neceſſary nearly to cloſe the mouth, whether the creature be immerſed in the fluid it is about to drink, or not: hence, when the child firſt attempts to ſuck, it does not ſlight⯑ly compreſs the nipple between its lips, and ſuck as an adult perſon would do, by abſorbing the milk; but it takes the whole nipple into its mouth for this purpoſe, compreſſes it between its gums, and thus repeatedly chewing (as it were) the nipple, preſſes out the milk; ex⯑actly in the ſame manner as it is drawn from the teats of cows by the hands of the milkmaid. The celebrated Harvey obſerves, that the foetus in the womb muſt have ſucked in a part of its nouriſhment, becauſe it knows how to ſuck the minute it is born, as any one may experience by putting a finger between its lips, and becauſe in a few days it forgets this art of ſucking, and cannot without ſome difficulty again acquire it, (Exercit. de Gener. Anim. 48). The ſame obſerva⯑tion is made by Hippocrates.

A little further experience teaches the young animal to ſuck by ab⯑ſorption, as well as by compreſſion; that is, to open the cheſt as in the beginning of reſpiration, and thus to rarefy the air in the mouth, that the preſſure of the denſer external atmoſphere may contribute to force out the milk.

The chick yet in the ſhell has learnt to drink by ſwallowing a part of the white of the egg for its food; but not having experienced how [141] to take up and ſwallow ſolid ſeeds, or grains, is either taught by the ſolicitous induſtry of its mother; or by many repeated attempts is enabled at length to diſtinguiſh and to ſwallow this kind of nutri⯑ment.

And puppies, though they know how to ſuck like other animals from their previous experience in ſwallowing, and in reſpiration; yet are they long in acquiring the art of lapping with their tongues, which from the flaccidity of their cheeks, and length of their mouths, is after⯑wards a more convenient way for them to take in water.

V. The ſenſes of ſmell and taſte in many other animals greatly ex⯑cel thoſe of mankind, for in civilized ſociety, as our victuals are ge⯑nerally prepared by others, and are adulterated with ſalt, ſpice, oil, and empyreuma, we do not heſitate about eating whatever is ſet before us, and neglect to cultivate theſe ſenſes: whereas other animals try every morſel by the ſmell, before they take it into their mouths, and by the taſte before they ſwallow it: and are led not only each to his proper nouriſhment by this organ of ſenſe, but it alſo at a maturer age directs them in the gratification of their appetite of love. Which may be further underſtood by conſidering the ſympathies of theſe parts de⯑ſcribed in Claſs IV. 2. 1. 7. While the human animal is directed to the object of his love by his ſenſe of beauty, as mentioned in No. VI. of this Section. Thus Virgil. Georg. III. 250.

[142] The following curious experiment is related by Galen. "On diſ⯑ſecting a goat great with young I found a briſk embryon, and having detached it from the matrix, and ſnatching it away before it ſaw its dam, I brought it into a certain room, where there were many veſſels, ſome filled with wine, others with oil, ſome with honey, others with milk, or ſome other liquor; and in others were grains and fruits; we firſt obſerved the young animal get upon its feet, and walk; then it ſhook itſelf, and afterwards ſcratched its ſide with one of its feet: then we ſaw it ſmelling to every one of theſe things, that were ſet in the room; and when it had ſmelt to them all, it drank up the milk." L. 6. de locis. cap. 6.

Parturient quadrupeds, as cats, and bitches, and ſows, are led by their ſenſe of ſmell to eat the placenta as other common food; why then do they not devour their whole progeny, as is repreſented in an antient emblem of TIME? This is ſaid ſometimes to happen in the unnatural ſtate in which we confine ſows; and indeed nature would ſeem to have endangered her offspring in this nice circumſtance! But at this time the ſtimulus of the milk in the tumid teats of the mother excites her to look out for, and to deſire ſome unknown circumſtance to relieve her. At the ſame time the ſmell of the milk attracts the exertions of the young animals towards its ſource, and thus the de⯑lighted mother diſcovers a new appetite, as mentioned in Sect. XIV. 8. and her little progeny are led to receive and to communicate pleaſure by this moſt beautiful contrivance.

VI. But though the human ſpecies in ſome of their ſenſations are much inferior to other animals, yet the accuracy of the ſenſe of touch, which they poſſeſs in ſo eminent a degree, gives them a great ſuperi⯑ority of underſtanding; as is well obſerved by the ingenious Mr. Buf⯑fon. The extremities of other animals terminate in horns, and hoofs, and claws, very unfit for the ſenſation of touch; whilſt the human hand is finely adapted to encompaſs its object with this organ of ſenſe.

[143] The elephant is indeed endued with a fine ſenſe of feeling at the extremity of his proboſcis, and hence has acquired much more accu⯑rate ideas of touch and of ſight than moſt other creatures. The two following inſtances of the ſagacity of theſe animals may entertain the reader, as they were told me by ſome gentlemen of diſtinct obſerva⯑tion, and undoubted veracity, who had been much converſant with our eaſtern ſettlements. Firſt, the elephants that are uſed to carry the baggage of our armies, are put each under the care of one of the natives of Indoſtan, and whilſt himſelf and his wife go into the woods to collect leaves and branches of trees for his food, they fix him to the ground by a length of chain, and frequently leave a child yet unable to walk, under his protection: and the intelligent animal not only de⯑fends it, but as it creeps about, when it arrives near the extremity of his chain, he wraps his trunk gently round its body, and brings it again into the centre of his circle. Secondly, the traitor elephants are taught to walk on a narrow path between two pit-falls, which are covered with turf, and then to go into the woods, and to ſeduce the wild elephants to come that way, who fall into theſe wells, whilſt he paſſes ſafe between them: and it is univerſally obſerved, that thoſe wild elephants that eſcape the ſnare, purſue the traitor with the ut⯑moſt vehemence, and if they can overtake him, which ſometimes hap⯑pens, they always beat him to death.

The monkey has a hand well enough adapted for the ſenſe of touch, which contributes to his great facility of imitation; but in taking ob⯑jects with his hands, as a ſtick or an apple, he puts his thumb on the ſame ſide of them with his fingers, inſtead of counteracting the preſ⯑ſure of his fingers with it: from this neglect he is much ſlower in ac⯑quiring the figures of objects, as he is leſs able to determine the diſ⯑tances or diameters of their parts, or to diſtinguiſh their vis inertiae from their hardneſs. Helvetius adds, that the ſhortneſs of his life, his being fugitive before mankind, and his not inhabiting all climates, combine to prevent his improvement. (De PEſprit. T. 1. p.) There [144] is however at this time an old monkey ſhewn in Exeter Change, London, who having loſt his teeth, when nuts are given him, takes a ſtone into his hand, and cracks them with it one by one; thus uſing tools to effect his purpoſe like mankind.

The beaver is another animal that makes much uſe of his hands, and if we may credit the reports of travellers, is poſſeſſed of amazing ingenuity. This however, M. Buffon affirms, is only where they exiſt in large numbers, and in countries thinly peopled with men; while in France in their ſolitary ſtate they ſhew no uncommon in⯑genuity.

Indeed all the quadrupeds, that have collar-bones, (claviculae) uſe their fore-limbs in ſome meaſure as we uſe our hands, as the cat, ſquirrel, tyger, bear and lion; and as they exerciſe the ſenſe of touch more univerſally than other animals, ſo are they more ſagacious in watching and ſurpriſing their prey. All thoſe birds, that uſe their claws for hands, as the hawk, parrot, and cuckoo, appear to be more docile and intelligent; though the gregarious tribes of birds have more acquired knowledge.

Now as the images, that are painted on the retina of the eye, are no other than ſigns, which recall to our imaginations the objects we had before examined by the organ of touch, as is fully demonſtrated by Dr. Berkley in his treatiſe on viſion; it follows that the human crea⯑ture has greatly more accurate and diſtinct ſenſe of viſion than that of any other animal. Whence as he advances to maturity he gradually acquires a ſenſe of female beauty, which at this time directs him to the object of his new paſſion.

Sentimental love, as diſtinguiſhed from the animal paſſion of that name, with which it is frequently accompanied, conſiſts in the deſire or ſenſation of beholding, embracing, and faluting a beautiful object.

The characteriſtic of beauty therefore is that it is the object of love; and though many other objects are in common language called beauti⯑ful, [145] yet they are only called ſo metaphorically, and ought to be termed agreeable. A Grecian temple may give us the pleaſurable idea of ſublimity, a Gothic temple may give us the pleaſurable idea of va⯑riety, and a modern houſe the pleaſurable idea of utility; muſic and poetry may inſpire our love by aſſociation of ideas; but none of theſe, except metaphorically, can be termed beautiful, as we have no wiſh to embrace or ſalute them.

Our perception of beauty conſiſts in our recognition by the ſenſe of viſion of thoſe objects, firſt, which have before inſpired our love by the pleaſure, which they have afforded to many of our ſenſes; as to our ſenſe of warmth, of touch, of ſmell, of taſte, hunger and thirſt; and, ſecondly, which bear any analogy of form to ſuch objects.

When the babe, ſoon after it is born into this cold world, is applied to its mother's boſom; its ſenſe of perceiving warmth is firſt agree⯑ably affected; next its ſenſe of ſmell is delighted with the odour of her milk; then its taſte is gratified by the flavour of it; after⯑wards the appetites of hunger and of thirſt afford pleaſure by the poſ⯑ſeſſion of their objects, and by the ſubſequent digeſtion of the ali⯑ment; and, laſtly, the ſenſe of touch is delighted by the ſoftneſs and ſmoothneſs of the milky fountain, the ſource of ſuch variety of hap⯑pineſs.

All theſe various kinds of pleaſure at length become aſſociated with the form of the mother's breaſt; which the infant embraces with its hands, preſſes with its lips, and watches with its eyes; and thus ac⯑quires more accurate ideas of the form of its mother's boſom, than of the odour and flavour or warmth, which it perceives by its other ſenſes. And hence at our maturer years, when any object of viſion is preſented to us, which by its waving or ſpiral lines bears any ſimili⯑tude to the form of the female boſom, whether it be found in a land⯑ſcape with ſoft gradations of riſing and deſcending ſurface, or in the forms of ſome antique vaſes, or in other works of the pencil or the chiſſel, we feel a general glow of delight, which ſeems to influence all our ſenſes; and, if the object be not too large, we experience an at⯑traction [146] to embrace it with our arms, and to ſalute it with our lips, as we did in our early infancy the boſom of our mother. And thus we find, according to the ingenious idea of Hogarth, that the waving lines of beauty were originally taken from the temple of Venus.

This animal attraction is love; which is a ſenſation, when the ob⯑ject is preſent; and a deſire, when it is abſent. Which conſtitutes the pureſt ſource of human felicity, the cordial drop in the otherwiſe vapid cup of life, and which overpays mankind for the care and la⯑bour, which are attached to the pre-eminence of his ſituation above other animals.

It ſhould have been obſerved, that colour as well as form ſometimes enters into our idea of a beautiful object, as a good complexion for in⯑ſtance, becauſe a fine or fair colour is in general a ſign of health, and conveys to us an idea of the warmth of the object; and a pale countenance on the contrary gives an idea of its being cold to the touch.

It was before remarked, that young animals uſe their lips to diſ⯑tinguiſh the forms of things, as well as their fingers, and hence we learn the origin of our inclination to ſalute beautiful objects with our lips.

VII. There are two ways by which we become acquainted with the paſſions of others: firſt, by having obſerved the effects of them, as of fear or anger, on our own bodies, we know at ſight when others are under the influence of theſe affections. So when two cocks are preparing to ſight, each feels the feathers riſe round his own neck, and knows from the ſame ſign the diſpoſition of his adverſary: and children long before they can ſpeak, or underſtand the language of their parents, may be frightened by an angry countenance, or ſoothed by ſmiles and blandiſhments.

Secondly, when we put ourſelves into the attitude that any paſſion naturally occaſions, we ſoon in ſome degree acquire that paſſion; hence when thoſe that ſcold indulge themſelves in loud oaths, and [147] violent actions of the arms, they increaſe their anger by the mode of expreſſing themſelves: and on the contrary the counterfeited ſmile of pleaſure in diſagreeable company ſoon brings along with it a portion of the reality, as is well illuſtrated by Mr. Burke. (Eſſay on the Su⯑blime and Beautiful.)

This latter method of entering into the paſſions of others is ren⯑dered of very extenſive uſe by the pleaſure we take in imitation, which is every day preſented before our eyes, in the actions of chil⯑dren, and indeed in all the cuſtoms and faſhions of the world. From this our aptitude to imitation, ariſes what is generally underſtood by the word ſympathy ſo well explained by Dr. Smith of Glaſgow. Thus the appearance of a cheerful countenance gives us pleaſure, and of a melancholy one makes us ſorrowful. Yawning and ſometimes vomiting are thus propagated by ſympathy, and ſome people of deli⯑cate fibres, at the preſence of a ſpectacle of miſery, have felt pain in the ſame parts of their own bodies, that were diſeaſed or mangled in the other. Amongſt the writers of antiquity Ariſtotle thought this aptitude to imitation an eſſential property of the human ſpecies, and calls man an imitative animal. To [...].

Theſe then are the natural ſigns by which we underſtand each other, and on this ſlender baſis is built all human language. For without ſome natural ſigns, no artificial ones could have been invented or underſtood, as is very ingeniouſly obſerved by Dr. Reid. (Inquiry into the Human Mind.)

VIII. The origin of this univerſal language is a ſubject of the high⯑eſt curioſity, the knowledge of which has always been thought utterly inacceſſible. A part of which we ſhall however here attempt.

Light, ſound, and odours, are unknown to the foetus in the womb, which, except the few ſenſations and motions already mentioned, ſleeps away its time inſenſible of the buſy world. But the moment he arrives into day, he begins to experience many vivid pains and plea⯑ſures; theſe are at the ſame time attended with certain muſcular mo⯑tions, [148] and from this their early, and individual aſſociation, they ac⯑quire habits of occurring together, that are afterwards indiſſoluble.

IX. It muſt have already appeared to the reader, that all other ani⯑mals, as well as man, are poſſeſſed of this natural language of the paſſions, expreſſed in ſigns or tones; and we ſhall endeavour to evince, that thoſe animals, which have preſerved themſelves from being en⯑ſlaved by mankind, and are aſſociated in flocks, are alſo poſſeſſed of ſome artificial language, and of ſome traditional knowledge.

The mother-turkey, when ſhe eyes a kite hovering high in air, has either ſeen her own parents thrown into fear at his preſence, or has by obſervation been acquainted with his dangerous deſigns upon her young. She becomes agitated with fear, and uſes the natural language of that paſſion, her young ones catch the fear by imitation, and in an inſtant conceal themſelves in the graſs.

At the ſame time that ſhe ſhews her fears by her geſture and de⯑portment, ſhe uſes a certain exclamation, Koe-ut, Koe-ut, and the young ones afterwards know, when they hear this note, though they do not ſee their dam, that the preſence of their adverſary is denounced, and hide themſelves as before.

The wild tribes of birds have very frequent opportunities of know⯑ing their enemies, by obſerving the deſtruction they make among their progeny, of which every year but a ſmall part eſcapes to maturity: but to our domeſtic birds theſe opportunities ſo rarely occur, that their knowledge of their diſtant enemies muſt frequently be de⯑livered [154] by tradition in the manner above explained, through many generations.

This note of danger, as well as the other notes of the mother⯑turkey, when ſhe calls her flock to their flood, or to ſleep under her wings, appears to be an artificial language, both as expreſſed by the mother, and as underſtood by the progeny. For a hen teaches this language with equal eaſe to the ducklings, ſhe has hatched from ſup⯑poſitious eggs, and educates as her own offspring: and the wagtails, or hedge-ſparrows, learn it from the young cuckoo their foſter nurſ⯑ling, and ſupply him with food long after he can fly about, whenever they hear his cuckooing, which Linneus tells us, is his call of hun⯑ger, (Syſt. Nat.) And all our domeſtic animals are readily taught to come to us for food, when we uſe one tone of voice, and to fly from our anger, when we uſe another.

Rabbits, as they cannot eaſily articulate ſounds, and are formed into ſocieties, that live under ground, have a very different method of giving alarm. When danger is threatened, they thump on the ground with one of their hinder feet, and produce a ſound, that can be heard a great way by animals near the ſurface of the earth, which would ſeem to be an artificial ſign both from its ſingularity and its aptneſs to the ſituation of the animal.

The rabbits on the iſland of Sor, near Senegal, have white fleſh, and are well taſted, but do not burrow in the earth, ſo that we may ſuſpect their digging themſelves houſes in this cold climate is an ac⯑quired art, as well as their note of alarm, (Adanſon's Voyage to Senegal).

The barking of dogs is another curious note of alarm, and would ſeem to be an acquired language, rather than a natural ſign: for "in the iſland of Juan Fernandes, the dogs did not attempt to bark, till ſome European dogs were put among them, and then they gradually begun to imitate them, but in a ſtrange manner at firſt, as if they were learning a thing that was not natural to them," (Voyage to [155] South America by Don G. Juan, and Don Ant. de Ulloa. B. 2. c. 4).

Linnaeus alſo obſerves, that the dogs of South America do not bark at ſtrangers, (Syſt. Nat.) And the European dogs, that have been carried to Guinea, are ſaid in three or four generations to ceaſe to bark, and only howl, like the dogs that are natives of that coaſt, (World Diſplayed, Vol. XVII. p. 26.)

A circumſtance not diſſimilar to this, and equally curious, is men⯑tioned by Kircherus. de Muſurgia, in his Chapter de Luſciniis. "That the young nightingales, that are hatched under other birds, never ſing till they are inſtructed by the company of other nightin⯑gales." And Jonſton affirms, that the nightingales that viſit Scot⯑land, have not the ſame harmony as thoſe of Italy, (Pennant's Zoo⯑logy, octavo, p. 255); which would lead us to ſuſpect that the ſing⯑ing of birds, like human muſic, is an artificial language rather than a natural expreſſion of paſſion.

X. Our muſic like our language, is perhaps entirely conſtituted of artificial tones, which by habit ſuggeſt certain agreeable paſſions. For the ſame combination of notes and tones do not excite devotion, love, or poetic melancholy in a native of Indoſtan and of Europe. And "the Highlander has the ſame warlike ideas annexed to the ſound of a bagpipe (an inſtrument which an Engliſhman derides), as the Engliſhman has to that of a trumpet or fife," (Dr. Brown's Union of Poetry and Muſic, p. 58.) So "the muſic of the Turks is very different from the Italian, and the people of Fez and Morocco have again a different kind, which to us appears very rough and hor⯑rid, but is highly pleaſing to them," (L' Arte Armoniaca a Giorgio Antoniotto). Hence we ſee why the Italian opera does not delight an untutored Engliſhman; and why thoſe, who are unaccuſtomed to muſic, are more pleaſed with a tune, the ſecond or third time they hear it, than the firſt. For then the ſame melodious train of ſounds excites the melancholy, they had learned from the ſong; or the ſame [156] vivid combination of them recalls all the mirthful ideas of the dance and company.

Even the ſounds, that were once diſagreeable to us, may by habit be aſſociated with other ideas, ſo as to become agreeable. Father Lafitau, in his account of the Iroquois, ſays "the muſic and dance of thoſe Americans, have ſomething in them extremely barbarous, which at firſt diſguſts. We grow reconciled to them by degrees, and in the end partake of them with pleaſure, the ſavages themſelves are fond of them to diſtraction," (Moeurs des Savages, Tom. ii.)

There are indeed a few ſounds, that we very generally aſſociate with agreeable ideas, as the whiſtling of birds, or purring of animals, that are delighted; and ſome others, that we as generally aſſociate with diſagreeable ideas, as the cries of animals in pain, the hiſs of ſome of them in anger, and the midnight howl of beaſts of prey. Yet we receive no terrible or ſublime ideas from the lowing of a cow, or the braying of an aſs. Which evinces, that theſe emotions are owing to previous aſſociations. So if the rumbling of a carriage in the ſtreet be for a moment miſtaken for thunder, we receive a ſub⯑lime ſenſation, which ceaſes as ſoon as we know it is the noiſe of a coach and ſix.

There are other diſagreeable ſounds, that are ſaid to ſet the teeth on edge; which, as they have always been thought a neceſſary effect of certain diſcordant notes, become a proper ſubject of our enquiry. Every one in his childhood has repeatedly bit a part of the glaſs or earthen veſſel, in which his food has been given him, and has thence had a very diſagreeable ſenſation in the teeth, which ſenſation was de⯑ſigned by nature to prevent us from exerting them on objects harder than themſelves. The jarring ſound produced between the cup and the teeth is always attendant on this diſagreeable ſenſation: and ever after when ſuch a ſound is accidentally produced by the conflict of two hard bodies, we feel by aſſociation of ideas the concomitant diſ⯑agreeable ſenſation in our teeth.

[157] Others have in their infancy frequently held the corner of a ſilk handkerchief in their mouth, or the end of the velvet cape of their coat, whilſt their companions in play have plucked it from them, and have given another diſagreeable ſenſation to their teeth, which has afterwards recurred on touching thoſe materials. And the ſight of a knife drawn along a china plate, though no ſound is excited by it, and even the imagination of ſuch a knife and plate ſo ſcraped together, I know by repeated experience will produce the ſame diſagreeable ſenſation of the teeth.

Theſe circumſtances indiſputably prove, that this ſenſation of the tooth-edge is owing to aſſociated ideas; as it is equally excitable by ſight, touch, hearing, or imagination.

In reſpect to the artificial proportions of ſound excited by muſical inſtruments, thoſe, who have early in life aſſociated them with agree⯑able ideas, and have nicely attended to diſtinguiſh them from each other, are ſaid to have a good ear, in that country where ſuch pro⯑portions are in faſhion: and not from any ſuperior perfection in the organ of hearing, or any inſtinctive ſympathy between certain ſounds and paſſions.

I have obſerved a child to be exquiſitely delighted with muſic, and who could with great facility learn to ſing any tune that he heard diſtinctly, and yet whoſe organ of hearing was ſo imperfect, that it was neceſſary to ſpeak louder to him in common converſation than to others.

Our muſic, like our architecture, ſeems to have no foundation in nature, they are both arts purely of human creation, as they imitate nothing. And the profeſſors of them have only claſſed thoſe circum⯑ſtances, that are moſt agreeable to the accidental taſte of their age, or country; and have called it Proportion. But this proportion muſt always fluctuate, as it reſts on the caprices, that are introduced into our minds by our various modes of education. And theſe fluctuations [158] of taſte muſt become more frequent in the preſent age, where man⯑kind have enfranchiſed themſelves from the blind obedience to the rules of antiquity in perhaps every ſcience, but that of architecture. See Sect. XII. No. 7. 3.

XI. There are many articles of knowledge, which the animals in cultivated countries ſeem to learn very early in their lives, either from each other, or from experience, or obſervation: one of the moſt general of theſe is to avoid mankind. There is ſo great a reſemblance in the natural language of the paſſions of all animals, that we gene⯑rally know, when they are in a pacific, or in a malevolent humour, they have the ſame knowledge of us; and hence we can ſcold them from us by ſome tones and geſtures, and could poſſibly attract them to us by others, if they were not already apprized of our general ma⯑levolence towards them. Mr. Gmelin, Profeſſor at Peterſburg, aſ⯑ſures us, that in his journey into Siberia, undertaken by order of the Empreſs of Ruſſia, he ſaw foxes, that expreſſed no fear of himſelf or companions, but permitted him to come quite near them, having never ſeen the human creature before. And Mr. Bongainville relates, that at his arrival at the Malouine, or Falkland's Iſlands, which were not inhabited by men, all the animals came about himſelf and his people; the fowls ſettling upon their heads and ſhoulders, and the quadrupeds running about their feet. From the difficulty of acquir⯑ing the confidence of old animals, and the eaſe of taming young ones, it appears that the fear, they all conceive at the ſight of mankind, is an acquired article of knowledge.

This knowledge is more nicely underſtood by rooks, who are formed into ſocieties, and build, as it were, cities over our heads; they evidently diſtinguiſh, that the danger is greater when a man is armed with a gun. Every one has ſeen this, who in the ſpring of the year has walked under a rookery with a gun in his hand: the in⯑habitants of the trees riſe on their wings, and ſcream to the unfledged [159] young to ſhrink into their neſts from the ſight of the enemy. The vulgar obſerving this circumſtance ſo uniformly to occur, aſſert that rooks can ſmell gun-powder.

The fieldfairs, (turdus pilarus) which breed in Norway, and come hither in the cold ſeaſon for our winter berries; as they are aſſociated in flocks, and are in a foreign country, have evident marks of keeping a kind of watch, to remark and announce the appearance of danger. On approaching a tree, that is covered with them, they continue fear⯑leſs till one at the extremity of the buſh riſing on his wings gives a loud and peculiar note of alarm, when they all immediately fly, ex⯑cept one other, who continues till you approach ſtill nearer, to cer⯑tify as it were the reality of the danger, and then he alſo flies off re⯑peating the note of alarm.

And in the woods about Senegal there is a bird called uett-uett by the negroes, and ſquallers by the French, which, as ſoon as they ſee a man, ſet up a loud ſcream, and keep flying round him, as if their intent was to warn other birds, which upon hearing the cry im⯑mediately take wing. Theſe birds are the bane of ſportſmen, and frequently put me into a paſſion, and obliged me to ſhoot them, (Adanſon's Voyage to Senegal, 78). For the ſame intent the leſſer birds of our climate ſeem to fly after a hawk, cuckoo, or owl, and ſcream to prevent their companions from being ſurpriſed by the general enemies of themſelves, or of their eggs and pro⯑geny.

But the lapwing, (charadrius pluvialis Lin.) when her unfledged offspring run about the marſhes, where they were hatched, not only gives the note of alarm at the approach of men or dogs, that her young may conceal themſelves; but flying and ſcreaming near the adverſary, ſhe appears more ſolicitous and impatient, as he recedes from her family, and thus endeavours to miſlead him, and frequently ſucceeds in her deſign. Theſe laſt inſtances are ſo appoſite to the ſitu⯑ation, rather than to the natures of the creatures, that uſe them; and [160] are ſo ſimilar to the actions of men in the ſame circumſtances, that we cannot but believe, that they proceed from a ſimilar principle.

On the northern coaſt of Ireland a friend of mine ſaw above a hundred crows at once preying upon muſcles; each crow took a muſcle up into the air twenty or forty yards high, and let it fall on the ſtones, and thus by breaking the ſhell, got poſſeſſion of the ani⯑mal.—A certain philoſopher (I think it was Anaxagoras) walking along the ſea-ſhore to gather ſhells, one of theſe unlucky birds miſ⯑taking his bald head for a ſtone, dropped a ſhell-fiſh upon it, and killed at once a philoſopher and an oyſter.

Our domeſtic animals, that have ſome liberty, are alſo poſſeſſed of ſome peculiar traditional knowledge: dogs and cats have been forced into each other's ſociety, though naturally animals of a very different kind, and have hence learned from each other to eat the knot-graſs, when they are ſick, to promote vomiting. I have ſeen a cat miſtake the blade of barley for this graſs, which evinces it is an acquired know⯑ledge. They have alſo learnt of each other to cover their excrement and urine;—about a ſpoonful of water was ſpilt upon my hearth from the tea-kettle, and I obſerved a kitten cover it with aſhes. Hence this muſt alſo be an acquired art, as the creature miſtook the applica⯑tion of it.

To preſerve their fur clean, and eſpecially their whiſkers, cats waſh their faces, and generally quite behind their ears, every time they eat. As they cannot lick thoſe places with their tongues, they firſt wet the inſide of the leg with ſaliva, and then repeatedly waſh their faces with it, which muſt originally be an effect of reaſoning, becauſe a means is uſed to produce an effect; and ſeems afterwards to be taught or acquired by imitation, like the greateſt part of human arts.

Mr. Leonard, a very intelligent friend of mine, ſaw a cat catch a trout by darting upon it in a deep clear water at the mill at Weaford, near Lichfield. The cat belonged to Mr. Stanley, who had often [161] ſeen her catch fiſh in the ſame manner in ſummer, when the mill-pool was drawn ſo low, that the fiſh could be ſeen. I have heard of other cats taking fiſh in ſhallow water, as they ſtood on the bank. This ſeems a natural art of taking their prey in cats, which their acquired delicacy by domeſtication has in general prevented them from uſing, though their deſire of eating fiſh continues in its original ſtrength.

Mr. White, in his ingenious Hiſtory of Selbourn, was witneſs to a cat's ſuckling a young hare, which followed her about the garden, and came jumping to her call of affection. At Elford, near Lich⯑field, the Rev. Mr. Sawley had taken the young ones out of a hare, which was ſhot; they were alive, and the cat, who had juſt loſt her own kittens, carried them away, as it was ſuppoſed to eat them; but it preſently appeared, that it was affection not hunger which incited her, as ſhe ſuckled them, and brought them up as their mother.

Other inſtances of the miſtaken application of what has been termed inſtinct may be obſerved in flies in the night, who miſtaking a candle for day-light, approach and periſh in the flame. So the pu⯑trid ſmell of the ſtapelia, or carrion-flower, allures the large fleſh-fly to depoſit its young worms on its beautiful petals, which periſh there for want of nouriſhment. This therefore cannot be a neceſſary in⯑ſtinct, becauſe the creature miſtakes the application of it.

Though in this country horſes ſhew little veſtiges of policy, yet in the deſerts of Tartary, and Siberia, when hunted by the Tartars they are ſeen to form a kind of community, ſet watches to prevent their being ſurpriſed, and have commanders, who direct, and haſten their flight, Origin of Language, Vol. I. p. 212. In this country, where four or five horſes travel in a line, the firſt always points his ears for⯑ward, and the laſt points his backward, while the intermediate ones ſeem quite careleſs in this reſpect; which ſeems a part of policy to prevent ſurpriſe. As all animals depend moſt on the ear to apprize them of the approach of danger, the eye taking in only half the ho⯑rizon at once, and horſes poſſeſs a great nicety of this ſenſe; as ap⯑pears [162] from their mode of fighting mentioned No. 8. 5. of this Section, as well as by common obſervation.

There are ſome parts of a horſe, which he cannot conveniently rub, when they itch, as about the ſhoulder, which he can neither bite with his teeth, nor ſcratch with his hind foot; when this part itches, he goes to another horſe, and gently bites him in the part which he wiſhes to be bitten, which is immediately done by his intelligent friend. I once obſerved a young foal thus bite its large mother, who did not chooſe to drop the graſs ſhe had in her mouth, and rubbed her noſe againſt the foal's neck inſtead of biting it; which evinces that ſhe knew the deſign of her progeny, and was not governed by a ne⯑ceſſary inſtinct to bite where ſhe was bitten.

Many of our ſhrubs, which would otherwiſe afford an agreeable food to horſes, are armed with thorns or prickles, which ſecure them from thoſe animals; as the holly, hawthorn, gooſeberry, gorſe. In the extenſive moorlands of Staffordſhire, the horſes have learnt to ſtamp upon a gorſe-buſh with one of their fore-fect for a minute to⯑gether, and when the points are broken, they eat it without injury. Which is an art other horſes in the fertile parts of the county do not poſſeſs, and prick their mouths till they bleed, if they are induced by hunger or caprice to attempt eating gorſe.

Swine have a ſenſe of touch as well as of ſmell at the end of their noſe, which they uſe as a hand, both to root up the ſoil, and to turn over and examine objects of food, ſomewhat like the proboſcis of an elephant. As they require ſhelter from the cold in this climate, they have learnt to collect ſtraw in their mouths to make their neſt, when the wind blows cold; and to call their companions by repeated cries to aſſiſt in the work, and add to their warmth by their numerous bed⯑fellows. Hence theſe animals, which are eſteemed ſo unclean, have alſo learned never to befoul their dens, where they have liberty, with their own excrement; an art, which cows and horſes, which have open hovels to run into, have never acquired. I have obſerved great [163] ſagacity in ſwine; but the ſhort lives we allow them, and their ge⯑neral confinement, prevents their improvement, which might pro⯑bably be otherwiſe greater than that of dogs.

Inſtances of the ſagacity and knowledge of animals are very nu⯑merous to every obſerver, and their docility in learning various arts from mankind, evinces that they may learn ſimilar arts from their own ſpecies, and thus be poſſeſſed of much acquired and traditional knowledge.

A dog whoſe natural prey is ſheep, is taught by mankind, not only to leave them unmoleſted, but to guard them; and to hunt, to ſet, or to deſtroy other kinds of animals, as birds, or vermin; and in ſome countries to catch fiſh, in others to find truffles, and to practiſe a great variety of tricks; is it more ſurpriſing that the crows ſhould teach each other, that the hawk can catch leſs birds, by the ſuperior ſwiftneſs of his wing, and if two of them follow him, till he ſucceeds in his deſign, that they can by force ſhare a part of the capture? This I have formerly obſerved with attention and aſtoniſhment.

There is one kind of pelican mentioned by Mr. Oſbeck, one of Linnaeus's travelling pupils (the pelicanus aquilus), whoſe food is fiſh; and which it takes from other birds, becauſe it is not formed to catch them itſelf; hence it is called by the Engliſh a Man-of-war-bird, Voyage to China, p. 88. There are many other intereſting anec⯑dotes of the pelican and cormorant, collected from authors of the beſt authority, in a well-managed Natural Hiſtory for Children, publiſhed by Mr. Galton. Johnſon. London.

And the following narration from the very accurate Monſ. Adan⯑ſon, in his Voyage to Senegal, may gain credit with the reader: as his employment in this country was ſolely to make obſervations in natu⯑ral hiſtory. On the river Niger, in his road to the iſland Griel, he ſaw a great number of pelicans, or wide throats. "They moved with great ſtate like ſwans upon the water, and are the largeſt bird next to the oſtrich; the bill of the one I killed was upwards of a foot [164] and half long, and the bag faſtened underneath it held two and twenty⯑pints of water. They ſwim in flocks, and form a large circle, which they contract afterwards, driving the fiſh before them with their legs: when they ſee the fiſh in ſufficient number confined in this ſpace, they plunge their bill wide open into the water, and ſhut it again with great quickneſs. They thus get fiſh into their throat⯑bag, which they eat afterwards on ſhore at their leiſure." P. 247.

XII. The knowledge and language of thoſe birds, that frequently change their climate with the ſeaſons, is ſtill more extenſive: as they perform theſe migrations in large ſocieties, and are leſs ſubject to the power of man, than the reſident tribes of birds. They are ſaid to follow a leader during the day, who is occaſionally changed, and to keep a continual cry during the night to keep themſelves together. It is probable that theſe emigrations were at firſt undertaken as acci⯑dent directed, by the more adventurous of their ſpecies, and learned from one another like the diſcoveries of mankind in navigation. The following circumſtances ſtrongly ſupport this opinion.

1. Nature has provided theſe animals, in the climates where they are produced, with another reſource: when the ſeaſon becomes too cold for their conſtitutions, or the food they were ſupported with ceaſes to be ſupplied, I mean that of ſleeping. Dormice, ſnakes, and bats, have not the means of changing their country; the two former from the want of wings, and the latter from his being not able to bear the light of the day. Hence theſe animals are obliged to make uſe of this reſource, and ſleep during the winter. And thoſe ſwallows that have been hatched too late in the year to acquire their full ſtrength of pinion, or that have been maimed by accident or diſeaſe, have been frequently found in the hollows of rocks on the ſea coaſts, and even under water in this torpid ſtate, from which they have been revived by the warmth of a fire. This torpid ſtate of ſwal⯑lows is teſtified by innumerable evidences both of antient and modern names. Ariſtotle ſpeaking of the ſwallows ſays, "They paſs into [165] warmer climates in winter, if ſuch places are at no great diſtance; if they are, they bury themſelves in the climates where they dwell," (8. Hiſt. c. 16. See alſo Derham's Phyſ. Theol. v. ii. p. 177.)

Hence their emigrations cannot depend on a neceſſary inſtinct, as the emigrations themſelves are not neceſſary!

2. When the weather becomes cold, the ſwallows in the neigh⯑bourhood aſſemble in large flocks; that is, the unexperienced attend thoſe that have before experienced the journey they are about to un⯑dertake: they are then ſeen ſome time to hover on the coaſt, till there is calm weather, or a wind, that ſuits the direction of their flight. Other birds of paſſage have been drowned by thouſands in the ſea, or have ſettled on ſhips quite exhauſted with fatigue. And others, either by miſtaking their courſe, or by diſtreſs of weather, have arrived in countries where they were never ſeen before: and thus are evidently ſubject to the ſame hazards that the human ſpecies undergo, in the execution of their artificial purpoſes.

3. The ſame birds are emigrant from ſome countries and not ſo from others: the ſwallows were ſeen at Goree in January by an in⯑genious philoſopher of my acquaintance, and he was told that they continued there all the year; as the warmth of the climate was at all ſeaſons ſufficient for their own conſtitutions, and for the production of the flies that ſupply them with nouriſhment. Herodotus ſays, that in Libya, about the ſprings of the Nile, the ſwallows continue all the year. (L. 2.)

Quails (tetrao corturnix, Lin.) are birds of paſſage from the coaſt of Barbary to Italy, and have frequently ſettled in large ſhoals on ſhips fatigued with their flight. (Ray, Wiſdom of God, p. 129. Derham Phyſic. Theol. v. ii. p. 178.) Dr. Ruſſel, in his Hiſtory of Aleppo, obſerves that the ſwallows viſit that country about the end of Febru⯑ary, and having hatched their young diſappear about the end of July; and returning again about the beginning of October, continue about a fortnight, and then again diſappear. (P. 70.)

[166] When my late friend Dr. Chambres, of Derby, was on the iſland of Caprea in the bay of Naples, he was informed that great flights of quails annually ſettle on that iſland about the beginning of May, in their paſſage from Africa to Europe. And that they always come when the ſouth-eaſt wind blows, are fatigued when they reſt on this iſland, and are taken in ſuch amazing quantities and ſold to the Con⯑tinent, that the inhabitants pay the biſhop his ſtipend out of the pro⯑fits ariſing from the ſale of them.

The flights of theſe birds acroſs the Mediterranean are recorded near three thouſand years ago. "There went forth a wind from the Lord and brought quails from the ſea, and let them fall upon the camp, a day's journey round about it, and they were two cubits above the earth," (Numbers, chap. ii. ver. 31.)

In our country, Mr. Pennant informs us, that ſome quails migrate, and others only remove from the internal parts of the iſland to the coaſts, (Zoology, octavo, 210.) Some of the ringdoves and ſtares breed here, others migrate, (ibid. 510, 511.) And the ſlender billed ſmall birds do not all quit theſe kingdoms in the winter, though the difficulty of procuring the worms and inſects, that they feed on, ſup⯑plies the ſame reaſon for migration to them all, (ibid. 511.)

Linnaeus has obſerved, that in Sweden the female chaffinches quit that country in September, migrating into Holland, and leave their mates behind till their return in ſpring. Hence he has called them Fringilla caelebs, (Amaen. Acad. ii. 42. iv. 595.) Now in our cli⯑mate both ſexes of them are perennial birds. And Mr. Pennant ob⯑ſerves that the hoopoe, chatterer, hawfinch, and croſsbill, migrate into England ſo rarely, and at ſuch uncertain times, as not to deſerve to be ranked among our birds of paſſage, (ibid. 511.)

The water fowl, as geeſe and ducks, are better adapted for long migrations, than the other tribes of birds, as, when the weather is calm, they can not only reſt themſelves, or ſleep upon the ocean, but poſſibly procure ſome kind of food from it.

[167] Hence in Siberia, as ſoon as the lakes are frozen, the water ſowl, which are very numerous, all diſappear, and are ſuppoſed to fly to warmer climates, except the rail, which, from its inability for long flights, probably ſleeps, like our bat, in their winter. The follow⯑ing account from the Journey of Profeſſor Gmelin, may entertain the reader. "In the neighbourhood of Kraſnoiark, amongſt many other emigrant water fowls, we obſerved a great number of rails, which when purſued never took flight, but endeavoured to eſcape by run⯑ning. We enquired how theſe birds, that could not fly, could retire into other countries in the winter, and were told, both by the Tar⯑tars and Aſſanians, that they well knew thoſe birds could not alone paſs into other countries: but when the crains (les grues) retire in autumn, each one takes a rail (un rale) upon his back, and carries him to a warmer climate."

XIII. In that ſeaſon of the year which ſupplies the nouriſhment proper for the expected brood, the birds enter into a contract of mar⯑riage, and with joint labour conſtruct a bed for the reception of their [168] offspring. Their choice of the proper ſeaſon, their contracts of mar⯑riage, and the regularity with which they conſtruct their neſts, have in all ages excited the admiration of naturaliſts; and have always been attributed to the power of inſtinct, which, like the occult qualities of the antient philoſophers, prevented all further enquiry. We ſhall conſider them in their order.

XIV. The deep receſſes of the ocean are inacceſſible to mankind, which prevents us from having much knowledge of the arts and go⯑vernment of its inhabitants.

1. One of the baits uſed by the fiſherman is an animal called an Old Soldier, his ſize and form are ſomewhat like the craw-fiſh, with this difference, that his tail is covered with a tough membrane inſtead of a ſhell; and to obviate this defect, he ſeeks out the uninhabited ſhell of ſome dead fiſh, that is large enough to receive his tail, and carries it about with him as part of his clothing or armour.

2. On the coaſts about Scarborough, where the haddocks, cods, and dog-fiſh, are in great abundance, the fiſhermen univerſally be⯑lieve that the dog-fiſh make a line, or ſemicircle, to encompaſs a ſhoal of haddocks and cod, confining them within certain limits near the ſhore, and eating them as occaſion requires. For the haddocks and cod are always found near the ſhore without any dog-fiſh among them, and the dog-fiſh further off without any haddocks or cod; and yet the former are known to prey upon the latter, and in ſome years devour ſuch immenſe quantities as to render this fiſhery more expenſive than profitable.

[174] 3. The remora, when he wiſhes to remove his ſituation, as he is a very ſlow ſwimmer, is content to take an outſide place on whatever conveyance is going his way; nor can the cunning animal be tempted to quit his hold of a ſhip when ſhe is ſailing, not even for the lucre of a piece of pork, leſt it ſhould endanger the loſs of his paſſage: at other times he is eaſily caught with the hook.

4. The crab-fiſh, like many other teſtaceous animals, annually changes its ſhell; it is then in a ſoft ſtate, covered only with a mucous membrane, and conceals itſelf in holes in the ſand or under weeds; at this place a hard ſhelled crab always ſtands centinel, to prevent the ſea inſects from injuring the other in its defenceleſs ſtate; and the fiſhermen from his appearance know where to find the ſoft ones, which they uſe for baits in catching other fiſh.

And though the hard ſhelled crab, when he is on this duty, ad⯑vances boldly to meet the foe, and will with difficulty quit the field; yet at other times he ſhews great timidity, and has a wonderful ſpeed in attempting his eſcape; and, if often interrupted, will pretend death like the ſpider, and watch an opportunity to ſink himſelf into the ſand, keeping only his eyes above. My ingenious friend Mr. Burdett, who favoured me with theſe accounts at the time he was ſurveying the coaſts, thinks the commerce between the ſexes takes place at this time, and inſpires the courage of the creature.

5. The ſhoals of herrings, cods, haddocks, and other fiſh, which approach our ſhores at certain ſeaſons, and quit them at other ſeaſons without leaving one behind; and the ſalmon, that periodically fre⯑quent our rivers, evince, that there are vagrant tribes of fiſh, that perform as regular migrations as the birds of paſſage already men⯑tioned.

6. There is a cataract on the river Liffey in Ireland about nine⯑teen feet high: here in the ſalmon ſeaſon many of the inhabitants amuſe themſelves in obſerving theſe fiſh leap up the torrent. They dart themſelves quite out of the water as they aſcend, and frequently [175] fall back many times before they ſurmount it, and baſkets made of twigs are placed near the edge of the ſtream to catch them in their fall.

I have obſerved, as I have ſat by a ſpout of water, which deſcends from a ſtone trough about two feet into a ſtream below, at particular ſeaſons of the year, a great number of little fiſh called minums, or pinks, throw themſelves about twenty times their own length out of the water, expecting to get into the trough above.

This evinces that the ſtorgee, or attention of the dam to provide for the offspring, is ſtrongly exerted amongſt the nations of fiſh, where it would ſeem to be the moſt neglected; as theſe ſalmon can⯑not be ſuppoſed to attempt ſo difficult and dangerous a taſk without being conſcious of the purpoſe or end of their endeavours.

It is further remarkable, that moſt of the old ſalmon return to the ſea before it is proper for the young ſhoals to attend them, yet that a few old ones continue in the rivers ſo late, that they become perfectly emaciated by the inconvenience of their ſituation, and this apparently to guide or to protect the unexperienced brood.

Of the ſmaller water animals we have ſtill leſs knowledge, who nevertheleſs probably poſſeſs many ſuperior arts; ſome of theſe are mentioned in Botanic Garden, P. I. Add. Note XXVII. and XXVIII. The nympha of the water-moths of our rivers, which cover them⯑ſelves with caſes of ſtraw, gravel, and ſhell, contrive to make their habitations nearly in equilibrium with the water; when too heavy, they add a bit of wood or ſtraw; when too light, a bit of gravel. Edinb. Tranſ.

All theſe circumſtances bear a near reſemblance to the deliberate ac⯑tions of human reaſon.

XV. We have a very imperfect acquaintance with the various tribes of inſects: their occupations, manner of life, and even the number of their ſenſes, differ from our own, and from each other; but there is reaſon to imagine, that thoſe which poſſeſs the ſenſe of [176] touch in the moſt exquiſite degree, and whoſe occupations require the moſt conſtant exertion of their powers, are indued with a greater proportion of knowledge and ingenuity.

The ſpiders of this country manufacture nets of various forms, adapted to various ſituations, to arreſt the flies that are their food; and ſome of them have a houſe or lodging-place in the middle of the net, well contrived for warmth, ſecurity, or concealment. There is a large ſpider in South America, who conſtructs nets of ſo ſtrong a texture as to entangle ſmall birds, particularly the humming bird. And in Jamaica there is another ſpider, who digs a hole in the earth obliquely downwards, about three inches in length, and one inch in diameter, this cavity ſhe lines with a tough thick web, which when taken out reſembles a leathern purſe: but what is moſt curious, this houſe has a door with hinges, like the operculum of ſome ſea ſhells; and herſelf and family, who tenant this neſt, open and ſhut the door, whenever they paſs or repaſs. This hiſtory was told me, and the neſt with its operculum ſhewn me by the late Dr. Butt of Bath, who was ſome years phyſician in Jamaica.

The production of theſe nets is indeed a part of the nature or con⯑formation of the animal, and their natural uſe is to ſupply the place of wings, when ſhe wiſhes to remove to another ſituation. But when ſhe employs them to entangle her prey, there are marks of evident deſign, for ſhe adapts the form of each net to its ſituation, and ſtrengthens thoſe lines, that require it, by joining others to the middle of them, and attaching thoſe others to diſtant objects, with the ſame individual art, that is uſed by mankind in ſupporting the maſts and extending the ſails of ſhips. This work is executed with more ma⯑thematical exactneſs and ingenuity by the field ſpiders, than by thoſe in our houſes, as their conſtructions are more ſubjected to the injuries of dews and tempeſts.

Beſides the ingenuity ſhewn by theſe little creatures in taking their prey, the circumſtance of their counterfeiting death, when they are [177] put into terror, is truly wonderful; and as ſoon as the object of terror is removed, they recover and run away. Some beetles are alſo ſaid to poſſeſs this piece of hypocriſy.

The curious webs, or chords, conſtructed by ſome young cater⯑pillars to defend themſelves from cold, or from inſects of prey; and by ſilk-worms and ſome other caterpillars, when they tranſmigrate into aureliae or larvae, have deſervedly excited the admiration of the inquiſitive. But our ignorance of their manner of life, and even of the number of their ſenſes, totally precludes us from underſtanding the means by which they acquire this knowledge.

The care of the ſalmon in chooſing a proper ſituation for her ſpawn, the ſtructure of the neſts of birds, their patient incubation, and the art of the cuckoo in depoſiting her egg in her neighbour's nurſery, are inſtances of great ſagacity in thoſe creatures: and yet they are much inferior to the arts exerted by many of the inſect tribes on ſimi⯑lar occaſions. The hairy excreſcences on briars, the oak apples, the blaſted leaves of trees, and the lumps on the backs of cows, are ſitu⯑ations that are rather produced than choſen by the mother inſect for the convenience of her offspring. The cells of bees, waſps, ſpiders, and of the various coralline inſects, equally aſtoniſh us, whether we attend to the materials or to the architecture.

But the conduct of the ant, and of ſome ſpecies of the ichneumon fly in the incubation of their eggs, is equal to any exertion of human ſcience. The ants many times in a day move their eggs nearer the ſurface of their habitation, or deeper below it, as the heat of the weather varies; and in colder days lie upon them in heaps for the purpoſe of incubation: if their manſion is too dry, they carry them to places where there is moiſture, and you may diſtinctly ſee the little worms move and ſuck up the water. When too much moiſture ap⯑proaches their neſt, they convey their eggs deeper in the earth, or to ſome other place of ſafety. (Swammerd, Epil. ad Hiſt. Inſects, p. 153. Phil. Tranſ. No. 23. Lowthrop. V. 2. p. 7.)

[178] There is one ſpecies of ichneumon-fly, that digs a hole in the earth, and carrying into it two or three living caterpillars, depoſits her eggs, and nicely cloſing up the neſt leaves them there; partly doubtleſs to aſſiſt the incubation, and partly to ſupply food to her future young, (Derham. B. 4. c. 13. Ariſtotle Hiſt. Animal, L. 5. c. 20.)

A friend of mine put about fifty large caterpillars collected from cabbages on ſome bran and a few leaves into a box, and covered it with gauze to prevent their eſcape. After a few days we ſaw, from more than three fourths of them, about eight or ten little caterpillars of the ichneumon-fly come out of their backs, and ſpin each a ſmall cocoon of ſilk, and in a few days the large caterpillars died. This ſmall fly it ſeems lays its egg in the back of the cabbage caterpillar, which when hatched preys upon the material, which is produced there for the purpoſe of making ſilk for the future neſt of the cabbage cater⯑pillar; of which being deprived, the creature wanders about till it dies, and thus our gardens are preſerved by the ingenuity of this cruel fly. This curious property of producing a ſilk thread, which is com⯑mon to ſome ſea animals, ſee Botanic Garden, Part I. Note XXVII. and is deſigned for the purpoſe of their transformation as in the ſilk⯑worm, is uſed for conveying themſelves from higher branches to lower ones of trees by ſome caterpillars, and to make themſelves temporary neſts or tents, and by the ſpider for entangling his prey. Nor is it ſtrange that ſo much knowledge ſhould be acquired by ſuch ſmall ani⯑mals; ſince there is reaſon to imagine, that theſe inſects have the ſenſe of touch, either in their proboſcis, or their antennae, to a great degree of perfection; and thence may poſſeſs, as far as their ſphere extends, as accurate knowledge, and as ſubtle invention, as the diſcoverers of human arts.

XVI. 1. If we were better acquainted with the hiſtories of thoſe inſects that are formed into ſocieties, as the bees, waſps, and ants, I make no doubt but we ſhould find, that their arts and improvements are not ſo ſimilar and uniform as they now appear to us, but that they [179] aroſe in the ſame manner from experience and tradition, as the arts of our own ſpecies; though their reaſoning is from fewer ideas, is buſied about fewer objects, and is exerted with leſs energy.

There are ſome kinds of inſects that migrate like the birds before mentioned. The locuſt of warmer climates has ſometimes come over to England; it is ſhaped like a graſshopper, with very large wings, and a body above an inch in length. It is mentioned as coming into Egypt with an eaſt wind, "The Lord brought an eaſt wind upon the land all that day and night, and in the morning the eaſt wind brought the locuſts, and covered the face of the earth, ſo that the land was dark," Exod. x. 13. The migrations of theſe inſects are mentioned in another part of the ſcripture, "The locuſts have no king, yet go they forth all of them in bands," Prov. xxx. 27.

The accurate Mr. Adanſon, near the river Gambia in Africa, was witneſs to the migration of theſe inſects. "About eight in the morning, in the month of February, there ſuddenly aroſe over our heads a thick cloud, which darkened the air, and deprived us of the rays of the ſun. We found it was a cloud of locuſts raiſed about twenty or thirty fathoms from the ground, and covering an extent of ſeveral leagues; at length a ſhower of theſe inſects deſcended, and after devouring every green herb, while they reſted, again reſumed their flight. This cloud was brought by a ſtrong eaſt-wind, and was all the morning in paſſing over the adjacent country." (Voyage to Senegal, 158.)

In this country the gnats are ſometimes ſeen to migrate in clouds, like the muſketoes of warmer climates, and our ſwarms of bees fre⯑quently travel many miles, and are ſaid in North America always to fly towards the ſouth. The prophet Iſaiah has a beautiful alluſion to theſe migrations, "The Lord ſhall call the fly from the rivers of Egypt, and ſhall hiſs for the bee that is in the land of Aſſyria," Iſa. vii. 18. which has been lately explained by Mr. Bruce, in his tra⯑vels to diſcover the ſource of the Nile.

[180] 2. I am well informed that the bees that were carried into Barba⯑does, and other weſtern iſlands, ceaſed to lay up any honey after the firſt year, as they found it not uſeful to them: and are now become very troubleſome to the inhabitants of thoſe iſlands by infeſting their ſugar⯑houſes; but thoſe in Jamaica continue to make honey, as the cold north winds, or rainy ſeaſons of that iſland, confine them at home for ſeveral weeks together. And the bees of Senegal, which differ from thoſe of Europe only in ſize, make their honey not only ſuperior to ours in delicacy of flavour, but it has this ſingularity, that it never concretes, but remains liquid as ſyrup, (Adanſon). From ſome obſervations of Mr. Wildman, and of other people of veracity, it ap⯑pears, that during the ſevere part of the winter ſeaſon for weeks to⯑gether the bees are quite benumbed and torpid from the cold, and do not conſume any of their proviſion. This ſtate of ſleep, like that of ſwallows and bats, ſeems to be the natural reſource of thoſe creatures in cold climates, and the making of honey to be an artificial improve⯑ment.

As the death of our hives of bees appears to be owing to their being kept ſo warm, as to require food when their ſtock is exhauſted; a very obſerving gentleman at my requeſt put two hives for many weeks into a dry cellar, and obſerved, during all that time, they did not conſume any of their proviſion, for their weight did not decreaſe, as it had done when they were kept in the open air. The ſame ob⯑ſervation is made in the Annual Regiſter for 1768, p. 113. And the Rev. Mr. White, in his Method of preſerving Bees, adds, that thoſe on the north ſide of his houſe conſumed leſs honey in the winter than thoſe on the ſouth ſide.

There is another obſervation on bees well aſcertained, that they at various times, when the ſeaſon begins to be cold, by a general mo⯑tion of their legs as they hang in cluſters produce a degree of warmth, which is eaſily perceptible by the hand. Hence by this ingenious ex⯑ertion, [181] they for a long time prevent the torpid ſtate they would na⯑turally fall into.

According to the late obſervations of Mr. Hunter, it appears that the bee's-wax is not made from the duſt of the anthers of flowers, which they bring home on their thighs, but that this makes what is termed bee-bread, and is uſed for the purpoſe of feeding the bee-mag⯑gots; in the ſame manner butterflies live on honey, but the previous caterpillar lives on vegetable leaves, while the maggots of large flies require fleſh for their food, and thoſe of the ichneumon fly require inſects for their food. What induces the bee who lives on honey to lay up vegetable powder for its young? What induces the butterfly to lay its eggs on leaves, when itſelf feeds on honey? What induces the other flies to ſeek a food for their progeny different from what they conſume themſelves? If theſe are not deductions from their own previous experience or obſervation, all the actions of mankind muſt be reſolved into inſtinct.

3. The dormouſe conſumes but little of its food during the rigour of the ſeaſon, for they roll themſelves up, or ſleep, or lie torpid the greateſt part of the time; but on warm ſunny days experience a ſhort revival, and take a little food, and then relapſe into their former ſtate." (Pennant Zoolog. p. 67.) Other animals, that ſleep in winter without laying up any provender, are obſerved to go into their winter beds fat and ſtrong, but return to day-light in the ſpring ſeaſon very lean and feeble. The common flies ſleep during the winter without any proviſion for their nouriſhment, and are daily revived by the warmth of the ſun, or of our fires. Theſe whenever they ſee light endeavour to approach it, having obſerved, that by its greater vicinity they get free from the degree of torpor, that the cold produces; and are hence induced perpetually to burn themſelves in our candles: de⯑ceived, like mankind, by the miſapplication of their knowledge. Whilſt many of the ſubterraneous inſects, as the common worms, ſeem to retreat ſo deep into the earth as not to be enlivened or awak⯑ened [182] by the difference of our winter days; and ſtop up their holes with leaves or ſtraws, to prevent the froſts from injuring them, or the centipes from devouring them. The habits of peace, or the ſtra⯑tagems of war, of theſe ſubterranean nations are covered from our view; but a friend of mine prevailed on a diſtreſſed worm to enter the hole of another worm on a bowling-green, and he preſently re⯑turned much wounded about his head. And I once ſaw a worm riſe haſtily out of the earth into the ſunſhine, and obſerved a centipes hanging at its tail; the centipes nimbly quitted the tail, and ſeizing the worm about its middle cut it in half with its forceps, and preyed upon one part, while the other eſcaped. Which evinces they have deſign in ſtopping the mouths of their habitations.

4. The waſp of this country fixes his habitation under ground, that he may not be affected with the various changes of our climate; but in Jamaica he hangs it on the bough of a tree, where the ſeaſons are leſs ſevere. He weaves a very curious paper of vegetable fibres to cover his neſt, which is conſtructed on the ſame principle with that of the bee, but with a different material; but as his prey conſiſts of fleſh, fruits, and inſects, which are periſhable commodities, he can lay up no provender for the winter.

M. de la Loubiere, in his relation of Siam, ſays, "That in a part of that kingdom, which lies open to great inundations, all the ants make their ſettlements upon trees; no ants' neſts are to be ſeen any where elſe." Whereas in our country the ground is their only ſitu⯑ation. From the ſcriptural account of theſe inſects, one might be led to ſuſpect, that in ſome climates they lay up a proviſion for the winter. Origen affirms the ſame, (Cont. Celſ. L. 4.) But it is gene⯑rally believed that in this country they do not, (Prov. vi. 6. xxx. 25.) The white ants of the coaſt of Africa make themſelves pyramids eight or ten feet high, on a baſe of about the ſame width, with a ſmooth ſurface of rich clay, exceſſively hard and well built, which appear at a diſtance like an aſſemblage of the huts of the negroes, (Adanſon). [183] The hiſtory of theſe have been lately well deſcribed in the Philoſoph. Tranſactions, under the name of termes, or termites. Theſe differ very much from the neſts of our large ant; but the real hiſtory of this creature, as well as of the waſp, is yet very imperfectly known.

Waſps are ſaid to catch large ſpiders, and to cut off their legs, and carry their mutilated bodies to their young, Dict. Raiſon. Tom. I. p. 152.

One circumſtance I ſhall relate which fell under my own eye, and ſhewed the power of reaſon in a waſp, as it is exerciſed among men. A waſp, on a gravel walk, had caught a fly nearly as large as himſelf; kneeling on the ground I obſerved him ſeparate the tail and the head from the body part, to which the wings were attached. He then took the body part in his paws, and roſe about two feet from the ground with it; but a gentle breeze wafting the wings of the fly turned him round in the air, and he ſettled again with his prey upon the gravel. I then diſtinctly obſerved him cut off with his mouth, firſt one of the wings, and then the other, after which he flew away with it unmoleſted by the wind.

Go, thou ſluggard, learn arts and induſtry from the bee, and from the ant!

Go, proud reaſoner, and call the worm thy ſiſter!

XVII. Concluſion.

It was before obſerved how much the ſuperior accuracy of our ſenſe of touch contributes to increaſe our knowledge; but it is the greater energy and activity of the power of volition (as explained in the former Sections of this work) that marks mankind, and has given him the empire of the world.

[184] There is a criterion by which we may diſtinguiſh our voluntary acts or thoughts from thoſe that are excited by our ſenſations: "The former are always employed about the means to acquire pleaſureable ob⯑jects, or to avoid painful ones: while the latter are employed about the poſſeſſion of thoſe that are already in our power."

If we turn our eyes upon the fabric of our fellow animals, we find they are ſupported with bones, covered with ſkins, moved by muſ⯑cles; that they poſſeſs the ſame ſenſes, acknowledge the ſame appetites, and are nouriſhed by the ſame aliment with ourſelves; and we ſhould hence conclude from the ſtrongeſt analogy, that their internal faculties were alſo in ſome meaſure ſimilar to our own.

Mr. Locke indeed publiſhed an opinion, that other animals poſſeſſed no abſtract or general ideas, and thought this circumſtance was the barrier between the brute and the human world. But theſe abſtracted ideas have been ſince demonſtrated by Biſhop Berkley, and allowed by Mr. Hume, to have no exiſtence in nature, not even in the mind of their inventor, and we are hence neceſſitated to look for ſome other mark of diſtinction.

The ideas and actions of brutes, like thoſe of children, are almoſt perpetually produced by their preſent pleaſures, or their preſent pains; and, except in the few inſtances that have been mentioned in this Section, they ſeldom buſy themſelves about the means of procuring future bliſs, or of avoiding future miſery.

Whilſt the acquiring of languages, the making of tools, and the labouring for money; which are all only the means of procuring plea⯑ſure: and the praying to the Deity, as another means to procure hap⯑pineſs, are characteriſtic of human nature.

I. 1. TO inveſtigate with preciſion the catenations of animal mo⯑tions, it would be well to attend to the manner of their production; but we cannot begin this diſquiſition early enough for this purpoſe, as the catenations of motion ſeem to begin with life, and are only ex⯑tinguiſhable with it. We have ſpoken of the power of irritation, of ſenſation, of volition, and of aſſociation, as preceding the fibrous [186] motions; we now ſtep forwards, and conſider, that converſely they are in their turn preceded by thoſe motions; and that all the ſuc⯑ceſſive trains or circles of our actions are compoſed of this twofold concatenation. Thoſe we ſhall call trains of action, which continue to proceed without any ſtated repetitions; and thoſe circles of action, when the parts of them return at certain periods, though the trains, of which they conſiſt, are not exactly ſimilar. The reading an epic poem is a train of actions; the reading a ſong with a chorus at equal diſtances in the meaſure conſtitutes ſo many circles of action.

2. Some catenations of animal motion are produced by reiterated ſucceſſive irritations, as when we learn to repeat the alphabet in its order by frequently reading the letters of it. Thus the vermicular motions of the bowels were originally produced by the ſucceſſive ir⯑ritations of the paſſing aliment; and the ſucceſſion of actions of the auricles and ventricles of the heart was originally formed by ſucceſſive ſtimulus of the blood, theſe afterwards become part of the diurnal cir⯑cles of animal actions, as appears by the periodical returns of hunger, and the quickened pulſe of weak people in the evening.

Other catenations of animal motion are gradually acquired by ſuc⯑ceſſive agreeable ſenſations, as in learning a favourite ſong or dance; others by diſagreeable ſenſations, as in coughing or nictitation; theſe become aſſociated by frequent repetition, and afterwards compoſe parts of greater circles of action like thoſe above mentioned.

Other catenations of motions are gradually acquired by frequent vo⯑luntary repetitions; as when we deliberately learn to march, read, fence, or any mechanic art, the motions of many of our muſcles be⯑come gradually linked together in trains, tribes, or circles of action. Thus when any one at firſt begins to uſe the tools in turning wood or metals in a lathe, he wills the motions of his hand or fingers, till at length theſe actions become ſo connected with the effect, that he ſeems only to will the point of the chiſſel. Theſe are cauſed by vo⯑lition, [187] connected by aſſociation like thoſe above deſcribed, and after⯑wards become parts of our diurnal trains or circles of action.

3. All theſe catenations of animal motions are liable to proceed ſome time after they are excited, unleſs they are diſturbed or impeded by other irritations, ſenſations, or volitions; and in many inſtances in ſpite of our endeavours to ſtop them; and this property of animal motions is probably the cauſe of their catenation. Thus when a child revolves ſome minutes on one foot, the ſpectra of the ambient objects appear to circulate round him ſome time after he falls upon the ground. Thus the palpitation of the heart continues ſome time after the object of fear, which occaſioned it, is removed. The bluſh of ſhame, which is an exceſs of ſenſation, and the glow of anger, which is an exceſs of volition, continue ſome time, though the affected per⯑ſon finds, that thoſe emotions were cauſed by miſtaken facts, and en⯑deavours to extinguiſh their appearance. See Sect. XII. 1. 5.

4. When a circle of motions becomes connected by frequent repe⯑titions as above, we can exert our attention ſtrongly on other objects, and the concatenated circle of motions will nevertheleſs proceed in due order; as whilſt you are thinking on this ſubject, you uſe variety of muſcles in walking about your parlour, or in ſitting at your writ⯑ing-table.

5. Innumerable catenations of motions may proceed at the ſame time, without incommoding each other. Of theſe are the motions of the heart and arteries; thoſe of digeſtion and glandular ſecretion; of the ideas, or ſenſual motions; thoſe of progreſſion, and of ſpeak⯑ing; the great annual circle of actions ſo apparent in birds in their times of breeding and moulting; the monthly circles of many female animals; and the diurnal circles of ſleeping and waking, of fulneſs and inanition.

6. Some links of ſucceſſive trains or of ſynchronous tribes of action may be left out without disjoining the whole. Such are our uſual trains of recollection; after having travelled through an entertaining [188] country, and viewed many delightful lawns, rolling rivers, and echo⯑ing rocks; in the recollection of our journey we leave out the many diſtricts, that we croſſed, which were marked with no peculiar plea⯑ſure. Such alſo are our complex ideas, they are catenated tribes of ideas, which do not perfectly reſemble their correſpondent perceptions, becauſe ſome of the parts are omitted.

7. If an interrupted circle of actions is not entirely diſſevered, it will continue to proceed confuſedly, till it comes to the part of the circle, where it was interrupted.

The vital motions in a fever from drunkenneſs, and in other peri⯑odical diſeaſes, are inſtances of this circumſtance. The accidental in⯑ebriate does not recover himſelf perfectly till about the ſame hour on the ſucceeding day. The accuſtomed drunkard is diſordered, if he has not his uſual potation of fermented liquor. So if a conſiderable part of a connected tribe of action be diſturbed, that whole tribe goes on with confuſion, till the part of the tribe affected regains its ac⯑cuſtomed catenations. So vertigo produces vomiting, and a great ſe⯑cretion of bile, as in ſea-ſickneſs, all theſe being parts of the tribe of irritative catenations.

8. Weaker catenated trains may be diſſevered by the ſudden ex⯑ertion of the ſtronger. When a child firſt attempts to walk acroſs a room, call to him, and he inſtantly falls upon the ground. So while I am thinking over the virtues of my friends, if the tea-kettle ſpurt out ſome hot water on my ſtocking; the ſudden pain breaks the weaker chain of ideas, and introduces a new group of figures of its own. This circumſtance is extended to ſome unnatural trains of ac⯑tion, which have not been confirmed by long habit; as the hiccough, or an ague-fit, which are frequently curable by ſurpriſe. A young lady about eleven years old had for five days had a contraction of one muſcle in her fore arm, and another in her arm, which occurred four or five times every minute; the muſcles were ſeen to leap, but with⯑out bending the arm. To counteract this new morbid habit, an iſſue [189] was placed over the convulſed muſcle of her arm, and an adheſive plaſter wrapped tight like a bandage over the whole fore arm, by which the new motions were immediately deſtroyed, but the means were continued ſome weeks to prevent a return.

9. If any circle of actions is diſſevered, either by omiſſion of ſome of the links, as in ſleep, or by inſertion of other links, as in ſurpriſe, new catenations take place in a greater or leſs degree. The laſt link of the broken chain of actions becomes connected with the new mo⯑tion which has broken it, or with that which was neareſt the link omitted; and theſe new catenations proceed inſtead of the old ones. Hence the periodic returns of ague-fits, and the chimeras of our dreams.

10. If a train of actions is diſſevered, much effort of volition or ſenſation will prevent its being reſtored. Thus in the common im⯑pediment of ſpeech, when the aſſociation of the motions of the muſ⯑cles of enunciation with the idea of the word to be ſpoken is diſor⯑dered, the great voluntary efforts, which diſtort the countenance, prevent the rejoining of the broken aſſociations. See No. II. 10. of this Section. It is thus likewiſe obſervable in ſome inflammations of the bowels, the too ſtrong efforts made by the muſcles to carry for⯑wards the offending material fixes it more firmly in its place, and prevents the cure. So in endeavouring to recal to our memory ſome particular word of a ſentence, if we exert ourſelves too ſtrongly about it, we are leſs likely to regain it.

11. Catenated trains or tribes of action are eaſier diſſevered than catenated circles of action. Hence in epileptic fits the ſynchronous connected tribes of action, which keep the body erect, are diſſevered, but the circle of vital motions continues undiſturbed.

12. Sleep deſtroys the power of volition, and precludes the ſtimuli of external objects, and thence diſſevers the trains, of which theſe are a part; which confirms the other catenations, as thoſe of the vital [190] motions, ſecretions, and abſorptions; and produces the new trains of ideas, which conſtitute our dreams.

II. 1. All the preceding circumſtances of the catenations of animal motions will be more clearly underſtood by the following example of a perſon learning muſic; and when we recollect the variety of me⯑chanic arts, which are performed by aſſociated trains of muſcular ac⯑tions catenated with the effects they produce, as in knitting, netting, weaving; and the greater variety of aſſociated trains of ideas cauſed or catenated by volitions or ſenſations, as in our hourly modes of rea⯑ſoning, or imagining, or recollecting, we ſhall gain ſome idea of the innumerable catenated trains and circles of action, which form the tenor of our lives, and which began, and will only ceaſe entirely with them.

2. When a young lady begins to learn muſic, ſhe voluntarily ap⯑plies herſelf to the characters of her muſic-book, and by many repe⯑titions endeavours to catenate them with the proportions of ſound, of which they are ſymbols. The ideas excited by the muſical charac⯑ters are ſlowly connected with the keys of the harpſichord, and much effort is neceſſary to produce every note with the proper finger, and in its due place and time; till at length a train of voluntary exertions becomes catenated with certain irritations. As the various notes by frequent repetitions become connected in the order, in which they are produced, a new catenation of ſenſitive exertions becomes mixed with the voluntary ones above deſcribed; and not only the muſical ſymbols of crotchets and quavers, but the auditory notes and tones at the ſame time, become ſo many ſucceſſive or ſynchronous links in thiſ circle of catenated actions.

At length the motions of her ſingers become catenated with the muſical characters; and theſe no ſooner ſtrike the eye, than the finger preſſes down the key without any voluntary attention between them; the activity of the hand being connected with the irritation of the [191] figure or place of the muſical ſymbol on the retina; till at length by frequent repetitions of the ſame tune the movements of her fingers in playing, and the muſcles of the larynx in ſinging, become aſſociated with each other, and form part of thoſe intricate trains and circles of catenated motions, according with the ſecond article of the preceding propoſitions in No. 1. of this Section.

3. Beſides the facility, which by habit attends the execution of this muſical performance, a curious circumſtance occurs, which is, that when our young muſician has began a tune, ſhe finds herſelf in⯑clined to continue it; and that even when ſhe is careleſsly ſinging alone without attending to her own ſong; according with the third preceding article.

4. At the ſame time that our young performer continues to play with great exactneſs this accuſtomed tune, ſhe can bend her mind, and that intenſely, on ſome other object, according with the fourth article of the preceding propoſitions.

The manuſcript copy of this work was lent to many of my friends at different times for the purpoſe of gaining their opinions and criti⯑ciſms on many parts of it, and I found the following anecdote writ⯑ten with a pencil oppoſite to this page, but am not certain by whom. "I remember ſeeing the pretty young actreſs, who ſucceeded Mrs. Arne in the performance of the celebrated Padlock, rehearſe the mu⯑ſical parts at her harpſichord under the eye of her maſter with great taſte and accuracy; though I obſerved her countenance full of emo⯑tion, which I could not account for; at laſt ſhe ſuddenly burſt into tears; for ſhe had all this time been eyeing a beloved canary bird, ſuffering great agonies, which at that inſtant fell dead from its perch."

5. At the ſame time many other catenated circles of action are going on in the perſon of our fair muſician, as well as the motions of her fingers, ſuch as the vital motions, reſpiration, the movements of [192] her eyes and eyelids, and of the intricate muſcles of vocality, accord⯑ing with the fifth preceding article.

6. If by any ſtrong impreſſion on the mind of our fair muſician ſhe ſhould be interrupted for a very inconſiderable time, ſhe can ſtill con⯑tinue her performance, according to the ſixth article.

7. If however this interruption be greater, though the chain of ac⯑tions be not diſſevered, it proceeds confuſedly, and our young per⯑former continues indeed to play, but in a hurry without accuracy and elegance, till ſhe begins the tune again, according to the ſeventh of the preceding articles.

8. But if this interruption be ſtill greater, the circle of actions be⯑comes entirely diſſevered, and ſhe finds herſelf immediately under the neceſſity to begin over again to recover the loſt catenation, according to the eighth preceding article.

9. Or in trying to recover it ſhe will ſing ſome diſſonant notes, or ſtrike ſome improper keys, according to the ninth preceding article.

10. A very remarkable thing attends this breach of catenation, if the performer has forgotten ſome word of her ſong, the more energy of mind ſhe uſes about it, the more diſtant is ſhe from regaining it; and artfully employs her mind in part on ſome other object, or endea⯑vours to dull its perceptions, continuing to repeat, as it were inconſci⯑ouſly, the former part of the ſong, that ſhe remembers, in hopes to regain the loſt connexion.

For if the activity of the mind itſelf be more energetic, or takes its attention more, than the connecting word, which is wanted; it will not perceive the ſlighter link of this loſt word; as who liſtens to a feeble ſound, muſt be very ſilent and motionleſs; ſo that in this caſe the very vigour of the mind itſelf ſeems to prevent it from regaining the loſt catenation, as well as the too great exertion in endeavouring to regain it, according to the tenth preceding article.

[193] We frequently experience, when we are doubtful about the ſpelling of a word, that the greater voluntary exertion we uſe, that is the more intenſely we think about it, the further are we from regaining the loſt aſſociation between the letters of it, but which readily recurs when we have become careleſs about it. In the ſame manner, after having for an hour laboured to recollect the name of ſome abſent per⯑ſon, it ſhall ſeem, particularly after ſleep, to come into the mind as it were ſpontaneouſly; that is, the word we are in ſearch of, was joined to the preceding one by aſſociation; this aſſociation being diſſevered, we endeavour to recover it by volition; this very action of the mind ſtrikes our attention more, than the faint link of aſſociation, and we find it impoſſible by this means to retrieve the loſt word. After ſleep, when volition is entirely ſuſpended, the mind becomes capable of per⯑ceiving the fainter link of aſſociation, and the word is regained.

On this circumſtance depends the impediment of ſpeech before mentioned; the firſt ſyllable of a word is cauſable by volition, but the remainder of it is in common converſation introduced by its aſſocia⯑tions with this firſt ſyllable acquired by long habit. Hence when the mind of the ſtammerer is vehemently employed on ſome idea of am⯑bition of ſhining, or fear of not ſucceeding, the aſſociations of the mo⯑tions of the muſcles of articulation with each other become diſſevered by this greater exertion, and he endeavours in vain by voluntary ef⯑forts to rejoin the broken aſſociation. For this purpoſe he continues to repeat the firſt ſyllable, which is cauſable by volition, and ſtrives in vain, by various diſtortions of countenance, to produce the next links, which are ſubject to aſſociation. See Claſs IV. 3. 1. 1.

11. After our accompliſhed muſician has acquired great variety of tunes and ſongs, ſo that ſome of them begin to ceaſe to be eaſily re⯑collected, ſhe finds progreſſive trains of muſical notes more frequently forgotten, than thoſe which are compoſed of reiterated circles, ac⯑cording with the eleventh preceding article.

[194] 12. To finiſh our example with the preceding articles we muſt at length ſuppoſe, that our fair performer falls aſleep over her harpſi⯑chord; and thus by the ſuſpenſion of volition, and the excluſion of ex⯑ternal ſtimuli, ſhe diſſevers the trains and circles of her muſical ex⯑ertions.

III. 1. Many of theſe circumſtances of catenations of motions re⯑ceive an eaſy explanation from the four following conſequences to the ſeventh law of animal cauſation in Sect. IV. Theſe are, firſt, that thoſe ſucceſſions or combinations of animal motions, whether they were united by cauſation, aſſociation, or catenation, which have been moſt frequently repeated, acquire the ſtrongeſt connection. Secondly, that of theſe, thoſe, which have been leſs frequently mixed with other trains or tribes of motion, have the ſtrongeſt connection. Thirdly, that of theſe, thoſe, which were firſt formed, have the ſtrongeſt connection. Fourthly, that if an animal motion be excited by more than one cauſation, aſſociation, or catenation, at the ſame time, it will be performed with greater energy.

2. Hence alſo we underſtand, why the catenations of irritative mo⯑tions are more ſtrongly connected than thoſe of the other claſſes, where the quantity of unmixed repetition has been equal; becauſe they were firſt formed. Such are thoſe of the ſecerning and abſorbent ſyſtems of veſſels, where the action of the gland produces a fluid, which ſtimulates the mouths of its correſpondent abſorbents. The aſſociated motions ſeem to be the next moſt ſtrongly united, from their frequent repetition; and where both theſe circumſtances unite, as in the vital motions, their catenations are indiſſoluble but by the deſtruction of the animal.

3. Where a new link has been introduced into a circle of actions by ſome accidental defect of ſtimulus; if that defect of ſtimulus be repeated at the ſame part of the circle a ſecond or a third time, the defective motions thus produced, both by the repeated defect of ſti⯑mulus [195] and by their catenation with the parts of the circle of actions, will be performed with leſs and leſs energy. Thus if any perſon is expoſed to cold at a certain hour to-day, ſo long as to render ſome part of the ſyſtem for a time torpid; and is again expoſed to it at the ſame hour to-morrow, and the next day; he will be more and more affected by it, till at length a cold fit of fever is completely formed, as happens at the beginning of many of thoſe fevers, which are called nervous or low fevers. Where the patient has ſlight periodical ſhiver⯑ings and paleneſs for many days before the febrile paroxiſm is com⯑pletely formed.

4. On the contrary, if the expoſure to cold be for ſo ſhort a time, as not to induce any conſiderable degree of torpor or quieſcence, and is repeated daily as above mentioned, it loſes its effect more and more at every repetition, till the conſtitution can bear it without inconveni⯑ence, or indeed without being conſcious of it. As in walking into the cold air in forſty weather. The ſame rule is applicable to increaſed ſtimulus, as of heat, or of vinous ſpirit, within certain limits, as is applied in the two laſt paragraphs to Deficient Stimulus, as is further explained in Sect. XXXVI. on the Periods of Diſeaſes.

5. Where irritation coincides with ſenſation to produce the ſame catenations of motion, as in inflammatory fevers, they are excited with ſtill greater energy than by the irritation alone. So when chil⯑dren expect to be tickled in play, by a feather lightly paſſed over the lips, or by gently vellicating the ſoles of their feet, laughter is moſt vehemently excited; though they can ſtimulate theſe parts with their own fingers unmoved. Here the pleaſureable idea of playfulneſs co⯑incides with the vellication; and there is no voluntary exertion uſed to diminiſh the ſenſation, as there would be, if a child ſhould endea⯑vour to tickle himſelf. See Sect. XXXIV. 1. 4.

6. And laſtly, the motions excited by the junction of voluntary exertion with irritation are performed with more energy, than thoſe [196] by irritation ſingly; as when we liſten to ſmall noiſes, as to the tick⯑ing of a watch in the night, we perceive the moſt weak ſounds, that are at other times unheeded. So when we attend to the irritative ideas of ſound in our ears, which are generally not attended to, we can hear them; and can ſee the ſpectra of objects, which remain in the eye, whenever we pleaſe to exert our voluntary power in aid of thoſe weak actions of the retina, or of the auditory nerve.

7. The temporary catenations of ideas, which are cauſed by the ſenſations of pleaſure or pain, are eaſily diſſevered either by irritations, as when a ſudden noiſe diſturbs a day-dream; or by the power of vo⯑lition, as when we awake from ſleep. Hence in our waking hours, whenever an idea occurs, which is incongruous to our former expe⯑rience, we inſtantly diſſever the train of imagination by the power of volition, and compare the incongruous idea with our previous knowledge of nature, and reject it. This operation of the mind has not yet acquired a ſpecific name, though it is exerted every minute of our waking hours; unleſs it may be termed INTUITIVE ANA⯑LOGY. It is an act of reaſoning of which we are unconſcious except from its effects in preſerving the congruity of our ideas, and bears the ſame relation to the ſenſorial power of volition, that irritative ideas, of which we are inconſcious except by their effects, do to the ſenſo⯑rial power of irritation; as the former is produced by volition without our attention to it, and the latter by irritation without our attention to them.

If on the other hand a train of imagination or of voluntary ideas are excited with great energy, and paſſing on with great vivacity, and be⯑come diſſevered by ſome violent ſtimulus, as the diſcharge of a piſtol near one's ear, another circumſtance takes place, which is termed SURPRISE; which by exciting violent irritation, and violent ſenſation, employs for a time the whole ſenſorial energy, and thus diſſevers the paſſing trains of ideas, before the power of volition has time to com⯑pare [197] them with the uſual phenomena of nature. In this caſe fear is generally the companion of ſurpriſe, and adds to our embarraſſment, as every one experiences in ſome degree when he hears a noiſe in the dark, which he cannot inſtantly account for. This catenation of fear with ſurpriſe is owing to our perpetual experience of injuries from external bodies in motion, unleſs we are upon our guard againſt them. See Sect. XVIII. 17. and XIX. 2.

Many other examples of the catenations of animal motions are ex⯑plained in Sect. XXXVI. on the Periods of Diſeaſes.

I. 1. IN the hemiplagia, when the limbs on one ſide have loſt their power of voluntary motion, the patient is for many days per⯑petually employed in moving thoſe of the other. 2. When the vo⯑luntary power is ſuſpended during ſleep, there commences a ceaſeleſs flow of ſenſitive motions, or ideas of imagination, which compoſe our dreams. 3. When in the cold fit of an intermittent fever ſome parts of the ſyſtem have for a time continued torpid, and have thus ex⯑pended leſs than their uſual expenditure of ſenſorial power; a hot fit ſucceeds, with violent action of thoſe veſſels, which had previouſly been quieſcent. All theſe are explained from an accumulation of ſenſorial power during the inactivity of ſome part of the ſyſtem.

Beſides the very great quantity of ſenſorial power perpetually pro⯑duced and expended in moving the arterial, venous, and glandular [250] ſyſtems, with the various organs of digeſtion, as deſcribed in Section XXXII. 3. 2. there is alſo a conſtant expenditure of it by the ac⯑tion of our locomotive muſcles and organs of ſenſe. Thus the thick⯑neſs of the optic nerves, where they enter the eye, and the great ex⯑panſion of the nerves of touch beneath the whole of the cuticle, evince the great conſumption of ſenſorial power by theſe ſenſes. And our perpetual muſcular actions in the common offices of life, and in conſtantly preſerving the perpendicularity of our bodies during the day, evince a conſiderable expenditure of the ſpirit of animation by our locomotive muſcles. It follows, that if the exertion of theſe organs of ſenſe and muſcles be for a while intermitted, that ſome quantity of ſenſorial power muſt be accumulated, and a propenſity to activity of ſome kind enſue from the increaſed excitability of the ſyſtem. Whence proceeds the irkſomeneſs of a continued attitude, and of an indolent life.

However ſmall this hourly accumulation of the ſpirit of animation may be, it produces a propenſity to ſome kind of action; but it never⯑theleſs requires either deſire or averſion, either pleaſure or pain, or ſome external ſtimulus, or a previous link of aſſociation, to excite the ſyſtem into activity; thus it frequently happens, when the mind and body are ſo unemployed as not to poſſeſs any of the three firſt kinds of ſtimuli, that the laſt takes place, and conſumes the ſmall but per⯑petual accumulation of ſenſorial power. Whence ſome indolent peo⯑ple repeat the ſame verſe for hours together, or hum the ſame tune. Thus the poet:

II. The repetitions of motions may be at firſt produced either by volition, or by ſenſation, or by irritation, but they ſoon become eaſier to perform than any other kinds of action, becauſe they ſoon become aſſociated together, according to Law the ſeventh, Section IV. on [251] Animal Cauſation. And becauſe their frequency of repetition, if as much ſenſorial power be produced during every reiteration as is ex⯑pended, adds to the facility of their production.

If a ſtimulus be repeated at uniform intervals of time, as deſcribed in Sect. XII. 3. 3. the action, whether of our muſcles or organs of ſenſe, is produced with ſtill greater facility or energy; becauſe the ſenſorial power of aſſociation, mentioned above, is combined with the ſenſorial power of irritation; that is, in common language, the ac⯑quired habit aſſiſts the power of the ſtimulus.

This not only obtains in the annual, lunar, and diurnal catenations of animal motions, as explained in Sect. XXXVI. which are thus performed with great facility and energy; but in every leſs circle of actions or ideas, as in the burthen of a ſong, or the reiterations of a dance. To the facility and diſtinctneſs, with which we hear ſounds at repeated intervals, we owe the pleaſure, which we receive from muſical time, and from poetic time; as deſcribed in Botanic Garden, P. 2. Interlude 3. And to this the pleaſure we receive from the rhimes and alliterations of modern verſification; the ſource of which without this key would be difficult to diſcover. And to this likewiſe ſhould be aſcribed the beauty of the duplicature in the perfect tenſe of the Greek verbs, and of ſome Latin ones, as tango tetegi, mordeo momordi.

There is no variety of notes referable to the gamut in the beating of the drum, yet if it be performed in muſical time, it is agreeable to our ears; and therefore this pleaſureable ſenſation muſt be owing to the repetition of the diviſions of the ſounds at certain intervals of time, or muſical bars. Whether theſe times or bars are diſtinguiſhed by a pauſe, or by an emphaſis, or accent, certain it is, that this diſtinction is perpetually repeated; otherwiſe the ear could not determine inſtant⯑ly, whether the ſucceſſions of ſound were in common or in triple time. In common time there is a diviſion between every two crotchets, or [252] other notes of equivalent time; though the bar in written muſic is put after every fourth crotchet, or notes equivalent in time; in triple time the diviſion or bar is after every three crotchets, or notes equi⯑valent; ſo that in common time the repetition recurs more frequently than in triple time. The grave or heroic verſes of the Greek and Latin poets are written in common time; the French heroic verſes, and Mr. Anſtie's humorous verſes in his bath guide, are written in the ſame time as the Greek and Latin verſes, but are one bar ſhorter. The Engliſh grave or heroic verſes are meaſured by triple time, as Mr. Pope's tranſlation of Homer.

But beſides theſe little circles of muſical time, there are the greater returning periods, and the ſtill more diſtant choruſes, which, like the rhimes at the ends of verſes, owe their beauty to repetition; that is, to the facility and diſtinctneſs with which we perceive ſounds, which we expect to perceive, or have perceived before; or in the language of this work, to the greater eaſe and energy with which our organ is excited by the combined ſenſorial powers of aſſociation and irritation, than by the latter ſingly.

A certain uniformity or repetition of parts enters the very compo⯑ſition of harmony. Thus two octaves neareſt to each other in the ſcale commence their vibrations together after every ſecond vibration of the higher one. And where the firſt, third, and fifth compoſe a chord the vibrations concur or coincide frequently, though leſs ſo than in the two octaves. It is probable that theſe chords bear ſome analogy to a mixture of three alternate colours in the ſun's ſpectrum ſeparated by a priſm.

The pleaſure we receive from a melodious ſucceſſion of notes refer⯑able to the gamut is derived from another ſource, viz. to the pandi⯑culation or counteraction of antagoniſt fibres. See Botanic Garden, P. 2. Interlude 3. If to theſe be added our early aſſociations of agree⯑able ideas with certain proportions of ſound, I ſuppoſe, from theſe [253] three ſources ſprings all the delight of muſic, ſo celebrated by ancient authors, and ſo enthuſiaſtically cultivated at preſent. See Sect. XVI. No. 10. on Inſtinct.

This kind of pleaſure ariſing from repetition, that is from the faci⯑lity and diſtinctneſs, with which we perceive and underſtand repeated ſenſations, enters into all the agreeable arts; and when it is carried to exceſs is termed formality. The art of dancing like that of muſic de⯑pends for a great part of the pleaſure, it affords, on repetition; ar⯑chitecture, eſpecially the Grecian, conſiſts of one part being a repe⯑tition of another; and hence the beauty of the pyramidal outline in landſcape-painting; where one ſide of the picture may be ſaid in ſome meaſure to balance the other. So univerſally does repetition contribute to our pleaſure in the fine arts, that beauty itſelf has been defined by ſome writers to conſiſt in a due combination of uniformity and variety. See Sect. XVI. 6.

III. 1. Man is termed by Ariſtotle an imitative animal; this pro⯑penſity to imitation not only appears in the actions of children, but in all the cuſtoms and faſhions of the world; many thouſands tread in the beaten paths of others, for one who traverſes regions of his own diſcovery. The origin of this propenſity to imitation has not, that I recollect, been deduced from any known principle; when any action preſents itſelf to the view of a child, as of whetting a knife, or thread⯑ing a needle, the parts of this action in reſpect of time, motion, fi⯑gure, is imitated by a part of the retina of his eye; to perform this action therefore with his hands is eaſier to him than to invent any new action, becauſe it conſiſts in repeating with another ſet of fibres, viz. with the moving muſcles, what he had juſt performed by ſome parts of the retina; juſt as in dancing we transfer the times of mo⯑tion from the actions of the auditory nerves to the muſcles of the limbs. Imitation therefore conſiſts of repetition, which we have ſhewn above to be the eaſieſt kind of animal action, and which we [254] perpetually fall into, when we poſſeſs an accumulation of ſenſorial power, which is not otherwiſe called into exertion.

It has been ſhewn, that our ideas are configurations of the organs of ſenſe, produced originally in conſequence of the ſtimulus of exter⯑nal bodies. And that theſe ideas, or configurations of the organs of ſenſe, reſemble in ſome property a correſpondent property of external matter; as the parts of the ſenſes of ſight and of touch, which are excited into action, reſemble in figure the figure of the ſtimulating body; and probably alſo the colour, and the quantity of denſity, which they perceive. As explained in Sect. XIV. 2. 2. Hence it ap⯑pears, that our perceptions themſelves are copies, that is, imitations of ſome properties of external matter; and the propenſity to imitation is thus interwoven with our exiſtence, as it is produced by the ſtimuli of external bodies, and is afterwards repeated by our volitions and ſen⯑ſations, and thus conſtitutes all the operations of our minds.

2. Imitations reſolve themſelves into four kinds, voluntary, ſenſi⯑tive, irritative, and aſſociate. The voluntary imitations are, when we imitate deliberately the actions of others, either by mimicry, as in acting a play, or in delineating a flower; or in the common actions of our lives, as in our dreſs, cookery, language, manners, and even in our habits of thinking.

Not only the greateſt part of mankind learn all the common arts of life by imitating others, but brute animals ſeem capable of acquiring knowledge with greater facility by imitating each other, than by any methods by which we can teach them; as dogs and cats, when they are ſick, learn of each other to eat graſs; and I ſuppoſe, that by making an artificial dog perform certain tricks, as in dancing on his hinder legs, a living dog might be eaſily induced to imitate them; and that the readieſt way of inſtructing dumb animals is by practiſing them with others of the ſame ſpecies, which have already learned the arts we wiſh to teach them. The important uſe of imitation in ac⯑quiring [255] natural language is mentioned in Section XVI. 7. and 8. on Inſtinct.

3. The ſenſitive imitations are the immediate conſequences of plea⯑ſure or pain, and theſe are often produced even contrary to the efforts of the will. Thus many young men on ſeeing cruel ſurgical opera⯑tions become ſick, and ſome even feel pain in the parts of their own bodies, which they ſee tortured or wounded in others; that is, they in ſome meaſure imitate by the exertions of their own fibres the vio⯑lent actions, which they witneſſed in thoſe of others. In this caſe a double imitation takes place, firſt the obſerver imitates with the ex⯑tremities of the optic nerve the mangled limbs, which are preſent be⯑fore his eyes; then by a ſecond imitation he excites ſo violent action of the fibres of his own limbs as to produce pain in thoſe parts of his own body, which he ſaw wounded in another. In theſe pains pro⯑duced by imitation the effect has ſome ſimilarity to the cauſe, which diſtinguiſhes them from thoſe produced by aſſociation; as the pains of the teeth, called tooth-edge, which are produced by aſſociation with diſagreeable ſounds, as explained in Sect. XVI. 10.

The effect of this powerful agent, imitation, in the moral world, is mentioned in Sect. XVI. 7. as it is the foundation of all our intel⯑lectual ſympathies with the pains and pleaſures of others, and is in conſequence the ſource of all our virtues. For in what conſiſts our ſympathy with the miſeries, or with the joys, of our fellow creatures, but in an involuntary excitation of ideas in ſome meaſure ſimilar or imitative of thoſe, which we believe to exiſt in the minds of the per⯑ſons, whom we commiſerate or congratulate?

There are certain concurrent or ſucceſſive actions of ſome of the glands, or other parts of the body, which are poſſeſſed of ſenſation, which become intelligible from this propenſity to imitation. Of theſe are the production of matter by the membranes of the fauces, or by the ſkin, in conſequence of the venereal diſeaſe previouſly affecting [256] the parts of generation. Since as no fever is excited, and as neither the blood of ſuch patients, nor even the matter from ulcers of the throat, or from cutaneous ulcers, will by inoculation produce the ve⯑nereal diſeaſe in others, as obſerved by Mr. Hunter, there is reaſon to conclude, that no contagious matter is conveyed thither by the blood-veſſels, but that a milder matter is formed by the actions of the fine veſſels in thoſe membranes imitating each other. See Section XXXIII. 2. 9. In this diſeaſe the actions of theſe veſſels producing ulcers on the throat and ſkin are imperfect imitations of thoſe pro⯑ducing chanker, or gonorrhoea; ſince the matter produced by them is not infectious, while the imitative actions in the hydrophobia ap⯑pear to be perfect reſemblances, as they produce a material equally in⯑fectious with the original one, which induced them.

The contagion from the bite of a mad dog differs from other con⯑tagious materials, from its being communicable from other animals to mankind, and from many animals to each other; the phenomena at⯑tending the hydrophobia are in ſome degree explicable on the fore⯑going theory. The infectious matter does not appear to enter the circulation, as it cannot be traced along the courſe of the lymphatics from the wound, nor is there any ſwelling of the lymphatic glands, nor does any fever attend, as occurs in the ſmall-pox, and in many other contagious diſeaſes; yet by ſome unknown proceſs the diſeaſe is communicated from the wound to the throat, and that many months after the injury, ſo as to produce pain and hydrophobia, with a ſecre⯑tion of infectious ſaliva of the ſame kind, as that of the mad dog, which inflicted the wound.

This ſubject is very intricate.—It would appear, that by certain morbid actions of the ſalivary glands of the mad dog, a peculiar kind of ſaliva is produced; which being inſtilled into a wound of another animal ſtimulates the cutaneous or mucous glands into morbid actions, but which are ineffectual in reſpect to the production of a ſimilar con⯑tagious [257] material; but the ſalivary glands by irritative ſympathy are thrown into ſimilar action, and produce an infectious ſaliva ſimilar to that inſtilled into the wound.

Though in many contagious fevers a material ſimilar to that which produced the diſeaſe, is thus generated by imitation; yet there are other infectious materials, which do not thus propagate themſelves, but which ſeem to act like ſlow poiſons. Of this kind was the con⯑tagious matter, which produced the jail-fever at the aſſizes at Oxford about a century ago. Which, though fatal to ſo many, was not communicated to their nurſes or attendants. In theſe caſes, the imi⯑tations of the fine veſſels, as above deſcribed, appear to be imperfect, and do not therefore produce a matter ſimilar to that, which ſtimulates them; in this circumſtance reſembling the venereal matter in ulcers of the throat or ſkin, according to the curious diſcovery of Mr. Hun⯑ter above related, who found, by repeated inoculations, that it would not infect. Hunter on Venereal Diſeaſe, Part vi. ch. 1.

Another example of morbid imitation is in the production of a great quantity of contagious matter, as in the inoculated ſmall-pox, from a ſmall quantity of it inſerted into the arm, and probably diffuſed in the blood. Theſe particles of contagious matter ſtimulate the extremities of the fine arteries of the ſkin, and cauſe them to imitate ſome pro⯑perties of thoſe particles of contagious matter, ſo as to produce a thou⯑ſandfold of a ſimilar material. See Sect. XXXIII. 2. 6. Other in⯑ſtances are mentioned in the Section on Generation, which ſhew the probability that the extremities of the ſeminal glands may imitate cer⯑tain ideas of the mind, or actions of the organs of ſenſe, and thus oc⯑caſion the male or female ſex of the embryon. See Sect. XXXIX. 6.

4. We come now to thoſe imitations, which are not attended with ſenſation. Of theſe are all the irritative ideas already explained, as when the retina of the eye imitates by its action or configuration the tree or the bench, which I ſhun in walking paſt without attending to them. Other examples of theſe irritative imitations are daily ob⯑ſervable [258] in common life: thus one yawning perſon ſhall ſet a whole company a yawning; and ſome have acquired winking of the eyes or impediments of ſpeech by imitating their companions without being conſcious of it.

5. Beſides the three ſpecies of imitations above deſcribed there may be ſome aſſociate motions, which may imitate each other in the kind as well as in the quantity of their action; but it is difficult to diſ⯑tinguiſh them from the aſſociations of motions treated of in Section XXXV. Where the actions of other perſons are imitated there can be no doubt, or where we imitate a preconceived idea by exertion of our locomotive muſcles, as in painting a dragon; all theſe imitations may aptly be referred to the ſources above deſcribed of the propenſity to activity, and the facility of repetition; at the ſame time I do not af⯑firm, that all thoſe other apparent ſenſitive and irritative imitations may not be reſolvable into aſſociations of a peculiar kind, in which certain diſtant parts of ſimilar irritability or ſenſibility, and which have habitually acted together, may affect each other exactly with the ſame kinds of motion; as many parts are known to ſympathiſe in the quan⯑tity of their motions. And that therefore they may be ultimately re⯑ſolvable into aſſociations of action, as deſcribed in Sect. XXXV.

I. WE now ſtep forwards to illuſtrate ſome of the phaenomena of diſeaſes, and to trace out their moſt efficacious methods of cure; and ſhall commence this ſubject with a ſhort deſcription of the circulatory ſyſtem.

As the nerves, whoſe extremities form our various organs of ſenſe and muſcles, are all joined, or communicate, by means of the brain, for the convenience perhaps of the diſtribution of a ſubtile ethereal fluid for the purpoſe of motion; ſo all thoſe veſſels of the body, which carry the groſſer fluids for the purpoſes of nutrition, communi⯑cate with each other by the heart.

The heart and arteries are hollow muſcles, and are therefore in⯑dued with power of contraction in conſequence of ſtimulus, like all other muſcular ſibres; but, as they have no antagoniſt muſcles, the cavities of the veſſels, which they form, would remain for ever [260] cloſed, after they have contracted themſelves, unleſs ſome extraneous power be applied to again diſtend them. This extraneous power in reſpect to the heart is the current of blood, which is perpetually ab⯑ſorbed by the veins from the various glands and capillaries, and puſhed into the heart by a power probably very ſimilar to that, which raiſes the ſap in vegetables in the ſpring, which, according to Dr. Hale's experiment on the ſtump of a vine, exerted a force equal to a column of water above twenty feet high. This force of the current of blood in the veins is partly produced by their abſorbent power, ex⯑erted at the beginning of every fine ramification; which may be con⯑ceived to be a mouth abſorbing blood, as the mouths of the lacteals and lymphatics abſorb chyle and lymph. And partly by their inter⯑mitted compreſſion by the pulſations of their generally concomitant ar⯑teries; by which the blood is perpetually propelled towards the heart, as the valves in many veins, and the abſorbent mouths in them all, will not ſuffer it to return.

The blood, thus forcibly injected into the chambers of the heart, diſtends this combination of hollow muſcles; till by the ſtimulus of diſtention they contract themſelves; and, puſhing forwards the blood into the arteries, exert ſufficient force to overcome in leſs than a ſecond of time the vis inertiae, and perhaps ſome elaſticity, of the very extenſive ramifications of the two great ſyſtems of the aortal and pul⯑monary arteries. The power neceſſary to do this in ſo ſhort a time muſt be conſiderable, and has been variouſly eſtimated by different phyſiologiſts.

The muſcular coats of the arterial ſyſtem are then brought into ac⯑tion by the ſtimulus of diſtention, and propel the blood to the mouths, or through the convolutions, which precede the ſecretory apertures of the various glands and capillaries.

In the veſſels of the liver there is no intervention of the heart; but the vena portarum, which does the office of an artery, is diſtended by the blood poured into it from the meſenteric veins, and is by this [261] diſtention ſtimulated to contract itſelf, and propel the blood to the mouths of the numerous glands, which compoſe that viſcus.

II. The glandular ſyſtem of veſſels may be divided into thoſe, which take ſome fluid from the circulation; and thoſe, which give ſomething to it. Thoſe, which take their fluid from the circulation, are the various glands, by which the tears, bile, urine, perſpiration, and many other ſecretions are produced; theſe glands probably conſiſt of a mouth to ſelect, a belly to digeſt, and an excretory aperture to emit their appropriated fluids; the blood is conveyed by the power of the heart and arteries to the mouths of theſe glands, it is there taken up by the living power of the gland, and carried forwards to its belly, and excretory aperture, where a part is ſeparated, and the remainder abſorbed by the veins for further purpoſes.

Some of theſe glands are furniſhed with long convoluted necks or tubes, as the ſeminal ones, which are curiouſly ſeen when injected with quickſilver. Others ſeem to conſiſt of ſhorter tubes, as that great congeries of glands, which conſtitute the liver, and thoſe of the kidneys. Some have their excretory apertures opening into reſervoirs, as the urinary and gall-bladders. And others on the external body, as thoſe which ſecret the tears, and perſpirable matter.

Another great ſyſtem of glands, which have very ſhort necks, are the capillary veſſels; by which the inſenſible perſpiration is ſecreted on the ſkin; and the mucus of various conſiſtences, which lubricates the interſtices of the cellular membrane, of the muſcular fibres, and of all the larger cavities of the body. From the want of a long con⯑volution of veſſels ſome have doubted, whether theſe capillaries ſhould be conſidered as glands, and have been led to conclude, that the per⯑ſpirable matter rather exuded than was ſecreted. But the fluid of perſpiration is not ſimple water, though that part of it which exhales into the air may be ſuch; for there is another part of it, which in a ſtate of health is abſorbed again; but which, when the abſorbents are [262] diſeaſed, remains on the ſurface of the ſkin, in the form of ſcurf, or indurated mucus. Another thing, which ſhews their ſimilitude to other glands, is their ſenſibility to certain affections of the mind; as is ſeen in the deeper colour of the ſkin in the bluſh of ſhame, or the greater paleneſs of it from fear.

III. Another ſeries of glandular veſſels is called the abſorbent ſyſtem; theſe open their mouths into all the cavities, and upon all thoſe ſurfaces of the body, where the excretory apertures of the other glands pour out their fluids. The mouths of the abſorbent ſyſtem drink up a part or the whole of theſe fluids, and carry them forwards by their living power to their reſpective glands, which are called conglobate glands. There theſe fluids undergo ſome change, before they paſs on into the circulation; but if they are very acrid, the conglobate gland ſwells, and ſometimes ſuppurates, as in inocu⯑lation of the ſmall-pox, in the plague, and in venereal abſorptions; at other times the fluid may perhaps continue there, till it undergoes ſome chemical change, that renders it leſs noxious; or, what is more likely, till it is regurgitated by the retrograde motion of the gland in ſpontaneous ſweats or diarrhoeas, as diſagreeing food is vomited from the ſtomach.

IV. As all the fluids, that paſs through theſe glands, and capillary veſſels, undergo a chemical change, acquiring new combinations, the matter of heat is at the ſame time given out; this is apparent, ſince whatever increaſes inſenſible perſpiration, increaſes the heat of the ſkin; and when the action of theſe veſſels is much increaſed but for a moment, as in bluſhing, a vivid heat on the ſkin is the immedi⯑ate conſequence. So when great bilious ſecretions, or thoſe of any other gland, are produced, heat is generated in the part in proportion to the quantity of the ſecretion.

The heat produced on the ſkin by bluſhing may be thought by ſome too ſudden to be pronounced a chemical effect, as the fermenta⯑tions [263] or new combinations taking place in a fluid is in general a ſlower proceſs. Yet are there many chemical mixtures in which heat is given out as inſtantaneouſly; as in ſolutions of metals in acids, or in mixtures of eſſential oils and acids, as of oil of cloves and acid of nitre. So the bruiſed parts of an unripe apple become almoſt inſtan⯑taneouſly ſweet; and if the chemico-animal proceſs of digeſtion be ſtopped for but a moment, as by fear, or even by voluntary eructa⯑tion, a great quantity of air is generated, by the fermentation, which inſtantly ſucceeds the ſtop of digeſtion. By the experiments of Dr. Hales it appears, that an apple during fermentation gave up above ſix hundred times its bulk of air; and the materials in the ſtomach are ſuch, and in ſuch a ſituation, as immediately to run into fermenta⯑tion, when digeſtion is impeded.

As the blood paſſes through the ſmall veſſels of the lungs, which connect the pulmonary artery and vein, it undergoes a change of co⯑lour from a dark to a light red; which may be termed a chemical change, as it is known to be effected by an admixture of oxygene, or vital air; which, according to a diſcovery of Dr. Prieſtley, paſſes through the moiſt membranes, which conſtitute the ſides of theſe veſſels. As the blood paſſes through the capillary veſſels, and glands, which connect the aorta and its various branches with their corre⯑ſpondent veins in the extremities of the body, it again loſes the bright red colour, and undergoes ſome new combinations in the glands or capillaries, in which the matter of heat is given out from the ſecreted fluids. This proceſs therefore, as well as the proceſs of reſpiration, has ſome analogy to combuſtion, as the vital air or oxygene ſeems to become united to ſome inflammable baſe, and the matter of heat eſcapes from the new acid, which is thus produced.

V. After the blood has paſſed theſe glands and capillaries, and parted with whatever they choſe to take from it, the remainder is re⯑ceived by the veins, which are a ſet of blood-abſorbing veſſels in ge⯑neral [264] correſponding with the remifications of the arterial ſyſtem. At the extremity of the fine convolutions of the glands the arterial force ceaſes; this in reſpect to the capillary veſſels, which unite the extre⯑mities of the arteries with the commencement of the veins, is evident to the eye, on viewing the tail of a tadpole by means of a ſolar, or even by a common microſcope, for globules of blood are ſeen to en⯑deavour to paſs, and to return again and again, before they become abſorbed by the mouths of the veins; which returning of theſe glo⯑bules evinces, that the arterial force behind them has ceaſed. The veins are furniſhed with valves like the lymphatic abſorbents; and the great trunks of the veins, and of the lacteals and lymphatic, join together before the ingreſs of their fluids into the left chamber of the heart; both which evince, that the blood in the veins, and the lymph and chyle in the lacteals and lymphatics, are carried on by a ſimilar force; otherwiſe the ſtream, which was propelled with a leſs power, could not enter the veſſels, which contained the ſtream propelled with a greater power. From whence it appears, that the veins are a ſyſtem of veſſels abſorbing blood, as the lacteals and lymphatics are a ſyſtem of veſſels abſorbing chyle and lymph. See Sect. XXVII. 1.

VI. The movements of their adapted fluids in the various veſſels of the body are carried forwards by the actions of thoſe veſſels in conſe⯑quence of two kinds of ſtimulus, one of which may be compared to a pleaſureable ſenſation or deſire inducing the veſſel to ſeize, and, as it were, to ſwallow the particles thus ſelected from the blood; as is done by the mouths of the various glands, veins, and other abſorbents, which may be called glandular appetency. The other kind of ſtimu⯑lus may be compared to diſagreeable ſenſation, or averſion, as when the heart has received the blood, and is ſtimulated by it to puſh it for⯑wards into the arteries; the ſame again ſtimulates the arteries to con⯑tract, and carry forwards the blood to their extremities, the glands and capillaries. Thus the meſenteric veins abſorb the blood from the [265] inteſtines by glandular appetency, and carry it forward to the vena por⯑tarum; which acting as an artery contracts itſelf by diſagreeable ſti⯑mulus, and puſhes it to its ramified extremities, the various glands, which conſtitute the liver.

It ſeems probable, that at the beginning of the formation of theſe veſſels in the embryon, an agreeable ſenſation was in reality felt by the glands during ſecretion, as is now felt in the act of ſwallowing palatable food; and that a diſagreeable ſenſation was originally felt by the heart from the diſtention occaſioned by the blood, or by its che⯑mical ſtimulus; but that by habit theſe are all become irritative mo⯑tions; that is, ſuch motions as do not affect the whole ſyſtem, except when the veſſels are diſeaſed by inflammation.

I. THE ſalival glands drink up a certain fluid from the circumflu⯑ent blood, and pour it into the mouth. They are ſometimes ſtimulated into action by the blood, that ſurrounds their origin, or by ſome part of that heterogeneous fluid: for when mercurial ſalts, or oxydes, are mixed with the blood, they ſtimulate theſe glands into unnatural ex⯑ertions; and then an unuſual quantity of ſaliva is ſeparated.

As the ſaliva ſecreted by theſe glands is moſt wanted during the maſtication of our food, it happens, when the terminations of their ducts in the mouth are ſtimulated into action, the ſalival glands them⯑ſelves [267] are brought into increaſed action at the ſame time by aſſociation, and ſeparate a greater quantity of their juices from the blood; in the ſame manner as tears are produced in greater abundance during the ſtimulus of the vapour of onions, or of any other acrid material in the eye.

The ſaliva is thus naturally poured into the mouth only during the ſtimulus of our food in maſtication; for when there is too great an ex⯑halation of the mucilaginous ſecretion from the membranes, which line the mouth, or too great an abſorption of it, the mouth becomes dry, though there is no deficiency in the quantity of ſaliva; as in thoſe who ſleep with their mouths open, and in ſome fevers.

2. Though during the maſtication of our natural food the ſalival glands are excited into action by the ſtimulus on their excretory ducts, and a due quantity of ſaliva is ſeparated from the blood, and poured into the mouth; yet as this maſtication of our food is always attended with a degree of pleaſure; and that pleaſureable ſenſation is alſo con⯑nected with our ideas of certain kinds of aliment; it follows, that when theſe ideas are reproduced, the pleaſureable ſenſation ariſes along with them, and the ſalival glands are excited into action, and fill the mouth with ſaliva from this ſenſitive aſſociation, as is frequent⯑ly ſeen in dogs, who ſlaver at the ſight of food.

3. We have alſo a voluntary power over the action of theſe ſalival glands, for we can at any time produce a flow of ſaliva into our mouth, and ſpit out, or ſwallow it at will.

4. If any very acrid material be held in the mouth, as the root of pyrethrum, or the leaves of tobacco, the ſalival glands are ſtimulated into ſtronger action than is natural, and thence ſecrete a much larger quantity of ſaliva; which is at the ſame time more viſcid than in its natural ſtate; becauſe the lymphatics, that open their mouths into the ducts of the ſalival glands, and on the membranes, which line the mouth, are likewiſe ſtimulated into ſtronger action, and abſorb the [268] more liquid parts of the ſaliva with greater avidity; and the remainder is left both in greater quantity and more viſcid.

The increaſed abſorption in the mouth by ſome ſtimulating ſub⯑ſtances, which are called aſtringents, as crab juice, is evident from the inſtant dryneſs produced in the mouth by a ſmall quantity of them.

As the extremities of the glands are of exquiſite tenuity, as appears by their difficulty of injection, it was neceſſary for them to ſecrete their fluids in a very dilute ſtate; and, probably for the purpoſe of ſtimulating them into action, a quantity of neutral ſalt is likewiſe ſe⯑creted or formed by the gland. This aqueous and ſaline part of all ſecreted fluids is again reabſorbed into the habit. More than half of ſome ſecreted fluids is thus imbibed from the reſervoirs, into which they are poured; as in the urinary bladder much more than half of what is ſecreted by the kidneys becomes reabſorbed by the lymphatics, which are thickly diſperſed around the neck of the bladder. This ſeems to be the purpoſe of the urinary bladders of fiſh, as otherwiſe ſuch a receptacle for the urine could have been of no uſe to an animal immerſed in water.

5. The idea of ſubſtances diſagreeably acrid will alſo produce a quantity of ſaliva in the mouth; as when we ſmell very putrid va⯑pours, we are induced to ſpit out our ſaliva, as if ſomething diſagree⯑able was actually upon our palates.

6. When diſagreeable food in the ſtomach produces nauſea, a flow of ſaliva is excited in the mouth by aſſociation; as efforts to vomit are frequently produced by diſagreeable drugs in the mouth by the ſame kind of aſſociation.

7. A preternatural flow of ſaliva is likewiſe ſometimes occaſioned by a diſeaſe of the voluntary power; for if we think about our ſaliva, and determine not to ſwallow it, or not to ſpit it out, an exertion is produced by the will, and more ſaliva is ſeoreted againſt our wiſh; [269] that is, by our averſion, which bears the ſame analogy to deſire, as pain does to pleaſure; as they are only modifications of the ſame diſ⯑poſition of the ſenſorium. See Claſs IV. 3. 2. 1.

8. The quantity of ſaliva may alſo be increaſed beyond what is na⯑tural, by the catenation of the motions of theſe glands with other mo⯑tions, or ſenſations, as by an extraneous body in the ear; of which I have known an inſtance; or by the application of ſtizolobium, ſiliqua hirſuta, cowhage, to the ſeat of the parotis, as ſome writers have af⯑firmed.

II. 1. The lacrymal gland drinks up a certain fluid from the cir⯑cumfluent blood, and pours it on the ball of the eye, on the upper part of the external corner of the eyelids. Though it may perhaps be ſtimulated into the performance of its natural action by the blood, which ſurrounds its origin, or by ſome part of that heterogeneous fluid; yet as the tears ſecreted by this gland are more wanted at ſome times than at others, its ſecretion is variable, like that of the ſaliva above mentioned, and is chiefly produced when its excretory duct is ſtimulated; for in our common ſleep there ſeems to be little or no ſe⯑cretion of tears; though they are occaſionally produced by our ſenſa⯑tions in dreams.

Thus when any extraneous material on the eye-ball, or the dryneſs of the external covering of it, or the coldneſs of the air, or the acri⯑mony of ſome vapours, as of onions, ſtimulates the excretory duct of the lacrymal gland, it diſcharges its contents upon the ball; a quicker ſecretion takes place in the gland, and abundant tears ſucceed, to moiſten, clean, and lubricate the eye. Theſe by frequent nictitation are diffuſed over the whole ball, and as the external angle of the eye in winking is cloſed ſooner than the internal angle, the tears are gra⯑dually driven forwards, and downwards from the lacrymal gland to the puncta lacrymalia.

2. The lacrymal ſack, with its puncta lacrymalia, and its naſal duct, is a complete gland; and is ſingular in this reſpect, that it nei⯑ther [270] derives its fluid from, nor diſgorges it into the circulation. The ſimplicity of the ſtructure of this gland, and both the extremities of it being on the ſurface of the body, makes it well worthy our minuter obſervation; as the actions of more intricate and concealed glands may be better underſtood from their analogy to this.

3. This ſimple gland conſiſts of two abſorbing mouths, a belly, and an excretory duct. As the tears are brought to the internal angle of the eye, theſe two mouths drink them up, being ſtimulated into ac⯑tion by this fluid, which they abſorb. The belly of the gland, or lacrymal ſack, is thus filled, in which the ſaline part of the tears is abſorbed, and when the other end of the gland, or naſal duct, is ſti⯑mulated by the dryneſs, or pained by the coldneſs of the air, or af⯑fected by any acrimonious duſt or vapour in the noſtrils, it is excited into action together with the ſack, and the tears are diſgorged upon the membrane, which lines the noſtrils; where they ſerve a ſecond purpoſe to moiſten, clean, and lubricate, the organ of ſmell.

4. When the naſal duct of this gland is ſtimulated by any very acrid material, as the powder of tobacco, or volatile ſpirits, it not only diſgorges the contents of its belly or receptacle (the lacrymal ſack), and abſorbs haſtily all the fluid, that is ready for it in the cor⯑ner of the eye; but by the aſſociation of its motions with thoſe of the lacrymal gland, it excites that alſo into increaſed action, and a large flow of tears is poured into the eye.

5. This naſal duct is likewiſe excited into ſtrong action by ſenſitive ideas, as in grief, or joy, and then alſo by its aſſociations with the la⯑crymal gland it produces a great flow of tears without any external ſti⯑mulus; as is more fully explained in Sect. XVI. 8. on Inſtinct.

6. There are ſome, famous in the arts of exciting compaſſion, who are ſaid to have acquired a voluntary power of producing a flow of tears in the eye; which, from what has been ſaid in the ſection on Inſtinct above-mentioned, I ſhould ſuſpect, is performed by acquiring a voluntary power over the action of this naſal duct.

[271] 7. There is another circumſtance well worthy our attention, that when by any accident this naſal duct is obſtructed, the lacrymal ſack, which is the belly or receptacle of this gland, by ſlight preſſure of the finger is enabled to diſgorge its contents again into the eye; perhaps the bile in the ſame manner, when the biliary ducts are obſtructed, is returned into the blood by the veſſels which ſecrete it?

8. A very important though minute occurrence muſt here be ob⯑ſerved, that though the lacrymal gland is only excited into action, when we weep at a diſtreſsful tale, by its aſſociation with this naſal duct, as is more fully explained in Sect. XVI. 8; yet the quantity of tears ſecreted at once is more than the puncta lacrymalia can readily abſorb; which ſhews that the motions occaſioned by aſſociations are fre⯑quently more energetic than the original motions, by which they were oc⯑caſioned. Which we ſhall have occaſion to mention hereafter, to il⯑luſtrate, why pains frequently exiſt in a part diſtant from the cauſe of them, as in the other end of the urethra, when a ſtone ſtimulates the neck of the bladder. And why inflammations frequently ariſe in parts diſtant from their cauſe, as the gutta roſea of drinking people, from an inflamed liver.

The inflammation of a part is generally preceded by a torpor or quieſcence of it; if this exiſts in any large congeries of glands, as in the liver, or any membranous part, as the ſtomach, pain is produced, and chillineſs in conſequence of the torpor of the veſſels. In this ſitu⯑ation ſometimes an inflammation of the parts ſucceeds the torpor; at other times a diſtant more ſenſible part becomes inflamed; whoſe ac⯑tions have previouſly been aſſociated with it; and the torpor of the firſt part ceaſes. This I apprehend happens, when the gout of the foot ſucceeds a pain of the biliary duct, or of the ſtomach. Laſtly, it ſome⯑times happens, that the pain of torpor exiſts without any conſequent inflammation of the affected part, or of any diſtant part aſſociated with it, as in the membranes about the temple and eye-brows in hemicra⯑nia, and in thoſe pains, which occaſion convulſions; if this happens [272] to gouty people, when it affects the liver, I ſuppoſe epileptic fits are produced; and, when it affects the ſtomach, death is the conſequence. In theſe caſes the pulſe is weak, and the extremities cold, and ſuch medicines as ſtimulate the quieſcent parts into action, or which induce inflammation in them, or in any diſtant part, which is aſſociated with them, cures the preſent pain of torpor, and ſaves the patient.

I have twice ſeen a gouty inflammation of the liver, attended with jaundice; the patients after a few days were both of them affected with cold fits, like ague-fits, and their feet became affected with gout, and the inflammation of their livers ceaſed. It is probable, that the uneaſy ſenſations about the ſtomach, and indigeſtion, which precedes gouty paroxyſms, are generally owing to torpor or ſlight inflammation of the liver, and biliary ducts; but where great pain with continued ſickneſs, with feeble pulſe, and ſenſation of cold, affect the ſtomach in patients debilitated by the gout, that it is a torpor of the ſtomach itſelf, and deſtroys the patient from the great connexion of that viſcus with the vital organs. See Sect. XXV. 17.

I. 1. THE capillary veſſels are like all the other glands except the abſorbent ſyſtem, inaſmuch as they receive blood from the ar⯑teries, ſeparate a fluid from it, and return the remainder by the veins.

2. This ſeries of glands is of the moſt extenſive uſe, as their ex⯑cretory ducts open on the whole external ſkin forming its perſpirative pores, and on the internal ſurfaces of every cavity of the body. Their ſecretion on the ſkin is termed inſenſible perſpiration, which in health is in part reabſorbed by the mouths of the lymphatics, and in part evaporated in the air; the ſecretion on the membranes, which line the larger cavities of the body, which have external openings, as the mouth and inteſtinal canal, is termed mucus, but is not however co⯑agulable by heat; and the ſecretion on the membranes of thoſe cavi⯑ties of the body, which have no external openings, is called lymph or [286] water, as in the cavities of the cellular membrane, and of the abdo⯑men; this lymph however is coagulable by the heat of boiling wa⯑ter. Some mucus nearly as viſcid as the white of egg, which was diſcharged by ſtool, did not coagulate, though I evaporated it to one fourth of the quantity, nor did the aqueous and vitreous humours of a ſheep's eye coagulate by the like experiment: but the ſeroſity from an anaſarcous leg, and that from the abdomen of a dropſical perſon, and the cryſtalline humour of a ſheep's eye, coagulated in the ſame heat.

3. When any of theſe capillary glands are ſtimulated into greater irritative actions, than is natural, they ſecrete a more copious mate⯑rial; and as the mouths of the abſorbent ſyſtem, which open in their vicinity, are at the ſame time ſtimulated into greater action, the thinner and more ſaline part of the ſecreted fluid is taken up again; and the remainder is not only more copious but alſo more viſcid than natural. This is more or leſs troubleſome or noxious according to the importance of the functions of the part affected: on the ſkin and bronchiae, where this ſecretion ought naturally to evaporate, it be⯑comes ſo viſcid as to adhere to the membrane; on the tongue it forms a pellicle, which can with difficulty be ſcraped off; produces the ſcurf on the heads of many people; and the mucus, which is ſpit up by others in coughing. On the noſtrils and fauces, when the ſecretion of theſe capillary glands is increaſed, it is termed ſimple catarrh; when in the inteſtines, a mucous diarrhoea; and in the urethra, or vagina, it has the name of gonorrhoea, or fluor albus.

4. When theſe capillary glands become inflamed, a ſtill more viſcid or even cretaceous humour is produced upon the ſurfaces of the mem⯑branes, which is the cauſe or the effect of rheumatiſm, gout, leproſy, and of hard tumours of the legs, which are generally termed ſcorbutic; all which will be treated of hereafter.

II. 1. The whole ſurface of the body, with all its cavities and con⯑tents, [287] are covered with membrane. It lines every veſſel, forms every cell, and binds together all the muſcular and perhaps the oſſeous fibres of the body; and is itſelf therefore probably a ſimpler ſubſtance than thoſe fibres. And as the containing veſſels of the body from the largeſt to the leaſt are thus lined and connected with membranes, it follows that theſe membranes themſelves conſiſted of unorganized materials.

For however ſmall we may conceive the diameters of the minuteſt veſſels of the body, which eſcape our eyes and glaſſes, yet theſe veſ⯑ſels muſt conſiſt of coats or ſides, which are made up of an unorga⯑nized material, and which are probably produced from a gluten, which hardens after its production, like the ſilk or web of caterpillars and ſpiders. Of this material conſiſt the membranes, which line the ſhells of eggs, and the ſhell itſelf, both which are unorganized, and are formed from mucus, which hardens after it is formed, either by the abſorption of its more fluid part, or by its uniting with ſome part of the atmoſphere. Such is alſo the production of the ſhells of ſnails, and of ſhell-fiſh, and I ſuppoſe of the enamel of the teeth.

2. But though the membranes, that compoſe the ſides of the moſt minute veſſels, are in truth unorganized materials, yet the larger membranes, which are perceptible to the eye, ſeem to be com⯑poſed of an intertexture of the mouths of the abſorbent ſyſtem, and of the excretory ducts of the capillaries, with their concomitant arteries, veins, and nerves: and from this conſtruction it is evident, that theſe membranes muſt poſſeſs great irritability to peculiar ſti⯑muli, though they are incapable of any motions, that are viſible to the naked eye: and daily experience ſhews us, that in their inflamed ſtate they have the greateſt ſenſibility to pain, as in the pleuriſy and paronychia.

3. On all theſe membranes a mucilaginous or aqueous fluid is ſecreted, which moiſtens and lubricates their ſurfaces, as was ex⯑plained [288] in Section XXIII. 2. Some have doubted, whether this mucus is ſeparated from the blood by an appropriated ſet of glands, or exudes through the membranes, or is an abraſion or deſtruc⯑tion of the ſurface of the membrane itſelf, which is continually repaired on the other ſide of it, but the great analogy between the capillary veſſels, and the other glands, countenances the former opinion; and evinces, that theſe capillaries are the glands, that ſecrete it; to which we muſt add, that the blood in paſſing theſe capillary veſſels undergoes a change in its colour from florid to purple, and gives out a quantity of heat; from whence, as in other glands, we muſt conclude that ſomething is ſecreted from it.

III. The ſeat of rheumatiſm is in the membranes, or upon them; but there are three very diſtinct diſeaſes, which commonly are confounded under this name. Firſt, when a membrane becomes affected with torpor, or inactivity of the veſſels which compoſe it, pain and coldneſs ſucceed, as in the hemicrania, and other head⯑achs, which are generally termed nervous rheumatiſm; they exiſt whether the part be at reſt or in motion, and are generally attended with other marks of debility.

Another rheumatiſm is ſaid to exiſt, when inflammation and ſwelling, as well as pain, affect ſome of the membranes of the joints, as of the ancles, wriſts, knees, elbows, and ſometimes of the ribs. This is accompanied with fever, is analogous to pleu⯑riſy, and other inflammations, and is termed the acute rheuma⯑tiſm.

A third diſeaſe is called chronic rheumatiſm, which is diſtin⯑guiſhed from that firſt mentioned, as in this the pain only affects the patient during the motion of the part, and from the ſecond kind of rheumatiſm above deſcribed, as it is not attended with quick pulſe or inflammation. It is generally believed to ſucceed the acute rheu⯑matiſm [289] of the ſame part, and that ſome coagulable lymph, or creta⯑ceous, or calculous material, has been left on the membrane; which gives pain, when the muſcles move over it, as ſome extraneous body would do, which was too inſoluble to be abſorbed. Hence there is an analogy between this chronic rheumatiſm and the diſeaſes which produce gravel or gout-ſtones; and it may perhaps receive relief from the ſame remedies, ſuch as aerated ſal ſoda.

I. AS the imbibing mouths of the abſorbent ſyſtem already de⯑ſcribed open on the ſurface, and into the larger cavities of the body, ſo there is another ſyſtem of abſorbent veſſels, which are not com⯑monly eſteemed ſuch, I mean the veins, which take up the blood from the various glands and capillaries, after their proper fluids or ſe⯑cretions have been ſeparated from it.

The veins reſemble the other abſorbent veſſels; as the progreſſion of their contents is carried on in the ſame manner in both, they alike abſorb their appropriated fluids, and have valves to prevent its regur⯑gitation by the accidents of mechanical violence. This appears firſt, becauſe there is no pulſation in the very beginnings of the veins, as is ſeen by microſcopes; which muſt happen, if the blood was carried into them by the action of the arteries. For though the concurrence of various venous ſtreams of blood from different diſtances muſt pre⯑vent any pulſation in the larger branches, yet in the very beginnings of all theſe branches a pulſation muſt unavoidably exiſt, if the circu⯑lation [291] in them was owing to the intermitted force of the arteries. Secondly, the venous abſorption of blood from the penis, and from the teats of female animals after their erection, is ſtill more ſimilar to the lymphatic abſorption, as it is previouſly poured into cells, where all arterial impulſe muſt ceaſe.

There is an experiment, which ſeems to evince this venous abſorp⯑tion, which conſiſts in the external application of a ſtimulus to the lips, as of vinegar, by which they become inſtantly pale; that is, the bibulous mouths of the veins by this ſtimulus are excited to abſorb the blood faſter, than it can be ſupplied by the uſual arterial exertion. See Sect. XXIII. 5.

There are two kinds of haemorrhages frequent in diſeaſes, one is where the glandular or capillary action is too powerfully exerted, and propels the blood forwards more haſtily, than the veins can abſorb it; and the other is, where the abſorbent power of the veins is diminiſhed, or a branch of them is become totally paralytic.

The former of theſe caſes is known by the heat of the part, and the general fever or inflammation that accompanies the haemorrhage. An haemorrhage from the noſe or from the lungs is ſometimes a criſis of inflammatory diſeaſes, as of the hepatitis and gout, and generally ceaſes ſpontaneouſly, when the veſſels are conſiderably emptied. Sometimes the haemorrhage recurs by daily periods accompanying the hot fits of fever, and ceaſing in the cold fits, or in the intermiſſions; this is to be cured by removing the febrile paroxyſms, which will be treated of in their place. Otherwiſe it is cured by veneſection, by the internal or external preparations of lead, or by the application of cold, with an abſtemious diet, and diluting liquids, like other inflam⯑mations. Which by inducing a quieſcence on thoſe glandular parts, that are affected, prevents a greater quantity of blood from being pro⯑truded forwards, than the veins are capable of abſorbing.

Mr. B [...] had an haemorrhage from his kidney, and parted with not leſs than a pint of blood a day (by conjecture) along with his [292] urine for above a fortnight: veneſections, mucilages, balſams, pre⯑parations of lead, the bark, alum, and dragon's blood, opiates, with a large bliſter on his loins, were ſeparately tried, in large doſes, to no purpoſe. He was then directed to bathe in a cold ſpring up to the middle of his body only, the upper part being covered, and the haemorrhage diminiſhed at the firſt, and ceaſed at the ſecond im⯑merſion.

In this caſe the external capillaries were rendered quieſcent by the coldneſs of the water, and thence a leſs quantity of blood was cir⯑culated through them; and the internal capillaries, or other glands, became quieſcent from their irritative aſſociations with the external ones; and the haemorrhage was ſtopped a ſufficient time for the rup⯑tured veſſels to contract their apertures, or for the blood in thoſe aper⯑tures to coagulate.

Mrs. K [...] had a continued haemorrhage from her noſe for ſome days; the ruptured veſſel was not to be reached by plugs up the noſtrils, and the ſenſibility of her fauces was ſuch that nothing could be borne behind the uvula. After repeated veneſection, and other common applications, ſhe was directed to immerſe her whole head into a pail of water, which was made colder by the addi⯑tion of ſeveral handfuls of ſalt, and the haemorrhage immediately ceaſed, and returned no more; but her pulſe continued hard, and ſhe was neceſſitated to loſe blood from the arm on the ſucceed⯑ing day.

Query, might not the cold bath inſtantly ſtop haemorrhages from the lungs in inflammatory caſes?—for the ſhortneſs of breath of thoſe, who go ſuddenly into cold water, is not owing to the accumulation of blood in the lungs, but to the quieſcence of the pulmonary capillaries from aſſociation, as explained in Section XXXII. 3. 2.

II. The other kind of haemorrhage is known from its being at⯑tended with a weak pulſe, and other ſymptoms of general debility, [293] and very frequently occurs in thoſe, who have diſeaſed livers, owing to intemperance in the uſe of fermented liquors. Theſe conſtitutions are ſhewn to be liable to paralyſis of the lymphatic abſorbents, pro⯑ducing the various kinds of dropſies in Section XXIX. 5. Now if any branch of the venous ſyſtem loſes its power of abſorption, the part ſwells, and at length burſts and diſcharges the blood, which the ca⯑pillaries or other glands circulate through them.

It ſometimes happens that the large external veins of the legs burſt, and effuſe their blood; but this occurs moſt frequently in the veins of the inteſtines, as the vena portarum is liable to ſuffer from a ſchirrus of the liver oppoſing the progreſſion of the blood, which is abſorbed from the inteſtines. Hence the piles are a ſymptom of he⯑patic obſtruction, and hence the copious diſcharges downwards or upwards of a black material, which has been called melancholia, or black bile; but is no other than the blood, which is probably diſcharged from the veins of the inteſtines.

J. F. Meckel, in his Experimenta de Finibus Vaſorum, publiſhed at Berlin, 1772, mentions his diſcovery of a communication of a lymphatic veſſel with the gaſtric branch of the vena portarum. It is poſſible, that when the motion of the lymphatic becomes retro⯑grade in ſome diſeaſes, that blood may obtain a paſſage into it, where it anaſtomoſes with the vein, and thus be poured into the inteſtines. A diſcharge of blood with the urine ſometimes attends diabetes, and may have its ſource in the ſame manner.

Mr. A [...], who had been a hard drinker, and had the gutta roſacea on his face and breaſt, after a ſtroke of the palſy voided near a quart of a black viſcid material by ſtool: on diluting it with water it did not become yellow, as it muſt have done if it had been inſpiſſated bile, but continued black like the grounds of coffee.

But any other part of the venous ſyſtem may become quieſcent or totally paralytic as well as the veins of the inteſtines: all which [294] occur more frequently in thoſe who have diſeaſed livers, than in any others. Hence troubleſome bleedings of the noſe, or from the lungs with a weak pulſe; hence haemorrhages from the kidneys, too great menſtruation; and hence the oozing of blood from every part of the body, and the petechiae in thoſe fevers, which are termed putrid, and which is erroneouſly aſcribed to the thinneſs of the blood: for the blood in inflammatory diſeaſes is equally fluid before it coagulates in the cold air.

Is not that hereditary conſumption, which occurs chiefly in dark⯑eyed people about the age of twenty, and commences with ſlight pul⯑monary haemorrhages without fever, a diſeaſe of this kind?—Theſe haemorrhages frequently begin during ſleep, when the irritability of the lungs is not ſufficient in theſe patients to carry on the circula⯑tion without the aſſiſtance of volition; for in our waking hours, the motions of the lungs are in part voluntary, eſpecially if any difficulty of breathing renders the efforts of volition neceſſary. See Claſs I. 2. 1. 2. and Claſs III. 2. 1. 10. Another ſpecies of pulmo⯑nary conſumption which ſeems more certainly of ſcrophulous origin is deſcribed in the next Section, No. 2.

I have ſeen two caſes of women, of about forty years of age, both of whom were ſeized with quick weak pulſe, with difficult reſpiration, and who ſpit up by coughing much viſcid mucus mixed with dark coloured blood. They had both large vibices on their limbs, and petechiae; in one the feet were in danger of mortifica⯑tion, in the other the legs were oedematous. To relieve the diffi⯑cult reſpiration, about ſix ounces of blood were taken from one of them, which to my ſurpriſe was ſizy, like inflamed blood: they had both palpitations or unequal pulſations of the heart. They con⯑tinued four or five weeks with pale and bloated countenances, and did not ceaſe ſpitting phlegm mixed with black blood, and the pulſe ſeldom ſlower than 130 or 135 in a minute. This blood, from its [295] dark colour, and from the many vibices and petechiae, ſeems to have been venous blood; the quickneſs of the pulſe, and the irregularity of the motion of the heart, are to be aſcribed to debility of that part of the ſyſtem; as the extravaſation of blood originated from the de⯑fect of venous abſorption. The approximation of theſe two caſes to ſea-ſcurvy is peculiar, and may allow them to be called ſcorbutus pul⯑monalis. Had theſe been younger ſubjects, and the paralyſis of the veins had only affected the lungs, it is probable the diſeaſe would have been a pulmonary conſumption.

Laſt week I ſaw a gentleman of Birmingham, who had for ten days laboured under great palpitation of his heart, which was ſo diſ⯑tinctly felt by the hand, as to diſcountenance the idea of there being a fluid in the pericardium. He frequently ſpit up mucus ſtained with dark coloured blood, his pulſe very unequal and very weak, with cold hands and noſe. He could not lie down at all, and for about ten days paſt could not ſleep a minute together, but waked perpetually with great uneaſineſs. Could thoſe ſymptoms be owing to very ex⯑tenſive adheſions of the lungs? or is this a ſcorbutus pulmonalis? After a few days he ſuddenly got ſo much better as to be able to ſleep many hours at a time by the uſe of one grain of powder of foxglove twice a day, and a grain of opium at night. After a few days longer, the bark was exhibited, and the opium continued with ſome wine; and the palpitations of his heart became much relieved, and he reco⯑vered his uſual degree of health.

In epileptic fits the patients frequently become black in the face, from the temporary paralyſis of the venous ſyſtem of this part. I have known two inſtances where the blackneſs has continued many days. M. P [...], who had drank intemperately, was ſeized with the epi⯑lepſy when he was in his fortieth year; in one of theſe fits the white part of his eyes was left totally black with effuſed blood; which was attended with no pain or heat, and was in a few weeks gradually ab⯑ſorbed, changing colour as is uſual with vibices from bruiſes.

[296] The haemorrhages produced from the inability of the veins to ab⯑ſorb the refluent blood, is cured by opium, the preparations of ſteel, lead, the bark, vitriolic acid, and bliſters; but theſe have the effect with much more certainty, if a veneſection to a few ounces, and a moderate cathartic with four or ſix grains of calomel be premiſed, where the patient is not already too much debilitated; as one great means of promoting the abſorption of any fluid conſiſts in previouſly emptying the veſſels, which are to receive it.

I. There is a ſpecies of atrophy, which has not been well under⯑ſtood; when the abſorbent veſſels of the ſtomach and inteſtines have been long inured to the ſtimulus of too much ſpirituous liquor, they at length, either by the too ſudden omiſſion of fermented or ſpirituous potation, or from the gradual decay of nature, become in a certain degree paralytic; now it is obſerved in the larger muſcles of the body, when one ſide is paralytic, the other is more frequently in motion, owing to the leſs expenditure of ſenſorial power in the paralytic limbs; ſo in this caſe the other part of the abſorbent ſyſtem acts with greater force, or with greater perſeverance, in conſequence of the paralyſis of the lacteals; and the body becomes greatly emaciated in a ſmall time.

[298] I have ſeen ſeveral patients in this diſeaſe, of which the following are the circumſtances. 1. They were men about fifty years of age, and had lived freely in reſpect to fermented liquors. 2. They loſt their appetite to animal food. 3. They became ſuddenly emaciated to a great degree. 4. Their ſkins were dry and rough. 5. They coughed and expectorated with difficulty a viſcid phlegm. 6. The membrane of the tongue was dry and red, and liable to become ul⯑cerous.

The inability to digeſt animal food, and the conſequent diſtaſte to it, generally precedes the dropſy, and other diſeaſes, which originate from ſpirituous potation. I ſuppoſe when the ſtomach becomes in⯑irritable, that there is at the ſame time a deficiency of gaſtric acid; hence milk ſeldom agrees with theſe patients, unleſs it be previouſly curdled, as they have not ſufficient gaſtric acid to curdle it; and hence vegetable food, which is itſelf aceſcent, will agree with their ſtomachs longer than animal food, which requires more of the gaſtric acid for its digeſtion.

In this diſeaſe the ſkin is dry from the increaſed abſorption of the cutaneous lymphatics, the fat is abſorbed from the increaſed abſorption of the cellular lymphatics, the mucus of the lungs is too viſcid to be eaſily ſpit up by the increaſed abſorption of the thinner parts of it, the membrana ſneideriana becomes dry, covered with hardened mucus, and at length becomes inflamed and full of apthae, and either theſe ſloughs, or pulmonary ulcers, terminate the ſcene.

II. The immediate cauſe of dropſy is the paralyſis of ſome other branches of the abſorbent ſyſtem, which are called lymphatics, and which open into the larger cavities of the body, or into the cells of the cellular membrane; whence thoſe cavities or cells become diſtend⯑ed with the fluid, which is hourly ſecreted into them for the purpoſe of lubricating their ſurfaces. As is more fully explained in No. 5. of the next Section.

[299] As thoſe lymphatic veſſels conſiſt generally of a long neck or mouth, which drinks up its appropriated fluid, and of a conglobate gland, in which this fluid undergoes ſome change, it happens, that ſometimes the mouth of the lymphatic, and ſometimes the belly or glandular part of it, becomes totally or partially paralytic. In the former caſe, where the mouths of the cutaneous lymphatics become torpid or qui⯑eſcent, the fluid ſecreted on the ſkin ceaſes to be abſorbed, and erodes the ſkin by its ſaline acrimony, and produces eruptions termed herpes, the diſcharge from which is as ſalt, as the tears, which are ſecreted too faſt to be reabſorbed, as in grief, or when the puncta la⯑crymalia are obſtructed, and which running down the cheek redden and inflame the ſkin.

When the mouths of the lymphatics, which open on the mucous membrane of the noſtrils, become torpid, as on walking into the air in a froſty morning; the mucus, which continues to be ſecreted, has not its aqueous and ſaline part reabſorbed, which running over the upper lip inflames it, and has a ſalt taſte, if it falls on the tongue.

When the belly, or glandular part of theſe lymphatics, becomes torpid, the fluid abſorbed by its mouth ſtagnates, and forms a tumour in the gland. This diſeaſe is called the ſcrophula. If theſe glands ſuppurate externally, they gradually heal, as thoſe of the neck; if they ſuppurate without an opening on the external habit, as the meſenteric glands, a hectic fever enſues, which de⯑ſtroys the patient; if they ſuppurate in the lungs, a pulmonary conſumption enſues, which is believed thus to differ from that deſcribed in the preceding Section, in reſpect to its ſeat or proximate cauſe.

It is remarkable, that matter produced by ſuppuration will lie concealed in the body many weeks, or even months, without pro⯑ducing hectic fever; but as ſoon as the wound is opened, ſo as to [300] admit air to the ſurface of the ulcer, a hectic fever ſupervenes, even in very few hours, which is probably owing to the azotic part of the atmoſphere rather than to the oxygene; becauſe thoſe me⯑dicines, which contain much oxygene, as the calces or oxydes of metals, externally applied, greatly contribute to heal ulcers, of theſe are the ſolutions of lead and mercury, and copper in acids, or their precipitates.

Hence when ulcers are to be healed by the firſt intention, as it is called, it is neceſſary carefully to exclude the air from them. Hence we have one cauſe, which prevents pulmonary ulcers from healing, which is their being perpetually expoſed to the air.

Both the dark-eyed patients, which are affected with pulmonary ulcers from deficient venous abſorption, as deſcribed in Section XXVII. 2. and the light-eyed patients from deficient lymphatic ab⯑ſorption, which we are now treating of, have generally large aper⯑tures of the iris; theſe large pupils of the eyes are a common mark of want of irritability; and it generally happens, that an increaſe of ſenſibility, that is, of motions in conſequence of ſenſation, attends theſe conſtitutions. See Sect. XXXI. 2. Whence inflammations may occur in theſe from ſtagnated fluids more frequently than in thoſe conſtitutions, which poſſeſs more irritability and leſs ſen⯑ſibility.

Great expectations in reſpect to the cure of conſumptions, as well as of many other diſeaſes, are produced by the very ingenious exer⯑tions of DR. BEDDOES; who has eſtabliſhed an apparatus for breathing various mixtures of airs or gaſſes, at the hot-wells near Briſtol, which well deſerves the attention of the public.

DR. BEDDOES very ingeniouſly concludes, from the florid colour of the blood of conſumptive patients, that it abounds in oxygene; and that the redneſs of their tongues, and lips, and the ſine bluſh of their cheeks ſhew the preſence of the ſame principle, like fleſh reddened [301] by nitre. And adds, that the circumſtance of the conſumptions of pregnant women being ſtopped in their progreſs during pregnancy, at which time their blood may be ſuppoſed to be in part deprived of its oxygene, by oxygenating the blood of the foetus, is a forceable ar⯑gument in favour of this theory; which muſt ſoon be confirmed or confuted by his experiments. See Eſſay on Scurvy, Conſumption, &c. by Dr. Beddoes. Murray. London. Alſo Letter to Dr. Darwin, by the ſame. Murray. London.

I. Account of the Abſorbent Syſtem.

1. THE abſorbent ſyſtem of veſſels in animal bodies conſiſts of ſe⯑veral branches, differing in reſpect to their ſituations, and to the fluids, which they abſorb.

The inteſtinal abſorbents open their mouths on the internal ſur⯑faces of the inteſtines; their office is to drink up the chyle and the [303] other fluids from the alimentary canal; and they are termed lacteals, to diſtinguiſh them from the other abſorbent veſſels, which have been termed lymphatics.

Thoſe, whoſe mouths are diſperſed on the external ſkin, imbibe a great quantity of water from the atmoſphere, and a part of the per⯑ſpirable matter, which does not evaporate, and are termed cutaneous abſorbents.

Thoſe, which ariſe from the internal ſurface of the bronchia, and which imbibe moiſture from the atmoſphere, and a part of the bron⯑chial mucus, are called pulmonary abſorbents.

Thoſe, which open their innumerable mouths into the cells of the whole cellular membrane; and whoſe uſe is to take up the fluid, which is poured into thoſe cells, after it has done its office there; may be called cellular abſorbents.

Thoſe, which ariſe from the internal ſurfaces of the membranes, which line the larger cavities of the body, as the thorax, abdomen, ſcrotum, pericardium, take up the mucus poured into thoſe ca⯑vities; and are diſtinguiſhed by the names of their reſpective cavities.

Whilſt thoſe, which ariſe from the internal ſurfaces of the urinary bladder, gall-bladder, ſalivary ducts, or other receptacles of ſecreted fluids, may take their names from thoſe fluids; the thinner parts of which it is their office to abſorb: as urinary, bilious, or ſalivary ab⯑ſorbents.

2. Many of theſe abſorbent veſſels, both lacteals and lymphatics, like ſome of the veins, are replete with valves: which ſeem deſigned to aſſiſt the progreſs of their fluids, or at leaſt to prevent their regur⯑gitation; where they are ſubjected to the intermitted preſſure of the muſcular, or arterial actions in their neighbourhood.

Theſe valves do not however appear to be neceſſary to all the ab⯑ſorbents, any more than to all the veins; ſince they are not found to exiſt in the abſorbent ſyſtem of fiſh; according to the diſcoveries of the [304] ingenious, and much lamented Mr. Hewſon. Philoſ. Tranſ. v. 59, Enquiries into the Lymph. Syſt. p. 94.

3. Theſe abſorbent veſſels are alſo furniſhed with glands, which are called conglobate glands; whoſe uſe is not at preſent ſufficiently inveſtigated; but it is probable that they reſemble the conglomerate glands both in ſtructure and in uſe, except that their abſorbent mouths are for the conveniency of ſituation placed at a greater diſ⯑tance from the body of the gland. The conglomerate glands open their mouths immediately into the ſanguiferous veſſels, which bring the blood, from whence they abſorb their reſpective fluids, quite up to the gland: but theſe conglobate glands collect their adapted fluids from very diſtant membranes, or cyſts, by means of mouths furniſh⯑ed with long necks for this purpoſe; and which are called lacteals, or lymphatics.

4. The fluids, thus collected from various parts of the body, paſs by means of the thoracic duct into the left ſubclavian near the ju⯑gular vein; except indeed that thoſe collected from the right ſide of the head and neck, and from the right arm, are carried into the right ſubclavian vein: and ſometimes even the lymphatics from the right ſide of the lungs are inſerted into the right ſubclavian vein; whilſt thoſe of the left ſide of the head open but juſt into the ſummit of the thoracic duct.

5. In the abſorbent ſyſtem there are many anaſtomoſes of the veſ⯑ſels, which ſeem of great conſequence to the preſervation of health. Theſe anaſtomoſes are diſcovered by diſſection to be very frequent between the inteſtinal and urinary lymphatics, as mentioned by Mr. Hewſon, (Phil. Tranſ. v. 58).

6. Nor do all the inteſtinal abſorbents ſeem to terminate in the thoracic duct, as appears from ſome curious experiments of D. Monro, who gave madder to ſome animals, having previouſly put a ligature on the thoracic duct, and found their bones, and the ſerum of their blood, coloured red.

II. The Valves of the Abſorbent Syſtem may ſuffer their Fluids to regurgitate in ſome Diſeaſes.

1. THE many valves, which occur in the progreſs of the lymphatic and lacteal veſſels, would ſeem inſuperable obſtacles to the regurgita⯑tion of their contents. But as theſe valves are placed in veſſels, which are indued with life, and are themſelves indued with life alſo; and are very irritable into thoſe natural motions, which abſorb, or propel the fluids they contain; it is poſſible, in ſome diſeaſes, where theſe valves or veſſels are ſtimulated into unnatural exertions, or are become paralytic, that during the diaſtole of the part of the veſſel to which the valve is attached, the valve may not ſo completely cloſe, as to prevent the relapſe of the lymph or chyle. This is rendered more probable, by the experiments of injecting mercury, or water, or ſuet, or by blowing air down theſe veſſels; all which paſs the valves very eaſily, contrary to the natural courſe of their fluids, when the veſſels are thus a little forcibly dilated, as mentioned by Dr. Haller, Elem. Phyſiol. t. iii. ſ. 4.

"The valves of the thoracic duct are few, ſome aſſert they are not more than twelve, and that they do not very accurately perform their office, as they do not cloſe the whole area of the duct, and thence may permit chyle to repaſs them downwards. In living animals, however, though not always, yet more frequently than in the dead, they prevent the chyle from returning. The principal of theſe valves is that, which preſides over the inſertion of the thoracic duct, into the ſubclavian vein; many have believed this alſo to perform the of⯑fice of a valve, both to admit the chyle into the vein, and to preclude the blood from entering the duct; but in my opinion it is ſcarcely ſufficient for this purpoſe." Haller, Elem. Phyſ. t. vii. p. 226.

[306] 2. The mouths of the lymphatics ſeem to admit water to paſs through them after death, the inverted way, eaſier than the natural one; ſince an inverted bladder readily lets out the water with which it is filled; whence it may be inferred, that there is no obſtacle at the mouths of theſe veſſels to prevent the regurgitation of their con⯑tained fluids.

I was induced to repeat this experiment, and having accurately tied the ureters and neck of a freſh ox's bladder, I made an opening at the fundus of it; and then, having turned it inſide outwards, filled it half full with water, and was ſurpriſed to ſee it empty itſelf ſo haſtily. I thought the experiment more appoſite to my purpoſe by ſuſpending the bladder with its neck downwards, as the lymphatics are chiefly ſpread upon this part of it; as ſhewn by Dr. Watſon, Philoſ. Tranſ. v. 59. p. 392.

3. In ſome diſeaſes, as in the diabetes and ſcrophula, it is probable the valves themſelves are diſeaſed, and are thence incapable of pre⯑venting the return of the fluids they ſhould ſupport. Thus the valves of the aorta itſelf have frequently been found ſchirrous, according to the diſſections of Monſ. Lieutaud, and have given riſe to an inter⯑rupted pulſe, and laborious palpitations, by ſuffering a return of part of the blood into the heart. Nor are any parts of the body ſo liable to ſchirroſity as the lymphatic glands and veſſels, inſomuch that their ſchirroſities have acquired a diſtinct name, and been termed ſchrophula.

4. There are valves in other parts of the body, analogous to thoſe of the abſorbent ſyſtem, and which are liable, when diſeaſed, to re⯑gurgitate their contents: thus the upper and lower orifices of the ſtomach are cloſed by valves, which, when too great quantities of warm water have been drank with a deſign to promote vomiting, have ſometimes reſiſted the utmoſt efforts of the abdominal muſcles, and diaphragm: yet, at other times, the upper valve, or cardia, eaſily permits the evacuation of the contents of the ſtomach; whilſt [307] the inferior valve, or pylorus, permits the bile, and other contents of the duodenum, to regurgitate into the ſtomach.

5. The valve of the colon is well adapted to prevent the retrograde motion of the excrements; yet, as this valve is poſſeſſed of a living power, in the iliac paſſion, either from ſpaſm, or other unnatural ex⯑ertions, it keeps itſelf open, and either ſuffers or promotes the retro⯑grade movements of the contents of the inteſtines below; as in rumi⯑nating animals the mouth of the firſt ſtomach ſeems to be ſo con⯑ſtructed, as to facilitate or aſſiſt the regurgitation of the food; the rings of the oeſophagus afterwards contracting themſelves in inverted order. De Haen, by means of a ſyringe, forced ſo much water into the rectum inteſtinum of a dog, that he vomited it in a full ſtream from his mouth; and in the iliac paſſion above mentioned, excre⯑ments and clyſter are often evacuated by the mouth. See Section XXV. 15.

6. The puncta lacrymalia, with the lacrymal ſack and naſal duct, compoſe a complete gland, and much reſemble the inteſtinal canal the puncta lacrymalia are abſorbent mouths, that take up the tears from the eye, when they have done their office there, and convey them into the noſtrils; but when the naſal duct is obſtructed, and the lacrymal ſack diſtended with its fluid, on preſſure with the finger the mouths of this gland (puncta lacrymalia) will readily diſgorge the fluid, they had previouſly abſorbed, back into the eye.

7. As the capillary veſſels receive blood from the arteries, and ſe⯑parating the mucus, or perſpirable matter from it, convey the re⯑mainder back by the veins; theſe capillary veſſels are a ſet of glands, in every reſpect ſimilar to the ſecretory veſſels of the liver, or other large congeries of glands. The beginnings of theſe capillary veſſels have frequent anaſtomoſes into each other, in which circumſtance they are reſembled by the lacteals; and like the mouths or beginnings of other glands, they are a ſet of abſorbent veſſels, which drink up the blood which is brought to them by the arteries, as the chyle is drank [308] up by the lacteals: for the circulation of the blood through the capil⯑laries is proved to be independent of arterial impulſe; ſince in the bluſh of ſhame, and in partial inflammations, their action is increaſed, with⯑out any increaſe of the motion of the heart.

8. Yet not only the mouths, or beginnings of theſe anaſtomoſing capillaries are frequently ſeen by microſcopes, to regurgitate ſome par⯑ticles of blood, during the ſtruggles of the animal; but retrograde motion of the blood, in the veins of thoſe animals, from the very heart to the extremity of the limbs, is obſervable, by intervals, during the diſtreſſes of the dying creature. Haller, Elem. Phyſiol. t. i. p. 216. Now, as the veins have perhaps all of them a valve ſome⯑where between their extremities and the heart, here is ocular de⯑monſtration of the fluids in this diſeaſed condition of the animal, re⯑paſſing through venous valves: and it is hence highly probable, from the ſtricteſt analogy, that if the courſe of the fluids, in the lymphatic veſſels, could be ſubjected to microſcopic obſervation, they would alſo, in the diſeaſed ſtate of the animal, be ſeen to repaſs the valves, and the mouths of thoſe veſſels, which had previouſly abſorbed them, or pro⯑moted their progreſſion.

III. Communication from the Alimentary Canal to the Bladder, by means of the Abſorbent Veſſels.

MANY medical philoſophers, both ancient and modern, have ſuſ⯑pected that there was a nearer communication between the ſtomach and the urinary bladder, than that of the circulation: they were led into this opinion from the great expedition with which cold water, when drank to exceſs, paſſes off by the bladder; and from the ſimila⯑rity of the urine, when produced in this haſty manner, with the ma⯑terial that was drank.

[309] The former of theſe circumſtances happens perpetually to thoſe who drink abundance of cold water, when they are much heated by exerciſe, and to many at the beginning of intoxication.

Of the latter, many inſtances are recorded by Etmuller, t. xi. p. 716. where ſimple water, wine, and wine with ſugar, and emul⯑ſions, were returned by urine unchanged.

There are other experiments, that ſeem to demonſtrate the exiſtence of another paſſage to the bladder, beſides that through the kidneys. Thus Dr. Kratzenſtein put ligatures on the ureters of a dog, and then emptied the bladder by a catheter; yet in a little time the dog drank greedily, and made a quantity of water, (Diſputat. Morbor. Halleri. t. iv. p. 63.) A ſimilar experiment is related in the Philoſophical Tranſactions, with the ſame event, (No. 65, 67, for the year 1670.)

Add to this, that in ſome morbid caſes the urine has continued to paſs, after the ſuppuration or total deſtruction of the kidneys; of which many inſtances are referred to in the Elem. Phyſiol. t. vii. p. 379. of Dr. Haller.

From all which it muſt be concluded, that ſome fluids have paſſed from the ſtomach or abdomen, without having gone through the ſanguiferous circulation: and as the bladder is ſupplied with many lymphatics, as deſcribed by Dr. Watſon, in the Philoſ. Tranſ. v. 59. p. 392. and as no other veſſels open into it beſides theſe and the ure⯑ters, it ſeems evident, that the unnatural urine, produced as above de⯑ſcribed, when the ureters were tied, or the kidneys obliterated, was carried into the bladder by the retrograde motions of the urinary branch of the lymphatic ſyſtem.

The more certainly to aſcertain the exiſtence of another communi⯑cation between the ſtomach and bladder, beſides that of the circula⯑tion, the following experiment was made, to which I muſt beg your patient attention:—A friend of mine (June 14, 1772) on drinking re⯑peatedly of cold ſmall punch, till he began to be intoxicated, made a [310] quantity of colourleſs urine. He then drank about two drams of nitre diſſolved in ſome of the punch, and eat about twenty ſtalks of boiled aſparagus: on continuing to drink more of the punch, the next urine that he made was quite clear, and without ſmell; but in a little time another quantity was made, which was not quite ſo colourleſs, and had a ſtrong ſmell of the aſparagus: he then loſt about four ounces of blood from the arm.

The ſmell of aſparagus was not at all perceptible in the blood, nei⯑ther when freſh taken, nor the next morning, as myſelf and two others accurately attended to; yet this ſmell was ſtrongly perceived in the urine, which was made juſt before the blood was taken from his arm.

Some bibulous paper, moiſtened in the ſerum of this blood, and ſuffered to dry, ſhewed no ſigns of nitre by its manner of burning. But ſome of the ſame paper, moiſtened in the urine, and dried, on being ignited, evidently ſhewed the preſence of nitre. This blood and the urine ſtood ſome days expoſed to the ſun in the open air, till they were evaporated to about a fourth of their original quantity, and began to ſtink: the paper, which was then moiſtened with the con⯑centrated urine, ſhewed the preſence of much nitre by its manner of burning; whilſt that moiſtened with the blood ſhewed no ſuch ap⯑pearance at all.

Hence it appears, that certain fluids at the beginning of intoxica⯑tion, find another paſſage to the bladder beſides the long courſe of the arterial circulation; and as the inteſtinal abſorbents are joined with the urinary lymphatics by frequent anaſtomoſes, as Hewſon has demon⯑ſtrated; and as there is no other road, we may juſtly conclude, that theſe fluids paſs into the bladder by the urinary branch of the lym⯑phatics, which has its motions inverted during the diſeaſed ſtate of the animal.

A gentleman, who had been ſome weeks affected with jaundice, and whoſe urine was in conſequence of a very deep yellow, took ſome [311] cold ſmall punch, in which was diſſolved about a dram of nitre; he then took repeated draughts of the punch, and kept himſelf in a cool room, till on the approach of ſlight intoxication he made a large quantity of water; this water had a ſlight yellow tinge, as might be expected from a ſmall admixture of bile ſecreted from the kidneys; but if the whole of it had paſſed through the ſanguiferous veſſels, which were now replete with bile (his whole ſkin being as yellow as gold) would not this urine alſo, as well as that he had made for weeks before, have been of a deep yellow? Paper dipped in this water, and dryed, and ignited, ſhewed evident marks of the preſence of nitre, when the flame was blown out.

IV. The Phaenomena of the Diabetes explained, and of ſome Diarrhoeas.

THE phaenomena of many diſeaſes are only explicable from the re⯑trograde motions of ſome of the branches of the lymphatic ſyſtem; as the great and immediate flow of pale urine in the beginning of drunk⯑enneſs; in hyſteric paroxyſms; from being expoſed to cold air; or to the influence of fear or anxiety.

Before we endeavour to illuſtrate this doctrine, by deſcribing the phaenomena of theſe diſeaſes, we muſt premiſe one circumſtance; that all the branches of the lymphatic ſyſtem have a certain ſympathy with each other, inſomuch that when one branch is ſtimulated into unuſual kinds or quantities of motion, ſome other branch has its mo⯑tions either increaſed, or decreaſed, or inverted at the ſame time. This kind of ſympathy can only be proved by the concurrent teſti⯑mony of numerous facts, which will be related in the courſe of the work. I ſhall only add here, that it is probable, that this ſympathy does not depend on any communication of nervous filaments, but on [312] habit; owing to the various branches of this ſyſtem having frequently been ſtimulated into action at the ſame time.

There are a thouſand inſtances of involuntary motions aſſociated in this manner; as in the act of vomiting, while the motions of the ſto⯑mach and oeſophagus are inverted, the pulſations of the arterial ſyſtem by a certain ſympathy become weaker; and when the bowels or kid⯑neys are ſtimulated by poiſon, a ſtone, or inflammation, into more violent action; the ſtomach and oeſophagus by ſympathy invert their motions.

1. When any one drinks a moderate quantity of vinous ſpirit, the whole ſyſtem acts with more energy by conſent with the ſtomach and inteſtines, as is ſeen from the glow on the ſkin, and the increaſe of ſtrength and activity; but when a greater quantity of this inebriating material is drank, at the ſame time that the lacteals are excited into greater action to abſorb it; it frequently happens, that the urinary branch of abſorbents, which is connected with the lacteals by many anaſtomoſes, inverts its motions, and a great quantity of pale unani⯑malized urine is diſcharged. By this wiſe contrivance too much of an unneceſſary fluid is prevented from entering the circulation—This may be called the drunken diabetes, to diſtinguiſh it from the other temporary diabetes, which occur in hyſteric diſeaſes, and from con⯑tinued fear or anxiety.

2. If this idle ingurgitation of too much vinous ſpirit be daily prac⯑tiſed, the urinary branch of abſorbents at length gains an habit of in⯑verting its motions, whenever the lacteals are much ſtimulated; and the whole or a great part of the chyle is thus daily carried to the blad⯑der without entering the circulation, and the body becomes emaci⯑ated. This is one kind of chronic diabetes, and may be diſtinguiſhed from the others by the taſte and appearance of the urine; which is ſweet, and the colour of whey, and may be termed the chyliferous diabetes.

[313] 3. Many children have a ſimilar depoſition of chyle in their urine, from the irritation of worms in their inteſtines, which ſtimulating the mouths of the lacteals into unnatural action, the urinary branch of the abſorbents becomes inverted, and carries part of the chyle to the bladder: part of the chyle alſo has been carried to the iliac and lumbar glands, of which inſtances are recorded by Haller, t. vii. 225▪ and which can be explained on no other theory: but the diſſections of the lymphatic ſyſtem of the human body, which have yet been publiſhed, are not ſufficiently extenſive for our purpoſe; yet if we may reaſon from comparative anatomy, this tranſlation of chyle to the bladder is much illuſtrated by the account given of this ſyſtem of veſſels in a turtle, by Mr. Hewſon, who obſerved, "That the lacteals near the root of the meſentery anaſtomoſe, ſo as to form a net-work, from which ſeveral large branches go into ſome conſiderable lymphatics ly⯑ing near the ſpine; and which can be traced almoſt to the anus, and particularly to the kidneys. Philoſ. Tranſ. v. 59. p. 199—Enquiries, p. 74.

4. At the ſame time that the urinary branch of abſorbents, in the beginning of diabetes, is excited into inverted action, the cellular branch is excited by the ſympathy above mentioned, into more ener⯑getic action; and the fat, that was before depoſited, is reabſorbed and thrown into the blood veſſels; where it floats, and was miſtaken for chyle, till the late experiments of the ingenious Mr. Hewſon demon⯑ſtrated it to be fat.

This appearance of what was miſtaken for chyle in the blood, which was drawn from theſe patients, and the obſtructed liver, which very frequently accompanies this diſeaſe, ſeems to have led Dr. Mead to ſuſpect the diabetes was owing to a defect of ſanguifica⯑tion; and that the ſchirroſity of the liver was the original cauſe of it: but as the ſchirrhus of the liver is moſt frequently owing to the ſame cauſes, that produce the diabetes and dropſies; namely, the great uſe [314] of fermented liquors; there is no wonder they ſhould exiſt together, without being the conſequence of each other.

5. If the cutaneous branch of abſorbents gains a habit of being ex⯑cited into ſtronger action, and imbibes greater quantities of moiſture from the atmoſphere, at the ſame time that the urinary branch has its motions inverted, another kind of diabetes is formed, which may be termed the aqueous diabetes. In this diabetes the cutaneous ab⯑ſorbents frequently imbibe an amazing quantity of atmoſpheric moiſ⯑ture; inſomuch that there are authentic hiſtories, where many gallons a day, for many weeks together, above the quantity that has been drank, have been diſcharged by urine.

Dr. Keil, in his Medicina Statica, found that he gained eighteen ounces from the moiſt air of one night; and Dr. Percival affirms, that one of his hands imbibed, after being well chafed, near an ounce and half of water, in a quarter of an hour. (Tranſact. of the College, London, vol. ii. p. 102). Home's Medic. Facts, p. 2. ſect. 3.

The pale urine in hyſterical women, or which is produced by fear or anxiety, is a temporary complaint of this kind; and it would in re⯑ality be the ſame diſeaſe, if it was confirmed by habit.

6. The purging ſtools, and pale urine, occaſioned by expoſing the naked body to cold air, or ſprinkling it with cold water, originate from a ſimilar cauſe; for the mouths of the cutaneous lymphatics being ſuddenly expoſed to cold become torpid, and ceaſe, or nearly ceaſe, to act; whilſt, by the ſympathy above deſcribed, not only the lymphatics of the bladder and inteſtines ceaſe alſo to abſorb the more aqueous and ſaline part of the fluids ſecreted into them; but it is probable that theſe lymphatics invert their motions, and return the fluids, which were previouſly abſorbed, into the inteſtines and bladder. At the very inſtant that the body is expoſed naked to the cold air, an unuſual movement is felt in the bowels; as is experienced by boys going into the cold bath: this could not occur from an obſtruction of [315] the perſpirable matter, ſince there is not time for that to be returned to the bowels by the courſe of the circulation.

There is alſo a chronic aqueous diarrhoea, in which the atmoſpheric moiſture, drank up by the cutaneous and pulmonary lymphatics, is poured into the inteſtines, by the retrograde motions of the lacteals. This diſeaſe is moſt ſimilar to the aqueous diabetes, and is frequently exchanged for it: a diſtinct inſtance of this is recorded by Benninge⯑rus, Cent. v. Obſ. 98. in which an aqueous diarrhoea ſucceeded an aqueous diabetes, and deſtroyed the patient. There is a curious ex⯑ample of this, deſcribed by Sympſon (De Re Medica)—"A young man (ſays he) was ſeized with a fever, upon which a diarrhoea came on, with great ſtupor; and he refuſed to drink any thing, though he was parched up with exceſſive heat: the better to ſupply him with moiſ⯑ture, I directed his feet to be immerſed in cold water; immediately I obſerved a wonderful decreaſe of water in the veſſel, and then an im⯑petuous ſtream of a fluid, ſcarcely coloured, was diſcharged by ſtool, like a cataract."

7. There is another kind of diarrhoea, which has been called cae⯑liaca; in this diſeaſe the chyle, drank up by the lacteals of the ſmall inteſtines, is probably poured into the large inteſtines, by the retro⯑grade motions of their lacteals: as in the chyliferous diabetes, the chyle is poured into the bladder, by the retrograde motions of the urinary branch of abſorbents.

The chyliferous diabetes, like this chyliferous diarrhoea, produces ſudden atrophy; ſince the nouriſhment, which ought to ſupply the hourly waſte of the body, is expelled by the bladder, or rectum: whilſt the aqueous diabetes, and the aqueous diarrhoea produce exceſ⯑ſive thirſt; becauſe the moiſture, which is obtained from the atmo⯑ſphere, is not conveyed to the thoracic receptacle, as it ought to be, but to the bladder, or lower inteſtines; whence the chyle, blood, and whole ſyſtem of glands, are robbed of their proportion of hu⯑midity.

[316] 8. There is a third ſpecies of diabetes, in which the urine is mu⯑cilaginous, and appears ropy in pouring it from one veſſel into another; and will ſometimes coagulate over the fire. This diſeaſe appears by intervals, and ceaſes again, and ſeems to be occaſioned by a previous dropſy in ſome part of the body. When ſuch a collection is reab⯑ſorbed, it is not always returned into the circulation; but the ſame irritation that ſtimulates one lymphatic branch to reabſorb the depo⯑ſited fluid, inverts the urinary branch, and pours it into the bladder. Hence this mucilaginous diabetes is a cure, or the conſequence of a cure, of a worſe diſeaſe, rather than a diſeaſe itſelf.

Dr. Cotunnius gave half an ounce of cream of tartar, every morning, to a patient, who had the anaſarca; and he voided a great quantity of urine; a part of which, put over the fire, coagulated, on the evaporation of half of it, ſo as to look like the white of an egg. De Iſchiade Nervos.

This kind of diabetes frequently precedes a dropſy; and has this remarkable circumſtance attending it, that it generally happens in the night; as during the recumbent ſtate of the body, the fluid, that was accumulated in the cellular membrane, or in the lungs, is more rea⯑dily abſorbed, as it is leſs impeded by its gravity. I have ſeen more than one inſtance of this diſeaſe. Mr. D. a man in the decline of life, who had long accuſtomed himſelf to ſpirituous liquor, had ſwelled legs, and other ſymptoms of approaching anaſarca; about once in a week, or ten days, for ſeveral months, he was ſeized, on going to bed, with great general uneaſineſs, which his attendants reſembled to an hyſteric fit; and which terminated in a great diſcharge of viſcid urine; his legs became leſs ſwelled, and he continued in better health for ſome days afterwards. I had not the opportunity to try if this urine would coagulate over the fire, when part of it was evaporated, which I imagine would be the criterion of this kind of diabetes; as the mu⯑cilaginous fluid depoſited in the cells and cyſts of the body, which have no communication with the external air, ſeems to acquire, by ſtagna⯑tion, [317] this property of coagulation by heat, which the ſecreted mucus of the inteſtines and bladder do not appear to poſſeſs; as I have found by experiment: and if any one ſhould ſuppoſe this coagulable urine was ſeparated from the blood by the kidneys, he may recollect, that in the moſt inflammatory diſeaſes, in which the blood is moſt replete, or moſt ready to part with the coagulable lymph, none of this appears in the urine.

9. Different kinds of diabetes require different methods of cure. For the firſt kind, or chyliferous diabetes, after clearing the ſtomach and inteſtines, by ipecacuanha and rhubarb, to evacuate any acid ma⯑terial, which may too powerfully ſtimulate the mouths of the lacteals, repeated and large doſes of tincture of cantharides have been much re⯑commended. The ſpecific ſtimulus of this medicine, on the neck of the bladder, is likely to excite the numerous abſorbent veſſels, which are ſpread on that part, into ſtronger natural actions, and by that means prevent their retrograde ones; till, by perſiſting in the uſe of the medicine, their natural habits of motions might again be eſtabliſh⯑ed. Another indication of cure, requires ſuch medicines, as by lin⯑ing the inteſtines with mucilaginous ſubſtances, or with ſuch as con⯑ſiſt of ſmooth particles, or which chemically deſtroy the acrimony of their contents, may prevent the too great action of the inteſtinal ab⯑ſorbents. For this purpoſe, I have found the earth precipitated from a ſolution of alum, by means of fixed alcali, given in the doſe of half a dram every ſix hours, of great advantage, with a few grains of rhu⯑barb, ſo as to procure a daily evacuation.

The food ſhould conſiſt of materials that have the leaſt ſtimulus, with calcareous water, as of Briſtol and Matlock; that the mouths of the lactcals may be as little ſtimulated as is neceſſary for their proper abſorption; leſt with their greater exertions, ſhould be connected by ſympathy, the inverted motions of the urinary lymphatics.

The ſame method may be employed with equal advantage in the aqueous diabetes, ſo great is the ſympathy between the ſkin and the [318] ſtomach. To which, however, ſome application to the ſkin might be uſefully added; as rubbing the patient all over with oil, to prevent the too great action of the cutaneous abſorbents. I knew an ex⯑periment of this kind made upon one patient with apparent ad⯑vantage.

The mucilaginous diabetes will require the ſame treatment, which is moſt efficacious in the dropſy, and will be deſcribed below. I muſt add, that the diet and medicines above mentioned, are ſtrongly re⯑commended by various authors, as by Morgan, Willis, Harris, and Etmuller; but more hiſtories of the ſucceſsful treatment of theſe diſeaſes are wanting to fully aſcertain the moſt efficacious methods of cure.

In a letter from Mr. Charles Darwin, dated April 24, 1778, Edin⯑burgh, is the ſubſequent paſſage:—"A man who had long laboured under a diabetes died yeſterday in the clinical ward. He had for ſome time drank four, and paſſed twelve pounds of fluid daily; each pound of urine contained an ounce of ſugar. He took, without conſiderable relief, gum kino, ſanguis draconis melted with alum, tincture of can⯑tharides, iſinglaſs, gum arabic, crabs eyes, ſpirit of hartſhorn, and eat ten or fifteen oyſters thrice a day. Dr. Home, having read my theſis, bled him, and found that neither the freſh blood nor the ſerum taſted ſweet. His body was opened this morning—every viſcus appeared in a ſound and natural ſtate, except that the left kidney had a very ſmall pelvis, and that there was a conſiderable enlargement of moſt of the meſenteric lymphatic glands. I intend to inſert this in my theſis, as it coincides with the experiment, where ſome aſparagus was eaten at the beginning of intoxication, and its ſmell perceived in the urine, though not in the blood."

The following caſe of chyliferous diabetes is extracted from ſome letters of Mr. Hughs, to whoſe unremitted care the infirmary at Staf⯑ford for many years was much indebted. Dated October 10, 1778.

[319] Richard Davis, aged 33, a whiteſmith by trade, had drank hard by intervals; was much troubled with ſweating of his hands, which incommoded him in his occupation, but which ceaſed on his frequent⯑ly dipping them in lime. About ſeven months ago he began to make large quantities of water; his legs are oedematous, his belly tenſe, and he complains of a riſing in his throat, like the globus hyſtericus: he eats twice as much as other people, drinks about fourteen pints of ſmall beer a day, beſides a pint of ale, ſome milk-porridge, and a baſon of broth, and he makes about eighteen pints of water a day.

He tried alum, dragon's blood, ſteel, blue vitriol, and cantharides in large quantities, and duly repeated, under the care of Dr. Under⯑hill, but without any effect; except that on the day after he omitted the cantharides, he made but twelve pints of water, but on the next day this good effect ceaſed again.

November 21.—He made eighteen pints of water, and he now, at Dr. Darwin's requeſt, took a grain of opium every four hours, and five grains of aloes at night; and had a flannel ſhirt given him.

22.—Made ſixteen pints. 23.—Thirteen pints: drinks leſs.

24.—Increaſed the opium to a grain and quarter every four hours: he made twelve pints.

25.—Increaſed the opium to a grain and half: he now makes ten pints; and drinks eight pints in a day.

The opium was gradually increaſed during the next fortnight, till he took three grains every four hours, but without any further dimu⯑nition of his water. During the uſe of the opium he ſweat much in the nights, ſo as to have large drops ſtand on his face and all over him. The quantity of opium was then gradually decreaſed, but not to⯑tally omitted, as he continued to take about a grain morning and evening.

January 17.—He makes fourteen pints of water a day. Dr. Un⯑derhill now directed him two ſcruples of common roſin triturated [320] with as much ſugar, every ſix hours; and three grains of opium every night.

19.—Makes fifteen pints of water: ſweats at night.

21.—Makes ſeventeen pints of water; has twitchings of his limbs in a morning, and pains of his legs: he now takes a dram of roſin for a doſe, and continues the opium.

23.—Water more coloured, and reduced to ſixteen pints, and he thinks has a brackiſh taſte.

26.—Water reduced to fourteen pints.

28.—Water thireen pints: he continues the opium, and takes four ſcruples of the roſin for a doſe.

February 1.—Water twelve pints.

4.—Water eleven pints: twitchings leſs: takes five ſcruples for a doſe.

8.—Water ten pints: has had many ſtools.

12.—Appetite leſs: purges very much.

After this the roſin either purged him, or would not ſtay on his ſtomach; and he gradually relapſed nearly to his former condition, and in a few months ſunk under the diſeaſe.

October 3, Mr. Hughs evaporated two quarts of the water, and obtained from it four ounces and half of a hard and brittle ſaccharine maſs, like treacle which had been ſome time boiled. Four ounces of blood, which he took from his arm with deſign to examine it, had the common appearances, except that the ſerum reſembled cheeſe⯑whey; and that on the evidence of four perſons, two of whom did not know what it was they taſted, the ſerum had a ſaltiſh taſte.

From hence it appears, that the ſaccharine matter, with which the urine of theſe patients ſo much abounds, does not enter the blood⯑veſſels like the nitre and aſparagus mentioned above; but that the proceſs of digeſtion reſembles the proceſs of the germination of ve⯑getables, or of making barley into malt; as the vaſt quantity of ſugar [321] found in the urine muſt be made from the food which he took (which was double that taken by others), and from the fourteen pints of ſmall beer which he drank. And, ſecondly, as the ſerum of the blood was not ſweet, the chyle appears to have been conveyed to the bladder without entering the circulation of the blood, ſince ſo large a quantity of ſugar, as was found in the urine, namely, twenty ounces a day, could not have previouſly exiſted in the blood without being perceptible to the taſte.

November 1. Mr. Hughes diſſolved two drams of nitre in a pint of a decoction of the roots of aſparagus, and added to it two ounces of tincture of rhubarb: the patient took a fourth part of this mixture every five minutes, till he had taken the whole.—In about half an hour he made eighteen ounces of water, which was very manifeſtly tinged with the rhubarb; the ſmell of aſparagus was doubtful.

He then loſt four ounces of blood, the ſerum of which was not ſo opake as that drawn before, but of a yellowiſh caſt, as the ſerum of the blood uſually appears.

Paper, dipped three or four times in the tinged urine and dried again, did not ſcintillate when it was ſet on fire; but when the flame was blown out, the fire ran along the paper for half an inch; which, when the ſame paper was unimpregnated, it would not do; nor when the ſame paper was dipped in urine made before he took the nitre, and dried in the ſame manner.

Paper, dipped in the ſerum of the blood and dried in the ſame manner as in the urine, did not ſcintillate when the flame was blown out, but burnt exactly in the ſame manner as the ſame paper dipped in the ſerum of blood drawn from another perſon.

This experiment, which is copied from a letter of Mr. Hughes, as well as the former, ſeems to evince the exiſtence of another paſſage from the inteſtines to the bladder, in this diſeaſe, beſides that of the ſanguiferous ſyſtem; and coincides with the curious experiment re⯑lated in ſection the third, except that the ſmell of the aſparagus was [322] not here perceived, owing perhaps to the roots having been made uſe of inſtead of the heads.

The riſing in the throat of this patient, and the twitchings of his limbs, ſeem to indicate ſome ſimilarity between the diabetes and the hyſteric diſeaſe, beſides the great flow of pale urine, which is common to them both.

Perhaps if the meſenteric glands were nicely inſpected in the diſ⯑ſections of theſe patients; and if the thoracic duct, and the larger branches of the lacteals, and if the lymphatics, which ariſe from the bladder, were well examined by injection, or by the knife, the cauſe of diabetes might be more certainly underſtood.

The opium alone, and the opium with the roſin, ſeem much to have ſerved this patient, and might probably have effected a cure, if the diſeaſe had been ſlighter, or the medicine had been exhibited, be⯑fore it had been confirmed by habit during the ſeven months it had continued. The increaſe of the quantity of water on beginning the large doſes of roſin was probably owing to his omitting the morning doſes of opium.

V. The Phaenomena of Dropſies explained.

I. SOME inebriates have their paroxyſms of inebriety terminated by much pale urine, or profuſe ſweats, or vomiting, or ſtools; others have their paroxyſms terminated by ſtupor, or ſleep, without the above evacuations.

The former kind of theſe inebriates have been obſerved to be more liable to diabetes and dropſy; and the latter to gout, gravel, and le⯑proſy. Evoe! attend ye bacchanalians! ſtart at this dark train of evils, and, amid your immodeſt jeſts, and idiot laughter, recollect,

[323] In thoſe who are ſubject to diabetes and dropſy, the abſorbent veſ⯑ſels are naturally more irritable than in the latter; and by being fre⯑quently diſturbed or inverted by violent ſtimulus, and by their too great ſympathy with each other, they become at length either entire⯑ly paralytic, or are only ſuſceptible of motion from the ſtimulus of very acrid materials; as every part of the body, after having been uſed to great irritations, becomes leſs affected by ſmaller ones. Thus we cannot diſtinguiſh objects in the night, for ſome time after we come out of a ſtrong light, though the iris is preſently dilated; and the air of a ſummer evening appears cold, after we have been expoſed to the heat of the day.

There are no cells in the body, where dropſy may not be produced, if the lymphatics ceaſe to abſorb that mucilaginous fluid, which is perpetually depoſited in them, for the purpoſe of lubricating their ſurfaces.

If the lymphatic branch, which opens into the cellular membrane, either does its office imperfectly, or not at all; theſe cells become re⯑plete with a mucilaginous fluid, which, after it has ſtagnated ſome time in the cells, will coagulate over the fire; and is erroneouſly called water. Wherever the ſeat of this diſeaſe is, (unleſs in the lungs or other pendent viſcera) the mucilaginous liquid above mentioned will ſubſide to the moſt depending parts of the body, as the feet and legs, when thoſe are lower than the head and trunk; for all theſe cells have communications with each other.

When the cellular abſorbents are become inſenſible to their uſual irritations, it moſt frequently happens, but not always, that the cuta⯑neous branch of abſorbents, which is ſtrictly aſſociated with them, ſuffers the like inability. And then, as no water is abſorbed from the atmoſphere, the urine is not only leſs diluted at the time of its ſecre⯑tion, and conſequently in leſs quantity and higher coloured: but great thirſt is at the ſame time induced, for as no water is abſorbed from the atmoſphere to dilute the chyle and blood, the lacteals and other ab⯑ſorbent [324] veſſels, which have not loſt their powers, are excited into more conſtant or more violent action, to ſupply this deficiency; whence the urine becomes ſtill leſs in quantity, and of a deeper colour, and turbid like the yolk of an egg, owing to a greater abſorption of its thinner parts. From this ſtronger action of thoſe abſorbents, which ſtill retain their irritability, the fat is alſo abſorbed, and the whole body becomes emaciated. This increaſed exertion of ſome branches of the lymphatics, while others are totally or partially pa⯑ralytic, is reſembled by what conſtantly occurs in the hemiplagia; when the patient has loſt the uſe of the limbs on one ſide, he is in⯑ceſſantly moving thoſe of the other; for the moving power, not hav⯑ing acceſs to the paralytic limbs, becomes redundant in thoſe which are not diſeaſed.

The paucity of urine and thirſt cannot be explained from a greater quantity of mucilaginous fluid being depoſited in the cellular mem⯑brane: for though theſe ſymptoms have continued many weeks, or even months, this collection frequently does not amount to more than very few pints. Hence alſo the difficulty of promoting copious ſweats in anaſarca is accounted for, as well as the great thirſt, paucity of urine, and loſs of fat; ſince, when the cutaneous branch of abſor⯑bents is paralytic, or nearly ſo, there is already too ſmall a quantity of aqueous fluid in the blood: nor can theſe torpid cutaneous lympha⯑tics be readily excited into retrograde motions.

Hence likewiſe we underſtand, why in the aſcites, and ſome other dropſies, there is often no thirſt, and no paucity of urine; in theſe caſes the cutaneous abſorbents continue to do their office.

Some have believed, that dropſies were occaſioned by the inability of the kidneys, from having only obſerved the paucity of urine; and have thence laboured much to obtain diuretic medicines; but it is daily obſervable, that thoſe who die of a total inability to make water, do not become dropſical in conſequence of it: Fernelius mentions one, who laboured under a perfect ſuppreſſion of urine during twenty days [325] before his death, and yet had no ſymptoms of dropſy. Pathol. 1. vi. c. 8. From the ſame idea many phyſicians have reſtrained their pa⯑tients from drinking, though their thirſt has been very urgent; and ſome caſes have been publiſhed, where this cruel regimen has been thought advantageous: but others of nicer obſervation are of opinion, that it has always aggravated the diſtreſſes of the patient; and though it has abated his ſwellings, yet by inducing a fever it has haſtened his diſſolution. See Tranſactions of the College, London, vol. ii. p. 235. Caſes of Dropſy by Dr. G. Baker.

The cure of anaſarca, ſo far as reſpects the evacuation of the accu⯑mulated fluid, coincides with the idea of the retrograde action of the lymphatic ſyſtem. It is well known that vomits, and other drugs, which induce ſickneſs or nauſea; at the ſame time that they evacuate the ſtomach, produce a great abſorption of the lymph accumulated in the cellular membrane. In the operation of a vomit, not only the motions of the ſtomach and duodenum become inverted, but alſo thoſe of the lymphatics and lacteals, which belong to them; whence a great quantity of chyle and lymph is perpetually poured into the ſto⯑mach and inteſtines, during the operation, and evacuated by the mouth. Now at the ſame time, other branches of the lymphatic ſyſtem, viz. thoſe which open on the cellular membrane, are brought into more energetic action, by the ſympathy above mentioned, and an increaſe of their abſorption is produced.

Hence repeated vomits, and cupreous ſalts, and ſmall doſes of ſquill or foxglove, are ſo efficacious in this diſeaſe. And as draſtic purges act alſo by inverting the motions of the lacteals; and thence the other branches of lymphatics are induced into more powerful natural action, by ſympathy, and drink up the fluids from all the cells of the body; and by their anaſtomoſes, pour them into the lacteal branches; which, by their inverted actions, return them into the inteſtines; and they are thus evacuated from the body:—theſe purges alſo are uſed with ſucceſs in diſcharging the accumulated fluid in anaſarca.

[326] II. The following caſes are related with deſign to aſcertain the par⯑ticular kinds of dropſy in which the digitalis purpurea, or common foxglove, is preferable to ſquill, or other evacuants, and were firſt publiſhed in 1780, in a pamphlet entitled Experiments on mucila⯑ginous and purulent Matter, &c. Cadell. London. Other caſes of dropſy, treated with digitalis, were afterwards publiſhed by Dr. Darwin in the Medical Tranſactions, vol. iii. in which there is a miſtake in reſpect to the doſe of the powder of foxglove, which ſhould have been from five grains to one, inſtead of from five grains to ten.

VI. Of cold Sweats.

THERE have been hiſtories given of chronical immoderate ſweat⯑ings, which bear ſome analogy to the diabetes. Dr. Willis mentions a lady then living, whoſe ſweats were for many years ſo profuſe, that all her bed-clothes were not only moiſtened, but deluged with them every night; and that many ounces, and ſometimes pints, of this ſweat, were received in veſſels properly placed, as it trickled down her body. He adds, that ſhe had great thirſt, had taken many medicines, and ſubmitted to various rules of life, and changes of climate, but ſtill continued to have theſe immoderate ſweats. Pharmac. ration. de ſudore anglico.

Dr. Willis has alſo obſerved, that the ſudor anglicanus which ap⯑peared in England, in 1483, and continued till 1551, was in ſome reſpects ſimilar to the diabetes; and as Dr. Caius, who ſaw this diſ⯑eaſe, mentions the viſcidity, as well as the quantity of theſe ſweats, and adds, that the extremities were often cold, when the internal parts were burnt up with heat and thirſt, with great and ſpeedy emaciation and debility: there is great reaſon to believe, that the fluids were ab⯑ſorbed from the cells of the body by the cellular and cyſtic branches of the lymphatics, and poured on the ſkin by the retrograde motions of the cutaneous ones.

Sydenham has recorded, in the ſtationary fever of the year 1685, the viſcid ſweats flowing from the head, which were probably from the ſame ſource as thoſe in the ſweating plague above mentioned.

It is very common in dropſies of the cheſt or lungs to have the difficulty of breathing relieved by copious ſweats, flowing from the head and neck. Mr. P. about 50 years of age, had for many weeks [334] been afflicted with anaſarca of his legs and thighs, attended with dif⯑ficulty of breathing; and had repeatedly been relieved by ſquill, other bitters, and chalybeates.—One night the difficulty of breathing became ſo great, that it was thought he muſt have expired; but ſo copious a ſweat came out of his head and neck, that in a few hours ſome pints, by eſtimation, were wiped off from thoſe parts, and his breath was for a time relieved. This dyſpnoea and theſe ſweats recurred at intervals, and after ſome weeks he ceaſed to exiſt. The ſkin of his head and neck felt cold to the hand, and appeared pale at the time theſe ſweats flowed ſo abundantly; which is a proof, that they were produced by an inverted motion of the abſorbents of thoſe parts: for ſweats, which are the con⯑ſequence of an increaſed action of the ſanguiferous ſyſtem, are always at⯑tended with a warmth of the ſkin, greater than is natural, and a more florid colour; as the ſweats from exerciſe, or thoſe that ſucceed the cold fits of agues. Can any one explain how theſe partial ſweats ſhould relieve the difficulty of breathing in anaſarca, but by ſuppoſing that the pul⯑monary branch of abſorbents drank up the fluid in the cavity of the tho⯑rax, or in the cells of the lungs, and threw it on the ſkin, by the retro⯑grade motions of the cutaneous branch? for, if we could ſuppoſe, that the increaſed action of the cutaneous glands or capillaries poured upon the ſkin this fluid, previouſly abſorbed from the lungs; why is not the whole ſurface of the body covered with ſweat? why is not the ſkin warm? Add to this, that the ſweats above mentioned were clammy or glutinous, which the condenſed perſpirable matter is not; whence it would ſeem to have been a different fluid from that of common perſpiration.

Dr. Dobſon, of Liverpool, has given a very ingenious explanation of the acid ſweats, which he obſerved in a diabetic patient—he thinks part of the chyle is ſecreted by the ſkin, and afterwards undergoes an ace⯑tous ſermentation.—Can the chyle get thither, but by an inverted motion of the cutaneous lymphatics? in the ſame manner as it is car⯑ried [335] to the bladder, by the inverted motions of the urinary lymphatics. Medic. Obſervat. and Enq. London, vol. v.

Are not the cold ſweats in ſome fainting fits, and in dying people, owing to an inverted motion of the cutaneous lymphatics? for in theſe there can be no increaſed arterial or glandular action.

Is the difficulty of breathing, ariſing from anarſaca of the lungs, relieved by ſweats from the head and neck; whilſt that difficulty of breathing, which ariſes from a dropſy of the thorax, or pericardium, is never attended with theſe ſweats of the head? and thence can theſe diſeaſes be diſtinguiſhed from each other? Do the periodic returns of nocturnal aſthma riſe from a temporary dropſy of the lungs, collected during their more torpid ſtate in ſound ſleep, and then re-abſorbed by the vehement efforts of the diſordered organs of reſpiration, and car⯑ried off by the copious ſweats about the head and neck?

More extenſive and accurate diſſections of the lymphatic ſyſtem are wanting to enable us to unravel theſe knots of ſcience.

VII. Tranſlations of Matter, of Chyle, of Milk, of Urine. Operation of purging Drugs applied externally.

1. THE tranſlations of matter from one part of the body to another, can only receive an explanation from the doctrine of the occaſional re⯑trograde motions of ſome branches of the lymphatic ſyſtem: for how can matter, abſorbed and mixed with the whole maſs of blood, be ſo haſtily collected again in any one part? and is it not an immutable law, in animal bodies, that each gland can ſecrete no other, but its own proper fluid? which is, in part, fabricated in the very gland by an animal proceſs, which it there undergoes: of theſe purulent tranſ⯑lations innumerable and very remarkable inſtances are recorded.

2. The chyle, which is ſeen among the materials thrown up by vio⯑lent [336] vomiting, or in purging ſtools, can only come thither by its having been poured into the bowels by the inverted motions of the lacteals: for our aliment is not converted into chyle in the ſtomach or inteſtines by a chemical proceſs, but is made in the very mouths of the lacteals; or in the meſenteric glands; in the ſame manner as other ſecreted fluids are made by an animal proceſs in their adapted glands.

Here a curious phaenomenon in the exhibition of mercury is worth explaining:—If a moderate doſe of calomel, as ſix or ten grains, be ſwallowed, and within one or two days a cathartic is given, a ſaliva⯑tion is prevented: but after three or four days, a ſalivation having come on, repeated purges every day, for a week or two, are required to eliminate the mercury from the conſtitution. For this acrid me⯑tallic preparation, being abſorbed by the mouths of the lacteals, con⯑tinues, for a time arreſted by the meſenteric glands, (as the variolous or venereal poiſons ſwell the ſubaxillar or inguinal glands): which, during the operation of a cathartic, is returned into the inteſtines by the inverted action of the lacteals, and thus carried out of the ſyſtem.

Hence we underſtand the uſe of vomits or purges, to thoſe who have ſwallowed either contagious or poiſonous materials, even though exhibited a day or even two days after ſuch accidents; namely, that by the retrograde motions of the lacteals and lymphatics, the material ſtill arreſted in the meſenteric, or other glands, may be eliminated from the body.

3. Many inſtances of milk and chyle found in ulcers are given by Haller, El. Phyſiol. t. vii. p. 12, 23, which admit of no other expla⯑nation than by ſuppoſing, that the chyle, imbibed by one branch of the abſorbent ſyſtem, was carried to the ulcer, by the inverted motions of another branch of the ſame ſyſtem.

4. Mrs. P. on the ſecond day after delivery, was ſeized with a vio⯑lent purging, in which, though opiates, mucilages, the bark, and teſtacea were profuſely uſed, continued many days, till at length ſhe recovered. During the time of this purging, no milk could be drawn [337] from her breaſts; but the ſtools appeared like the curd of milk broken into ſmall pieces. In this caſe, was not the milk taken up from the follicles of the pectoral glands, and thrown on the inteſtines, by a re⯑trogreſſion of the inteſtinal abſorbents? for how can we for a moment ſuſpect that the mucous glands of the inteſtines could ſeparate pure milk from the blood? Doctor Smelly has obſerved, that looſe ſtools, mixed with milk, which is curdled in the inteſtines, frequently re⯑lieves the turgeſcency of the breaſts of thoſe who ſtudiouſly repel their milk. Caſes in Midwifery, 43, No. 2. 1.

5. J. F. Meckel obſerved in a patient, whoſe urine was in ſmall quantity and high coloured, that a copious ſweat under the arm-pits, of a perfectly urinous ſmell, ſtained the linen; which ceaſed again when the uſual quantity of urine was diſcharged by the urethra. Here we muſt believe from analogy, that the urine was firſt ſecreted in the kidneys, then re-abſorbed by the increaſed action of the urinary lym⯑phatics, and laſtly carried to the axillae by the retrograde motions of the lymphatic branches of thoſe parts. As in the jaundice it is ne⯑ceſſary, that the bile ſhould firſt be ſecreted by the liver, and re-ab⯑ſorbed into the circulation, to produce the yellowneſs of the ſkin; as was formerly demonſtrated by the late Dr. Monro, (Edin. Medical Eſſays) and if in this patient the urine had been re-abſorbed into the maſs of blood, as the bile in the jaundice, why was it not detected in other parts of the body, as well as in the arm-pits?

6. Cathartic and vermifuge medicines applied externally to the ab⯑domen, ſeem to be taken up by the cutaneous branch of lymphatics, and poured on the inteſtines by the retrograde motions of the lacteals, without having paſſed the circulation.

For when the draſtic purges are taken by the mouth, they excite the lacteals of the inteſtines into retrograde motions, as appears from the chyle, which is found coagulated among the faeces, as was ſhewn above, (ſect 2 and 4.) And as the cutaneous lymphatics are joined with the lacteals of the inteſtines, by frequent anaſtomoſes; it would [338] be more extraordinary, when a ſtrong purging drug, abſorbed by the ſkin, is carried to the anaſtomoſing branches of the lacteals unchanged, if it ſhould not excite them into retrograde action as efficaciouſly, as if it was taken by the mouth, and mixed with the food of the ſto⯑mach.

VIII. Circumſtances by which the Fluids, that are effuſed by the retro⯑grade Motions of the abſorbent Veſſels, are diſtinguiſhed.

1. WE frequently obſerve an unuſual quantity of mucus or other fluids in ſome diſeaſes, although the action of the glands, by which thoſe fluids are ſeparated from the blood, is not unuſually increaſed; but when the power of abſorption alone is diminiſhed. Thus the ca⯑tarrhal humour from the noſtrils of ſome, who ride in froſty weather; and the tears, which run down the cheeks of thoſe, who have an ob⯑ſtruction of the puncta lacrymalia; and the ichor of thoſe phagedenic ulcers, which are not attended with inflammation, are all inſtances of this circumſtance.

Theſe fluids however are eaſily diſtinguiſhed from others by their abounding in ammoniacal or muriatic ſalts; whence they inflame the circumjacent ſkin: thus in the catarrh the upper lip becomes red and ſwelled from the acrimony of the mucus, and patients complain of the ſaltneſs of its taſte. The eyes and cheeks are red with the corro⯑ſive tears, and the ichor of ſome herpetic eruptions erodes far and wide the contiguous parts, and is pungently ſalt to the taſte, as ſome pa⯑tients have informed me.

Whilſt, on the contrary, thoſe fluids, which are effuſed by the re⯑trograde action of the lymphatics, are for the moſt part mild and in⯑nocent; as water, chyle, and the natural mucus: or they take their properties from the materials previouſly abſorbed, as in the coloured [339] or vinous urine, or that ſcented with aſparagus, deſcribed be⯑fore.

2. Whenever the ſecretion of any fluid is increaſed, there is at the ſame time an increaſed heat in the part; for the ſecreted fluid, as the bile, did not previouſly exiſt in the maſs of blood, but a new combi⯑nation is produced in the gland. Now as ſolutions are attended with cold, ſo combinations are attended with heat; and it is probable the ſum of the heat given out by all the ſecreted fluids of animal bodies may be the cauſe of their general heat above that of the atmoſphere.

Hence the fluids derived from increaſed ſecretions are readily diſtin⯑guiſhed from thoſe originating from the retrograde motions of the lymphatics: thus an increaſe of heat either in the diſeaſed parts, or diffuſed over the whole body, is perceptible, when copious bilious ſtools are conſequent to an inflamed liver; or a copious mucous ſaliva⯑tion from the inflammatory angina.

3. When any ſecreted fluid is produced in an unuſual quantity, and at the ſame time the power of abſorption is increaſed in equal pro⯑portion, not only the heat of the gland becomes more intenſe, but the ſecreted fluid becomes thicker and milder, its thinner and ſaline parts being re-abſorbed: and theſe are diſtinguiſhable both by their greater conſiſtence, and by their heat, from the fluids, which are effuſed by the retrograde motions of the lymphatics; as is obſervable towards the termination of gonorrhoea, catarrh, chincough, and in thoſe ulcers, which are ſaid to abound with laudable pus.

4. When chyle is obſerved in ſtools, or among the materials eject⯑ed by vomit, we may be confident it muſt have been brought thither by the retrograde motions of the lacteals; for chyle does not previouſly exiſt amid the contents of the inteſtines, but is made in the very mouths of the lacteals, as was before explained.

5. When chyle, milk, or other extraneous fluids are found in the urinary bladder, or in any other excretory receptacle of a gland; no one can for a moment believe, that theſe have been collected from [340] the maſs of blood by a morbid ſecretion, as it contradicts all ana⯑logy.

IX. Retrograde Motions of Vegetable Juices.

THERE are beſides ſome motions of the ſap in vegetables, which bear analogy to our preſent ſubject; and as the vegetable tribes are by many philoſophers held to be inferior animals, it may be a matter of curioſity at leaſt to obſerve, that their abſorbent veſſels ſeem evidently, at times, to be capable of a retrograde motion. Mr. Perault cut off a forked branch of a tree, with the leaves on; and inverting one of the forks into a veſſel of water, obſerved, that the leaves on the other branch continued green much longer than thoſe of a ſimilar branch, cut off from the ſame tree; which ſhews, that the water from the veſſel was carried up one part of the forked branch, by the retrograde motion of its veſſels, and ſupplied nutriment ſome time to the other part of the branch, which was out of the water. And the celebrated Dr. Hales found, by numerous very accurate experiments, that the ſap of trees roſe upwards during the warmer hours of the day, and in part deſcended again during the cooler ones. Vegetable Statics.

It is well known that the branches of willows, and of many other trees, will either take root in the earth or engraft on other trees, ſo as to have their natural direction inverted, and yet flouriſh with vigour.

Dr. Hope has alſo made this pleaſing experiment, after the manner of Hales—he has placed a forked branch, cut from one tree, erect be⯑tween two others; then cutting off a part of the bark from one fork [341] applied it to a ſimilar branch of one of the trees in its vicinity; and the ſame of the other fork; ſo that a tree is ſeen to grow ſuſpended in the air, between two other trees; which ſupply their foſter friend with due nouriſhment.

All theſe experiments clearly evince, that the juices of vegetables can occaſionally paſs either upwards or downwards in their abſorbent ſyſtem of veſſels.

X. Objections anſwered.

THE following experiment, at firſt view, would ſeem to invalidate this opinion of the retrograde motions of the lymphatic veſſels, in ſome diſeaſes.

About a gallon of milk having been given to an hungry ſwine, he was ſuffered to live about an hour, and was then killed by a ſtroke or two on his head with an axe.—On opening his belly the lacteals were well ſeen filled with chyle; on irritating many of the branches of them with a knife, they did not appear to empty themſelves haſtily; but they did however carry forwards their contents in a little time.

I then paſſed a ligature round ſeveral branches of lacteals, and irri⯑tated them much with a knife beneath the ligature, but could not make them regurgitate their contained fluid into the bowels.

I am not indeed certain, that the nerve was not at the ſame time included in the ligature, and thus the lymphatic rendered unirritable or lifeleſs; but this however is certain, that it is not any quantity of any ſtimulus, which induces the veſſels of animal bodies to revert their motions; but a certain quantity of a certain ſtimulus, as appears from wounds in the ſtomach, which do not produce vomiting; and wounds of the inteſtines, which do not produce the cholera morbus.

[342] At Nottingham, a few years ago, two ſhoemakers quarrelled, and one of them with a knife, which they uſe in their occupation, ſtabbed his companion about the region of the ſtomach. On opening the ab⯑domen of the wounded man after his death the food and medicines he had taken were in part found in the cavity of the belly, on the outſide of the bowels; and there was a wound about half an inch long at the bottom of the ſtomach; which I ſuppoſe was diſtended with liquor and food at the time of the accident; and thence was more li⯑able to be injured at its bottom: but during the whole time he lived, which was about ten days, he had no efforts to vomit, nor ever even complained of being ſick at the ſtomach! Other caſes ſimilar to this are mentioned in the philoſophical tranſactions.

Thus, if you vellicate the throat with a feather, nauſea is produced; if you wound it with a penknife, pain is induced, but not ſickneſs. So if the ſoles of the feet of children or their armpits are tickled, convulſive laughter is excited, which ceaſes the moment the hand is applied, ſo as to rub them more forcibly.

The experiment therefore above related upon the lacteals of a dead pig, which were included in a ſtrict ligature, proves nothing; as it is not the quantity, but the kind of ſtimulus, which excites the lymphatic veſſels into retrograde motion.

XI. The Cauſes which induce the retrograde Motions of animal Veſſels; and the Medicines by which the natural Motions are reſtored.

1. SUCH is the conſtruction of animal bodies, that all their parts, which are ſubjected to leſs ſtimuli than nature deſigned, perform their functions with leſs accuracy: thus, when too watery or too aceſcent food is taken into the ſtomach, indigeſtion, and flatulency, and heart⯑burn ſucceed.

2. Another law of irritation, connate with our exiſtence, is, that [343] all thoſe parts of the body, which have previouſly been expoſed to too great a quantity of ſuch ſtimuli, as ſtrongly affect them, become for ſome time afterwards diſobedient to the natural quantity of their adapt⯑ed ſtimuli.—Thus the eye is incapable of ſeeing objects in an obſcure room, though the iris is quite dilated, after having been expoſed to the meridian ſun.

3. There is a third law of irritation, that all the parts of our bodies, which have been lately ſubjected to leſs ſtimulus, than they have been accuſtomed to, when they are expoſed to their uſual quantity of ſti⯑mulus, are excited into more energetic motions: thus when we come from a duſky cavern into the glare of daylight, our eyes are dazzled; and after emerging from the cold bath, the ſkin becomes warm and red.

4. There is a fourth law of irritation, that all the parts of our bodies, which are ſubjected to ſtill ſtronger ſtimuli for a length of time, become torpid, and refuſe to obey even theſe ſtronger ſtimuli; and thence do their offices very imperfectly.—Thus, if any one looks earneſtly for ſome minutes on an area, an inch diameter, of red ſilk, placed on a ſheet of white paper, the image of the ſilk will gradually become pale, and at length totally vaniſh.

5. Nor is it the nerves of ſenſe alone, as the optic and auditory nerves, that thus become torpid, when the ſtimulus is withdrawn or their irritability decreaſed; but the motive muſcles, when they are deprived of their natural ſtimuli, or of their irritability, become torpid and paralytic; as is ſeen in the tremulous hand of the drunkard in a morning; and in the awkward ſtep of age.

The hollow muſcles alſo, of which the various veſſels of the body are conſtructed, when they are deprived of their natural ſtimuli, or of their due degree of irritability, not only become tremulous, as the ar⯑terial pulſations of dying people; but alſo frequently invert their motions, as in vomiting, in hyſteric ſuffocations, and diabetes above deſcribed.

[344] I muſt beg your patient attention, for a few moments whilſt I en⯑deavour to explain, how the retrograde actions of our hollow muſcles are the conſequence of their debility; as the tremulous actions of the ſolid muſcles are the conſequence of their debility. When, through fatigue, a muſcle can act no longer; the antagoniſt muſcles, either by their inanimate elaſticity, or by their animal action, draw the limb into a contrary direction: in the ſolid muſcles, as thoſe of locomotion, their actions are aſſociated in tribes, which have been accuſtomed to ſynchronous action only; hence when they are fatigued, only a ſingle contrary effort takes place; which is either tremulous, when the fa⯑tigued muſcles are again immediately brought into action; or it is a pandiculation, or ſtretching, where they are not immediately again brought into action.

Now the motions of the hollow muſcles, as they in general pro⯑pel a fluid along their cavities, are aſſociated in trains, which have been accuſtomed to ſucceſſive actions: hence when one ring of ſuch a muſcle is fatigued from its too great debility, and is brought into retrograde action, the next ring from its aſſociation falls ſucceſſively into retrograde action; and ſo on throughout the whole canal. See Sect. XXV. 6.

6. But as the retrograde motions of the ſtomach, oeſophagus, and fauces in vomiting are, as it were, apparent to the eye; we ſhall conſider this operation more minutely, that the ſimilar operations in the more recondite parts of our ſyſtem may be eaſier underſtood.

From certain nauſeous ideas of the mind, from an ungrateful taſte in the mouth, or from foetid ſmells, vomiting is ſometimes inſtantly excited; or even from a ſtroke on the head, or from the vibratory motions of a ſhip; all which originate from aſſociation, or ſympathy. See Sect. XX. on Vertigo.

But when the ſtomach is ſubjected to a leſs ſtimulus than is na⯑tural, according to the firſt law of irritation mentioned above, its motions become diſturbed, as in hunger; firſt pain is p roduced, [345] then ſickneſs, and at length vain efforts to vomit, as many authors inform us.

But when a great quantity of wine, or of opium, is ſwallowed, the retrograde motions of the ſtomach do not occur till after ſeveral minutes, or even hours; for when the power of ſo ſtrong a ſtimulus ceaſes, according to the ſecond law of irritation, mentioned above, the periſtaltic motions become tremulous, and at length retrograde; as is well known to the drunkard, who on the next morning has ſickneſs and vomitings.

When a ſtill greater quantity of wine, or of opium, or when nau⯑ſeous vegetables, or ſtrong bitters, or metallic ſalts, are taken into the ſtomach, they quickly induce vomiting; though all theſe in leſs doſes excite the ſtomach into more energetic action, and ſtrengthen the digeſtion; as the flowers of chamomile, and the vitriol of zinc: for, according to the fourth law of irritation, the ſtomach will not long be obedient to a ſtimulus ſo much greater than is natural; but its action becomes firſt tremulous and then retrograde.

7. When the motions of any veſſels become retrograde, leſs heat of the body is produced; for in paroxyſms of vomiting, of hyſteric affections, of diabetes, of aſthma, the extremities of the body are cold: hence we may conclude, that theſe ſymptoms ariſe from the debility of the parts in action; for an increaſe of muſcular action is always attended with increaſe of heat.

8. But as animal debility is owing to defect of ſtimulus, or to defect of irritability, as ſhewn above, the method of cure is eaſily deduced: when the vaſcular muſcles are not excited into their due action by the natural ſtimuli, we ſhould exhibit thoſe medicines, which poſſeſs a ſtill greater degree of ſtimulus; amongſt theſe are the foetids, the vo⯑latiles, aromatics, bitters, metallic ſalts, opiates, wine, which indeed ſhould be given in ſmall doſes, and frequently repeated. To theſe ſhould be added conſtant, but moderate exerciſe, cheerfulneſs of mind, and change of country to a warmer climate; and perhaps occaſionally the external ſtimulus of bliſters.

[346] It is alſo frequently uſeful to diminiſh the quantity of natural ſtimulus for a ſhort time, by which afterwards the irritability of the ſyſtem becomes increaſed; according to the third law of irritation above-mentioned, hence the uſe of baths ſomewhat colder than ani⯑mal heat, and of equitation in the open air.

1. FROM the ingurgitation of ſpirituous liquors into the ſtomach and duodenum, the termination of the common bile-duct in that bowel becomes ſtimulated into unnatural action, and a greater quan⯑tity of bile is produced from all the ſecretory veſſels of the liver, by the aſſociation of their motions with thoſe of their excretory ducts; as has been explained in Section XXIV. and XXV. but as all parts of the body, that have been affected with ſtronger ſtimuli for any length of time, become leſs ſuſceptible of motion, from their natural weaker ſtimuli, it follows, that the motions of the ſecretory veſſels, and in conſequence the ſecretion of bile, is leſs than is natural during the intervals of ſobriety. 2. If this ingurgitation of ſpirituous liquors has been daily continued in conſiderable quantity, and is then ſud⯑denly intermitted, a languor or paralyſis of the common bile-duct is induced; the bile is prevented from being poured into the inteſtines; and as the bilious abſorbents are ſtimulated into ſtronger action by its accumulation, and by the acrimony or viſcidity, which it acquires by [348] delay, it is abſorbed, and carried to the receptacle of the chyle; or otherwiſe the ſecretory veſſels of the liver, by the above-mentioned ſtimulus, invert their motions, and regurgitate their contents into the blood, as ſometimes happens to the tears in the lachrymal ſack, ſee Sect. XXIV. 2. 7. and one kind of jaundice is brought on.

There is reaſon to believe, that the bile is moſt frequently returned into the circulation by the inverted motions of theſe hepatic glands, for the bile does not ſeem liable to be abſorbed by the lymphatics, for it ſoaks through the gall-ducts, and is frequently found in the cellular membrane. This kind of jaundice is not generally attended with pain, neither at the extremity of the bile-duct, where it enters the duodenum, nor on the region of the gall-bladder.

Mr. S. a gentleman between 40 and 50 years of age, had had the jaundice about ſix weeks, without pain, ſickneſs, or fever; and had taken emetics, cathartics, mercurials, bitters, chalybeates, eſſential oil, and ether, without apparent advantage. On a ſuppoſition that the obſtruction of the bile might be owing to the paralyſis, or torpid action of the common bile-duct, and the ſtimulants taken into the ſtomach ſeeming to have no effect, I directed half a ſcore ſmart electric ſhocks from a coated bottle, which held about a quart, to be paſſed through the liver, and along the courſe of the common gall⯑duct, as near as could be gueſſed, and on that very day the ſtools became yellow; he continued the electric ſhocks a few days more, and his ſkin gradually became clear.

3. The bilious vomiting and purging, that affects ſome people by intervals of a few weeks, is a leſs degree of this diſeaſe; the bile-duct is leſs irritable than natural, and hence the bile becomes accumulated in the gall-bladder, and hepatic ducts, till by its quantity, acrimony or viſcidity, a greater degree of irritation is produced, and it is ſuddenly evacuated, or laſtly from the abſorption of the more liquid parts of the bile, the remainder becomes inſpiſſated, and chryſtallizes into [349] maſſes too large to paſs, and forms another kind of jaundice, where the bile-duct is not quite paralytic, or has regained its irritability.

This diſeaſe is attended with much pain, which at firſt is felt at the pit of the ſtomach, exactly in the centre of the body, where the bile-duct enters the duodenum; afterwards, when the ſize of the bile-ſtones increaſe, it is alſo felt on the right ſide, where the gall-bladder is ſituated. The former pain at the pit of the ſtomach recurs by in⯑tervals, as the bile-ſtone is puſhed againſt the neck of the duct; like the paroxyſms of the ſtone in the urinary bladder, the other is a more dull and conſtant pain.

Where theſe bile-ſtones are too large to paſs, and the bile-ducts poſſeſs their ſenſibility, this becomes a very painful and hopeleſs diſeaſe. I made the following experiments with a view to their chemical ſolution.

Some fragments of the ſame bile-ſtone were put into the weak ſpirit of marine ſalt, which is ſold in the ſhops, and into ſolution of mild alcali; and into a ſolution of cauſtic alcali; and into oil of tur⯑pentine; without their being diſſolved. All theſe mixtures were after ſome time put into a heat of boiling water, and then the oil of turpentine diſſolved its fragments of bile-ſtone, but no alteration was produced upon thoſe in the other liquids except ſome change of their colour.

Some fragments of the ſame bile-ſtone were put into vitriolic aether, and were quickly diſſolved without additional heat. Might not aether mixed with yolk of egg or with honey be given advan⯑tageouſly in bilious concretions?

I have in two inſtances ſeen from 30 to 50 bile-ſtones come away by ſtool, about the ſize of large peaſe, after having given ſix grains of calomel in the evening, and four ounces of oil of almonds or olives on the ſucceeding morning. I have alſo given half a pint of good olive or almond oil as an emetic during the painful fit, and [350] repeated it in half an hour, if the firſt did not operate, with frequent good effect.

4. Another diſeaſe of the liver, which I have ſeveral times ob⯑ſerved, conſiſts in the inability or paralyſis of the ſecretory veſſels. This diſeaſe has generally the ſame cauſe as the preceding one, the too frequent potation of ſpirituous liquors, or the too ſudden omiſſion of them, after the habit is confined; and is greater or leſs in propor⯑tion, as the whole or a part of the liver is affected, and as the inability or paralyſis is more or leſs complete.

This palſy of the liver is known from theſe ſymptoms, the pa⯑tients have generally paſſed the meridian of life, have drank fer⯑mented liquors daily, but perhaps not been opprobrious drunkards; they loſe their appetite, then their fleſh and ſtrength diminiſh in con⯑ſequence, there appears no bile in their ſtools, nor in their urine, nor is any hardneſs or ſwelling perceptible on the region of the liver. But what is peculiar to this diſeaſe, and diſtinguiſhes it from all others at the firſt glance of the eye, is the bombycinous colour of the ſkin, which, like that of full-grown ſilkworms, has a degree of tranſ⯑parency with a yellow tint not greater than is natural to the ſerum of the blood.

Mr. C. and Mr. B. both very ſtrong men, between 50 and 60 years of age, who had drank ale at their meals inſtead of ſmall beer, but were not reputed hard-drinkers, ſuddenly became weak, loſt their appetite, fleſh, and ſtrength, with all the ſymptoms above enu⯑merated, and died in about two months from the beginning of their malady. Mr. C. became anaſarcous a few days before his death, and Mr. B. had frequent and great haemorrhages from an iſſue, and ſome parts of his mouth, a few days before his death. In both theſe caſes calomel, bitters and chalybeates were repeatedly uſed without effect.

One of the patients deſcribed above, Mr. C, was by trade a plumber; both of them could digeſt no food, and died apparently for [351] want of blood. Might not the transfuſion of blood be uſed in theſe caſes with advantage?

5. When the paralyſis of the hepatic glands is leſs complete, or leſs univerſal, a ſchirroſity of ſome part of the liver is induced; for the ſecretory veſſels retaining ſome of their living power take up a fluid from the circulation, without being ſufficiently irritable to carry it forwards to their excretory ducts; hence the body, or receptacle of each gland, becomes inflated, and this diſtenſion increaſes, till by its very great ſtimulus inflammation is produced, or till thoſe parts of the viſcus become totally paralytic. This diſeaſe is diſtinguiſhable from the foregoing by the palpable hardneſs or largeneſs of the liver; and as the hepatic glands are not totally paralytic, or the whole liver not affected, ſome bile continues to be made. The inflammations of this viſcus, conſequent to the ſchirroſity of it, belong to the diſeaſes of the ſenſitive motions, and will be treated of hereafter.

6. The ancients are ſaid to have poſſeſſed an art of increaſing the livers of geeſe to a ſize greater than the remainder of the gooſe. Martial. l. 13. epig. 58.—This is ſaid to have been done by fat and figs. Horace, l. 2. ſat. 8.—Juvenal ſets theſe large livers before an epicure as a great rarity. Sat. 5. l. 114; and Perſius, ſat. 6. l. 71. Pliny ſays theſe large gooſe-livers were ſoaked in mulled milk, that is, I ſuppoſe, milk mixed with honey and wine; and adds, "that it is uncertain whether Scipio Metellus, of conſular dignity, or M. Seſtius, a Roman knight, was the great diſcoverer of this excellent diſh." A modern traveller, I believe Mr. Brydone, aſſerts that the art of enlarging the livers of geeſe ſtill exiſts in Sicily; and it is to be lamented that he did not import it into his native country, as ſome method of affecting the human liver might perhaps have been col⯑lected from it; beſides the honour he might have acquired in improv⯑ing our giblet pies.

Our wiſer caupones, I am told, know how to fatten their fowls, as well as their geeſe, for the London markets, by mixing gin inſtead [352] of figs and fat with their food; by which they are ſaid to become ſleepy, and to fatten apace, and probably acquire enlarged livers; as the ſwine are aſſerted to do, which are fed on the ſediments of barrels in the diſtilleries; and which ſo frequently obtains in thoſe, who in⯑gurgitate much ale, or wine, or drams.

II. The irritative diſeaſes of the kidneys, pancreas, ſpleen, and other glands, are analogous to thoſe of the liver above deſcribed, differing only in the conſequences attending their inability to action. For inſtance, when the ſecretory veſſels of the kidneys become diſ⯑obedient to the ſtimulus of the paſſing current of blood, no urine is ſeparated or produced by them; their excretory mouths become filled with concreted mucus, or calculus matter, and in eight or ten days ſtupor and death ſupervenes in conſequence of the retention of the feculent part of the blood.

This diſeaſe in a ſlighter degree, or when only a part of the kidney is affected, is ſucceeded by partial inflammation of the kidney in con⯑ſequence of previous torpor. In that caſe greater actions of the ſecretory veſſels occur, and the nucleus of gravel is formed by the in⯑flamed mucous membranes of the tubuli uriniferi, as farther explained in its place.

This torpor, or paralyſis of the ſecretory veſſels of the kidneys, like that of the liver, owes its origin to their being previouſly habi⯑bituated to too great ſtimulus; which in this country is generally owing to the alcohol contained in ale or wine; and hence muſt be regiſtered amongſt the diſeaſes owing to inebriety; though it may be cauſed by whatever occaſionally inflames the kidney; as too violent riding on horſeback, or the cold from a damp bed, or by ſleeping on the cold ground; or perhaps by drinking in general too little aqueous fluids.

III. I ſhall conclude this ſection on the diſeaſes of the liver induced by ſpirituous liquors, with the well-known ſtory of Prometheus, which ſeems indeed to have been invented by phyſicians in thoſe [353] ancient times, when all things were clothed in hieroglyphic, or in fable. Prometheus was painted as ſtealing fire from heaven, which might well repreſent the inflammable ſpirit produced by fermentation; which may be ſaid to animate or enliven the man of clay: whence the conqueſts of Bacchus, as well as the temporary mirth and noiſe of his devotees. But the after puniſhment of thoſe, who ſteal this ac⯑curſed fire, is a vulture gnawing the liver; and well allegoriſes the poor inebriate lingering for years under painful hepatic diſ⯑eaſes. When the expediency of laying a further tax on the diſtilla⯑tion of ſpirituous liquors from grain was canvaſſed before the Houſe of Commons ſome years ago, it was ſaid of the diſtillers, with great truth, "They take the bread from the people, and convert it into poiſon!" Yet is this manufactory of diſeaſe permitted to continue, as appears by its paying into the treaſury above 900,000l. near a million of money annually. And thus, under the names of rum, brandy, gin, whiſky, uſquebaugh, wine, cyder, beer, and porter, alcohol is become the bane of the Chriſtian world, as opium of the Mahometan.

I. The Temperament of decreaſed Irritability.

THE diſeaſes, which are cauſed by irritation, moſt frequently ori⯑ginate from the defect of it; for thoſe, which are immediately owing to the exceſs of it, as the hot fits of fever, are generally occaſioned by an accumulation of ſenſorial power in conſequence of a previous defect of irritation, as in the preceding cold fits of fever. Whereas the diſeaſes, which are cauſed by ſenſation and volition, moſt fre⯑quently originate from the exceſs of thoſe ſenſorial powers, as will be explained below.

The temperament of decreaſed irritability appears from the follow⯑ing circumſtances, which ſhew that the muſcular fibres or organs of ſenſe are liable to become torpid or quieſcent from leſs defect of ſti⯑mulation than is productive of torpor or quieſcence in other con⯑ſtitutions.

1. The firſt is the weak pulſe, which in ſome conſtitutions is at the ſame time quick. 2. The next moſt marked criterion of this tem⯑perament is the largeneſs of the aperture of the iris, or pupil of the eye, which has been reckoned by ſome a beautiful feature in the fe⯑male countenance, as an indication of delicacy, but to an experienced obſerver it is an indication of debility, and is therefore a defect, not an excellence. The third moſt marked circumſtance in this conſti⯑tution is, that the extremities, as the hands and feet, or noſe and ears, are liable to become cold and pale in ſituations in reſpect to warmth, where thoſe of greater ſtrength are not affected. Thoſe of this temperament are ſubject to hyſteric affections, nervous fevers, hydrocephalus, ſcrophula, and conſumption, and to all other diſeaſes of debility.

Thoſe, who poſſeſs this kind of conſtitution, are popularly ſup⯑poſed to be more irritable than is natural, but are in reality leſs ſo. [356] This miſtake has ariſen from their generally having a greater quick⯑neſs of pulſe, as explained in Sect. XII. 1. 4. XII. 3. 3.; but this fre⯑quency of pulſe is not neceſſary to the temperament, like the debility of it.

Perſons of this temperament are frequently found amongſt the ſofter ſex, and amongſt narrow-ſhouldered men; who are ſaid to bear labour worſe, and pain better than others. This laſt circumſtance is ſuppoſed to have prevented the natives of North America from hav⯑ing been made ſlaves of by the Europeans. They are a narrow⯑ſhouldered race of people, and will rather expire under the laſh, than be made to labour. Some nations of Aſia have ſmall hands, as may be ſeen by the handles of their ſcymetars; which with their narrow ſhoulders ſhew, that they have not been accuſtomed to ſo great la⯑bour with their hands and arms, as the European nations in agricul⯑ture, and thoſe on the coaſts of Africa in ſwimming and rowing. Dr. Maningham, a popular accoucheur in the beginning of this century, obſerves in his aphoriſms, that broad-ſhouldered men pro⯑create broad-ſhouldered children. Now as labour ſtrengthens the muſcles employed, and increaſes their bulk, it would ſeem that a few generations of labour or of indolence may in this reſpect change the form and temperament of the body.

On the contrary, thoſe who are happily poſſeſſed of a great degree of irritability, bear labour better than pain; and are ſtrong, active, and ingenious. But there is not properly a temperament of in⯑creaſed irritability tending to diſeaſe, becauſe an increaſed quantity of irritative motions generally induces an increaſe of pleaſure or pain, as in intoxication, or inflammation; and then the new motions are the immediate conſequences of increaſed ſenſation, not of increaſed irritation; which have hence been ſo perpetually confounded with each other.

II. Temperament of Senſibility.

THERE is not properly a temperament, or prediſpoſition to diſeaſe, from decreaſed ſenſibility, ſince irritability and not ſenſibility is im⯑mediately neceſſary to bodily health. Hence it is the exceſs of ſen⯑ſation alone, as it is the defect of irritation, that moſt frequently pro⯑duces diſeaſe. This temperament of increaſed ſenſibility is known from the increaſed activity of all thoſe motions of the organs of ſenſe and muſcles, which are exerted in conſequence of pleaſure or pain, as in the beginning of drunkenneſs, and in inflammatory fever. Hence thoſe of this conſtitution are liable to inflammatory diſeaſes, as hepa⯑titis; and to that kind of conſumption which is hereditary, and com⯑mences with ſlight repeated hoemoptoe. They have high-coloured lips, frequently dark hair and dark eyes with large pupils, and are in that caſe ſubject to gutta ſerena. They are liable to enthuſiaſm, de⯑lirium, and reverie. In this laſt circumſtance they are liable to ſtart at the clapping of a door; becauſe the more intent any one is on the paſſing current of his ideas, the greater ſurpriſe he experiences on their being diſſevered by ſome external violence, as explained in Sect. XIX. on reverie.

As in theſe conſtitutions more than the natural quantities of ſen⯑ſitive motions are produced by the increaſed quantity of ſenſation exiſting in the habit, it follows, that the irritative motions will be performed in ſome degree with leſs energy, owing to the great ex⯑penditure of ſenſorial power on the ſenſitive ones. Hence thoſe of this temperament do not attend to ſlight ſtimulations, as explained in Sect. XIX. But when a ſtimulus is ſo great as to excite ſenſation, it produces greater ſenſitive actions of the ſyſtem than in others; ſuch as dehrium or inflammation. Hence they are liable to be abſent in company; fit or lie long in one poſture; and in winter have the ſkin [358] of their legs burnt into various colours by the fire. Hence alſo they are fearful of pain; covet muſic and ſleep; and delight in poetry and romance.

As the motions in conſequence of ſenſation are more than natural, it alſo happens from the greater expenditure of ſenſorial power on them, that the voluntary motions are leſs eaſily exerted. Hence the ſubjects of this temperament are indolent in reſpect to all voluntary exertions, whether of mind or body.

A race of people of this deſcription ſeems to have been found by the Spaniards in the iſlands of America, where they firſt landed, ten of whom are ſaid not to have conſumed more food than one Spaniard, nor to have been capable of more than one tenth of the exertion of a Spaniard. Robertſon's Hiſtory.—In a ſtate ſimilar to this the greateſt part of the animal world paſs their lives, between ſleep or inactive reverie, except when they are excited by the call of hunger.

III. The Temperament of increaſed Voluntarity.

THOSE of this conſtitution differ from both the laſt mentioned in this, that the pain, which gradually ſubſides in the firſt, and is pro⯑ductive of inflammation or delirium in the ſecond, is in this ſucceded by the exertion of the muſcles or ideas, which are moſt frequently connected with volition; and they are thence ſubject to locked jaw, convulſions, epilepſy, and mania, as explained in Sect. XXXIV. Thoſe of this temperament attend to the ſlighteſt irritations or ſenſations, and immediately exert themſelves to obtain or avoid the objects of them; they can at the ſame time bear cold and hunger better than others, of which Charles the Twelfth of Sweden was an inſtance. They are ſuited and generally prompted to all great exertions of ge⯑nius or labour, as their deſires are more extenſive and more vehement, and their powers of attention and of labour greater. It is this facility [359] of voluntary exertion, which diſtinguiſhes men from brutes, and which has made them lords of the creation.

IV. The Temperament of increaſed Aſſociation.

THIS conſtitution conſiſts in the too great facility, with which the fibrous motions acquire habits of aſſociation, and by which theſe aſ⯑ſociations become proportionably ſtronger than in thoſe of the other temperaments. Thoſe of this temperament are ſlow in voluntary exertions, or in thoſe dependent on ſenſation, or on irritation. Hence great memories have been ſaid to be attended with leſs ſenſe and leſs imagination from Ariſtotle down to the preſent time; for by the word memory theſe writers only underſtood the unmeaning repetition of words or numbers in the order they were received, without any vo⯑luntary efforts of the mind.

In this temperament thoſe aſſociations of motions, which are com⯑monly termed ſympathies, act with greater certainty and energy, as thoſe between diſturbed viſion and the inverſion of the motion of the ſtomach, as in ſea-ſickneſs; and the pains in the ſhoulder from hepatic inflammation. Add to this, that the catenated circles of actions are of greater extent than in the other conſtitutions. Thus if a ſtrong vo⯑mit or cathartic be exhibited in this temperament, a ſmaller quantity will produce as great an effect, if it be given ſome weeks afterwards; whereas in other temperaments this is only to be expected, if it be exhibited in a few days after the firſt doſe. Hence quartan agues are formed in thoſe of this temperament, as explained in Section XXXII. on diſeaſes from irritation, and other intermittents are liable to recur from ſlight cauſes many weeks after they have been cured by the bark.

V. The firſt of theſe temperaments differs from the ſtandard of health from defect, and the others from exceſs of ſenſorial power; but it ſometimes happens that the ſame individual, from the changes introduced into his habit by the different ſeaſons of the year, modes or periods of life, or by accidental diſeaſes, paſſes from one of theſe temperaments to another. Thus a long uſe of too much fermented liquor produces the temperament of increaſed ſenſibility; great indo⯑lence and ſolitude that of decreaſed irritability; and want of the ne⯑ceſſaries of life that of increaſed voluntarity.

I. WHEN the contractile ſides of the heart and arteries perform a greater number of pulſations in a given time, and move through a greater area at each pulſation, whether theſe motions are occaſioned by the ſtimulus of the acrimony or quantity of the blood, or by their aſſociation with other irritative motions, or by the increaſed irritability [362] of the arterial ſyſtem, that is, by an increaſed quantity of ſenſorial power, one kind of fever is produced; which may be called Synocha irritativa, or Febris irritativa pulſu forti, or irritative fever with ſtrong pulſe.

When the contractile ſides of the heart and arteries perform a greater number of pulſations in a given time, but move through a much leſs area at each pulſation, whether theſe motions are occaſioned by defect of their natural ſtimuli, or by the defect of other irritative motions with which they are aſſociated, or from the inirritability of the arterial ſyſtem, that is, from a decreaſed quantity of ſenſorial power, another kind of fever ariſes; which may be termed, Typhus irritativus, or Febris irritativa pulſu debili, or irritative fever with weak pulſe. The former of theſe fevers is the ſynocha of noſologiſts, and the latter the typhus mitior, or nervous fever. In the former there appears to be an increaſe of ſenſorial power, in the latter a deficiency of it; which is ſhewn to be the immediate cauſe of ſtrength and weakneſs, as defined in Sect. XII. 1. 3.

It ſhould be added, that a temporary quantity of ſtrength or debility may be induced by the defect or exceſs of ſtimulus above what is natural; and that in the ſame fever debility always exiſts during the cold fit, though ſtrength does not always exiſt during the hot fit.

Theſe fevers are always connected with, and generally induced by, the diſordered irritative motions of the organs of ſenſe, or of the in⯑teſtinal canal, or of the glandular ſyſtem, or of the abſorbent ſyſtem; and hence are always complicated with ſome or many of theſe diſ⯑ordered motions, which are termed the ſymptoms of the fever, and which compoſe the great variety in theſe diſeaſes.

The irritative fevers both with ſtrong and with weak pulſe, as well as the ſenſitive fevers with ſtrong and with weak pulſe, which are to be deſcribed in the next ſection, are liable to periodical remiſſions, and then they take the name of intermittent fevers, and are diſtinguiſhed by the periodical times of their acceſs.

II. For the better illuſtration of the phenomena of irritative fevers we muſt refer the reader to the circumſtances of irritation ex⯑plained in Sect. XII. and ſhall commence this intricate ſubject by ſpeaking of the quick pulſe, and proceed by conſidering many of the cauſes, which either ſeparately or in combination moſt frequently produce the cold fits of fevers.

1. If the arteries are dilated but to half their uſual diameters, though they contract twice as frequently in a given time, they will circulate only half their uſual quantity of blood; for as they are cy⯑linders, the blood which they contain muſt be as the ſquares of their diameters. Hence when the pulſe becomes quicker and ſmaller in the ſame proportion, the heart and arteries act with leſs energy than in their natural ſtate. See Sect. XII. 1. 4.

That this quick ſmall pulſe is owing to want of irritability, ap⯑pears, firſt, becauſe it attends other ſymptoms of want of irritability; and, ſecondly, becauſe on the application of a ſtimulus greater than uſual, it becomes ſlower and larger. Thus in cold fits of agues, in hyſteric palpitations of the heart, and when the body is much ex⯑hauſted by haemorrhages, or by fatigue, as well as in nervous fevers, the pulſe becomes quick and ſmall; and ſecondly, in all thoſe caſes if an increaſe of ſtimulus be added, by giving a little wine or opium; the quick ſmall pulſe becomes ſlower and larger, as any one may eaſily experience on himſelf, by counting his pulſe after drinking one or two glaſſes of wine, when he is faint from hunger or fatigue.

Now nothing can ſo ſtrongly evince that this quick ſmall pulſe is owing to defect of irritability, than that an additional ſtimulus, above what is natural, makes it become ſlower and larger immediately: for what is meant by a defect of irritability, but that the arteries and heart are not excited into their uſual exertions by their uſual quantity of ſtimulus? but if you increaſe the quantity of ſtimulus, and they immediately act with their uſual energy, this proves their previous want of their natural degree of irritability. Thus the trembling [364] hands of drunkards in a morning become ſteady, and acquire ſtrength to perform their uſual offices, by the accuſtomed ſtimulus of a glaſs or two of brandy.

2. In ſleep and in apoplexy the pulſe becomes ſlower, which is not owing to defect of irritability, for it is at the ſame time larger; and thence the quantity of the circulation is rather increaſed than diminiſhed. In theſe caſes the organs of ſenſe are cloſed, and the voluntary power is ſuſpended, while the motions dependent on internal irritations, as thoſe of digeſtion and ſecretion, are carried on with more than their uſual vigour; which has led ſuperficial obſervers to confound theſe caſes with thoſe ariſing from want of irritability. Thus if you lift up the eyelid of an apoplectic patient, who is not actually dying, the iris will, as uſual, contract itſelf, as this motion is aſſociated with the ſtimulus of light; but it is not ſo in the laſt ſtages of nervous fevers, where the pupil of the eye continues expanded in the broad day-light: in the former caſe there is a want of voluntary power, in the latter a want of irritability.

Hence alſo thoſe conſtitutions which are deficient in quantity of irritability, and which poſſeſs too great ſenſibility, as during the pain of hunger, of hyſteric ſpaſms, or nervous headachs, are generally ſuppoſed to have too much irritability; and opium, which in its due doſe is a moſt powerful ſtimulant, is erroneouſly called a ſedative; becauſe by increaſing the irritative motions it decreaſes the pains ariſing from defect of them.

Why the pulſe ſhould become quicker both from an increaſe of irritation, as in the ſynocha irritativa, or irritative fever with ſtrong pulſe; and from the decreaſe of it, as in the typhus irritativus, or irri⯑tative fever with weak pulſe; ſeems paradoxical. The former cir⯑cumſtance needs no illuſtration; ſince if the ſtimulus of the blood, or the irritability of the ſanguiferous ſyſtem be increaſed, and the ſtrength of the patient not diminiſhed, it is plain that the motions muſt be performed quicker and ſtronger.

[365] In the latter circumſtance the weakneſs of the muſcular power of the heart is ſoon over-balanced by the elaſticity of the coats of the ar⯑teries, which they poſſeſs beſides a muſcular power of contraction; and hence the arteries are diſtended to leſs than their uſual diameters. The heart being thus ſtopped when it is but half emptied, begins ſooner to dilate again; and the arteries being dilated to leſs than their uſual diameters, begin ſo much ſooner to contract themſelves; inſo⯑much, that in the laſt ſtages of fevers with weakneſs the frequency of pulſation of the heart and arteries becomes doubled; which, however, is never the caſe in fevers with ſtrength, in which they ſeldom ex⯑ceed 118 or 120 pulſations in a minute. It muſt be added, that in theſe caſes, while the pulſe is very ſmall and very quick, the heart often feels large, and labouring to one's hand; which coincides with the above explanation, ſhewing that it does not completely empty itſelf.

3. In caſes however of debility from paucity of blood, as in ani⯑mals which are bleeding to death in the ſlaughter-houſe, the quick pulſations of the heart and arteries may be owing to their not being diſtended to more than half their uſual diaſtole; and in conſequence they muſt contract ſooner, or more frequently, in a given time. As weak people are liable to a deficient quantity of blood, this cauſe may occaſionally contribute to quicken the pulſe in fevers with debility, which may be known by applying one's hand upon the heart as above; but the principal cauſe I ſuppoſe to conſiſt in the diminution of ſenſorial power. When a muſcle contains, or is ſupplied with but little ſenſorial power, its contraction ſoon ceaſes, and in conſe⯑quence may ſoon recur, as is ſeen in the trembling hands of people weakened by age or by drunkenneſs. See Sect. XII. 1. 4. XII. 3. 4.

It may nevertheleſs frequently happen, that both the deficiency of ſtimulus, as where the quantity of blood is leſſened (as deſcribed in No. 4. of this ſection), and the deficiency of ſenſorial power, as in [366] thoſe of the temperament of inirritability, deſcribed in Sect. XXXI. occur at the ſame time; which will thus add to the quickneſs of the pulſe and to the danger of the diſeaſe.

III. 1. A certain degree of heat is neceſſary to muſcular motion, and is, in conſequence, eſſential to life. This is obſerved in thoſe ani⯑mals and inſects which paſs the cold ſeaſon in a torpid ſtate, and which revive on being warmed by the fire. This neceſſary ſtimulus of heat has two ſources; one from the fluid atmoſphere of heat, in which all things are immerſed, and the other from the internal combinations of the particles, which form the various fluids, which are produced in the extenſive ſyſtems of the glands. When either the external heat, which ſurrounds us, or the internal production of it, becomes leſſened to a certain degree, the pain of cold is perceived.

This pain of cold is experienced moſt ſenſibly by our teeth, when ice is held in the mouth; or by our whole ſyſtem after having been previouſly accuſtomed to much warmth. It is probable, that this pain does not ariſe from the mechanical or chemical effects of a defi⯑ciency of heat; but that, like the organs of ſenſe by which we per⯑ceive hunger and thirſt, this ſenſe of heat ſuffers pain, when the ſtimu⯑lus of its object is wanting to excite the irritative motions of the or⯑gan; that is, when the ſenſorial power becomes too much accumu⯑lated in the quieſcent fibres. See Sect. XII. 5. 3. For as the pe⯑riſtaltic motions of the ſtomach are leſſened, when the pain of hunger is great, ſo the action of the cutaneous capillaries are leſſened during the pain of cold; as appears by the paleneſs of the ſkin, as explained in Sect. XIV. 6. on the production of ideas.

The pain in the ſmall of the back and forehead in the cold ſits of the ague, in nervous hemicrania, and in hyſteric paroxyſms, when all the irritative motions are much impaired, ſeems to ariſe from this cauſe; the veſſels of theſe membranes or muſcles become torpid by their irritative aſſociations with other parts of the body, and thence [367] produce leſs of their accuſtomed ſecretions, and in conſequence leſs heat is evolved, and they experience the pain of cold; which cold⯑neſs may often be felt by the hand applied upon the affected part.

2. The importance of a greater or leſs deduction of heat from the ſyſtem will be more eaſy to comprehend, if we firſt conſider the great expenſe of ſenſorial power uſed in carrying on the vital motions; that is, which circulates, abſorbs, ſecretes, aerates, and elaborates the whole maſs of fluids with unceaſing aſſiduity. The ſenſorial power, or ſpirit of animation, uſed in giving perpetual and ſtrong motion to the heart, which overcomes the elaſticity and vis inertiae of the whole arterial ſyſtem; next the expenſe of ſenſorial power in moving with great force and velocity the innumerable trunks and ramifications of the arterial ſyſtem; the expenſe of ſenſorial power in circulating the whole maſs of blood through the long and intricate intortions of the very fine veſſels, which compoſe the glands and capillaries; then the expenſe of ſenſorial power in the exertions of the abſorbent extremi⯑ties of all the lacteals, and of all the lymphatics, which open their mouths on the external ſurface of the ſkin, and on the internal ſur⯑faces of every cell or interſtice of the body; then the expenſe of ſen⯑ſorial power in the venous abſorption, by which the blood is received from the capillary veſſels, or glands, where the arterial power ceaſes, and is drank up, and returned to the heart; next the expenſe of ſen⯑ſorial power uſed by the muſcles of reſpiration in their office of perpetu⯑ally expanding the bronchia, or air-veſſels, of the lungs; and laſtly in the unceaſing periſtaltic motions of the ſtomach and whole ſyſtem of inteſtines, and in all the ſecretions of bile, gaſtric juice, mucus, per⯑ſpirable matter, and the various excretions from the ſyſtem. If we conſider the ceaſeleſs expenſe of ſenſorial power thus perpetually em⯑ployed, it will appear to be much greater in a day than all the volun⯑tary exertions of our muſcles and organs of ſenſe conſume in a week; and all this without any ſenſible fatigue! Now, if but a part of theſe vital motions are impeded, or totally ſtopped for but a ſhort time, we [368] gain an idea, that there muſt be a great accumulation of ſenſorial power; as its production in theſe organs, which are ſubject to perpe⯑tual activity, is continued during their quieſcence, and is in conſe⯑quence accumulated.

While, on the contrary, where thoſe vital organs act too forcibly by increaſe of ſtimulus without a proportionally-increaſed production of ſenſorial power in the brain, it is evident, that a great deficiency of action, that is torpor, muſt ſoon follow, as in fevers; whereas the locomotive muſcles, which act only by intervals, are neither liable to ſo great accumulation of ſenſorial power during their times of inacti⯑vity, nor to ſo great an exhauſtion of it during their times of action.

Thus, on going into a very cold bath, ſuppoſe at 33 degrees of heat on Fahrenheit's ſcale, the action of the ſubcutaneous capillaries, or glands, and of the mouths of the cutaneous abſorbents is diminiſhed, or ceaſes for a time. Hence leſs or no blood paſſes theſe capillaries, and paleneſs ſucceeds. But ſoon after emerging from the bath, a more florid colour and a greater degree of heat is generated on the ſkin than was poſſeſſed before immerſion; for the capillary glands, after this quieſcent ſtate, occaſioned by the want of ſtimulus, become more irritable than uſual to their natural ſtimuli, owing to the accu⯑mulation of ſenſorial power, and hence a greater quantity of blood is tranſmitted through them, and a greater ſecretion of perſpirable mat⯑ter; and, in conſequence, a greater degree of heat ſucceeds. During the continuance in cold water the breath is cold, and the act of re⯑ſpiration quick and laborious; which have generally been aſcribed to the obſtruction of the circulating fluid by a ſpaſm of the cutaneous veſſels, and by a conſequent accumulation of blood in the lungs, oc⯑caſioned by the preſſure as well as by the coldneſs of the water. This is not a ſatisfactory account of this curious phaenomenon, ſince at this time the whole circulation is leſs, as appears from the ſmallneſs of the pulſe and coldneſs of the breath; which ſhew that leſs blood paſſes through the lungs in a given time; the ſame laborious breathing [369] immediately occurs when the paleneſs of the ſkin is produced by fear, where no external cold or preſſure are applied.

The minute veſſels of the bronchia, through which the blood paſſes from the arterial to the venal ſyſtem, and which correſpond with the cutaneous capillaries, have frequently been expoſed to cold air, and become quieſcent along with thoſe of the ſkin; and hence their mo⯑tions are ſo aſſociated together, that when one is affected either with quieſcence or exertion, the other ſympathizes with it, according to the laws of irritative aſſociation. See Sect. XXVII. 1. on haemor⯑rhages.

Beſides the quieſcence of the minute veſſels of the lungs, there are many other ſyſtems of veſſels which become torpid from their irrita⯑tive aſſociations with thoſe of the ſkin, as the abſorbents of the blad⯑der and inteſtines; whence an evacuation of pale urine occurs, when the naked ſkin is expoſed only to the coldneſs of the atmoſphere; and ſprinkling the naked body with cold water is known to remove even pertinacious conſtipation of the bowels. From the quieſcence of ſuch extenſive ſyſtems of veſſels as the glands and capillaries of the ſkin, and the minute veſſels of the lungs, with their various abſorb⯑ent ſeries of veſſels, a great accumulation of ſenſorial powers is occa⯑ſioned; part of which is again expended in the increaſed exertion of all theſe veſſels, with an univerſal glow of heat in conſequence of this exertion, and the remainder of it adds vigour to both the vital and voluntary exertions of the whole day.

If the activity of the ſubcutaneous veſſels, and of thoſe with which their actions are aſſociated, was too great before cold immerſion, as in the hot days of ſummer, and by that means the ſenſorial power was previ⯑ouſly diminiſhed, we ſee the cauſe why the cold bath gives ſuch preſent ſtrength; namely, by ſtopping the unneceſſary activity of the ſubcu⯑taneous veſſels, and thus preventing the too great exhauſtion of ſen⯑ſorial power; which, in metaphorical language, has been called bracing the ſyſtem: which is, however, a mechanical term, only applicable [370] to drums, or muſical ſtrings: as on the contrary the word relaxa⯑tion, when applied to living animal bodies, can only mean too ſmall a quantity of ſtimulus, or too ſmall a quantity of ſenſorial power; as explained in Sect. XII. 1.

3. This experiment of cold bathing preſents us with a ſimple fe⯑ver-fit; for the pulſe is weak, ſmall, and quick during the cold im⯑merſion; and becomes ſtrong, full, and quick during the ſubſequent glow of heat; till in a few minutes theſe ſymptoms ſubſide, and the temporary fever ceaſes.

In thoſe conſtitutions where the degree of inirritability, or of debi⯑lity, is greater than natural, the coldneſs and paleneſs of the ſkin with the quick and weak pulſe continue a long time after the patient leaves the bath; and the ſubſequent heat approaches by unequal fluſhings, and he feels himſelf diſordered for many hours. Hence the bathing in a cold ſpring of water, where the heat is but forty-eight degrees on Fahrenheit's thermometer, much diſagrees with thoſe of weak or inirri⯑table habits of body; who poſſeſs ſo little ſenſorial power, that they cannot without injury bear to have it diminiſhed even for a ſhort time; but who can nevertheleſs bear the more temperate coldneſs of Buxton bath, which is about eighty degrees of heat, and which ſtrengthens them, and makes them by habit leſs liable to great quieſ⯑cence from ſmall variations of cold, and thence leſs liable to be diſor⯑dered by the unavoidable accidents of life. Hence it appears, why people of theſe inirritable conſtitutions, which is another expreſſion for ſenſorial deficiency, are often much injured by bathing in a cold ſpring of water; and why they ſhould continue but a very ſhort time in baths, which are colder than their bodies; and ſhould gradually in⯑creaſe both the degree of coldneſs of the water, and the time of their continuance in it, if they would obtain ſalutary effects from cold im⯑merſions. See Sect. XII. 2. 1.

On the other hand in all caſes where the heat of the external ſur⯑face of the body, or of the internal ſurface of the lungs, is greater than [371] natural, the uſe of expoſure to cool air may be deduced. In fever⯑fits attended with ſtrength, that is with great quantity of ſenſorial power, it removes the additional ſtimulus of heat from the ſurfaces above mentioned, and thus prevents their exceſs of uſeleſs motion; and in fever-fits attended with debility, that is with a deficiency of the quantity of ſenſorial power, it prevents the great and dangerous waſte of ſenſorial power expended in the unneceſſary increaſe of the actions of the glands and capillaries of the ſkin and lungs.

4. In the ſame manner, when any one is long expoſed to very cold air, a quieſcence is produced of the cutaneous and pulmonary ca⯑pillaries and abſorbents, owing to the deficiency of their uſual ſtimu⯑lus of heat; and this quieſcence of ſo great a quantity of veſſels af⯑fects, by irritative aſſociation, the whole abſorbent and glandular ſyſ⯑tem, which becomes in a greater or leſs degree quieſcent, and a cold fit of fever is produced.

If the deficiency of the ſtimulus of heat is very great, the quieſ⯑cence becomes ſo general as to extinguiſh life, as in thoſe who are fro⯑zen to death.

If the deficiency of heat be in leſs degree, but yet ſo great as in ſome meaſure to diſorder the ſyſtem, and ſhould occur the ſucceeding day, it will induce a greater degree of quieſcence than before, from its acting in concurrence with the period of the diurnal circle of ac⯑tions, explained in Sect. XXXVI. Hence from a ſmall beginning a greater and greater degree of quieſcence may be induced, till a com⯑plete fever-fit is formed; and which will continue to recur at the pe⯑riods by which it was produced. See Sect. XVII. 3. 6.

If the degree of quieſcence occaſioned by defect of the ſtimulus of heat be very great, it will recur a ſecond time by a ſlighter cauſe, than that which firſt induced it. If the cauſe, which induces the ſecond fit of quieſcence, recurs the ſucceeding day, the quotidian fever is pro⯑duced; if not till the alternate day, the tertian fever; and if not till after ſeventy-two hours from the firſt fit of quieſcence, the quartan [372] fever is formed. This laſt kind of fever recurs leſs frequently than the other, as it is a diſeaſe only of thoſe of the temperament of aſſoci⯑ability, as mentioned in Sect. XXXI.; for in other conſtitutions the capability of forming a habit ceaſes, before the new cauſe of quieſcence is again applied, if that does not occur ſooner than in ſeventy-two hours.

And hence thoſe fevers, whoſe cauſe is from cold air of the night or morning, are more liable to obſerve the ſolar day in their periods; while thoſe from other cauſes frequently obſerve the lunar day in their periods, their paroxyſms returning near an hour later every day, as explained in Sect. XXXVI.

IV. Another frequent cauſe of the cold fits of fever is the defect of the ſtimulus of diſtention. The whole arterial ſyſtem would appear, by the experiments of Haller, to be irritable by no other ſtimulus, and the motions of the heart and alimentary canal are certainly in ſome meaſure dependant on the ſame cauſe. See Sect. XIV. 7. Hence there can be no wonder, that the diminution of diſtention ſhould fre⯑quently induce the quieſcence, which conſtitutes the beginning of fe⯑ver-fits.

Monſieur Leiutaud has judiciouſly mentioned the deficiency of the quantity of blood amongſt the cauſes of diſeaſes, which he ſays is fre⯑quently evident in diſſections: fevers are hence brought on by great haemorrhages, diarrhoeas, or other evacuations; or from the continued uſe of diet, which contains but little nouriſhment; or from the exhauſ⯑tion occaſioned by violent fatigue, or by thoſe chronic diſeaſes in which the digeſtion is much impaired; as where the ſtomach has been long affected with the gout or ſchirrus; or in the paralyſis of the liver, as deſcribed in Sect. XXX. Hence a paroxyſm of gout is liable to recur on bleeding or purging; as the torpor of ſome viſcus, which precedes the inflammation of the foot, is thus induced by the want of the ſtimulus of diſtention. And hence the extremities of the body, as the noſe and fingers, are more liable to become cold, [373] when we have long abſtained from food; and hence the pulſe is in⯑creaſed both in ſtrength and velocity above the natural ſtandard after a full meal by the ſtimulus of diſtention.

However, this ſtimulus of diſtention, like the ſtimulus of heat above deſcribed, though it contributes much to the due action not only of the heart, arteries, and alimentary canal, but ſeems neceſſary to the proper ſecretion of all the various glands; yet perhaps it is not the ſole cauſe of any of theſe numerous motions: for as the lacteals, cu⯑taneous abſorbents, and the various glands appear to be ſtimulated into action by the peculiar pungency of the fluids they abſorb, ſo in the inteſtinal canal the pungency of the digeſting aliment, or the acri⯑mony of the faeces, ſeem to contribute, as well as their bulk, to pro⯑mote the periſtaltic motions; and in the arterial ſyſtem, the mo⯑mentum of the particles of the circulating blood, and their acrimony, ſtimulate the arteries, as well as the diſtention occaſioned by it. Where the pulſe is ſmall this defect of diſtention is preſent, and con⯑tributes much to produce the febris irritativa pulſu debili, or irritative fever with weak pulſe, called by modern writers nervous fever, as a prediſponent cauſe. See Sect. XII. 1. 4. Might not the transfuſion of blood, ſuppoſe of four ounces daily from a ſtrong man, or other healthful animal, as a ſheep or an aſs, be uſed in the early ſtate of ner⯑vous or putrid fevers with great proſpect of ſucceſs?

V. The defect of the momentum of the particles of the circu⯑lating blood is another cauſe of the quieſcence, with which the cold fits of fever commence. This ſtimulus of the momentum of the progreſſive particles of the blood does not act over the whole body like thoſe of heat and diſtention above deſcribed, but is confined to the arterial ſyſtem; and differs from the ſtimulus of the diſtention of the blood, as much as the vibration of the air does from the currents of it. Thus are the different organs of our bodies ſtimulated by four different mechanic properties of the external world: the ſenſe of touch by the preſſure of ſolid bodies ſo as to diſtinguiſh their figure; [374] the muſcular ſyſtem by the diſtention, which they occaſion; the in⯑ternal ſurface of the arteries, by the momentum of their moving par⯑ticles; and the auditory nerves, by the vibration of them: and theſe four mechanic properties are as different from each other as the vari⯑ous chemical ones, which are adapted to the numerous glands, and to the other organs of ſenſe.

2. The momentum of the progreſſive particles of blood is com⯑pounded of their velocity and their quantity of matter: hence what⯑ever circumſtances diminiſh either of theſe without proportionally in⯑creaſing the other, and without ſuperadding either of the general ſti⯑muli of heat or diſtention, will tend to produce a quieſcence of the ar⯑terial ſyſtem, and from thence of all the other irritative motions, which are connected with it.

Hence in all thoſe conſtitutions or diſeaſes where the blood con⯑tains a greater proportion of ſerum, which is the lighteſt part of its compoſition, the pulſations of the arteries are weaker, as in nervous fevers, chloroſis, and hyſteric complaints; for in theſe caſes the mo⯑mentum of the progreſſive particles of blood is leſs: and hence, where the denſer parts of its compoſition abound, as the red part of it, or the coagulable lymph, the arterial pulſations are ſtronger; as in thoſe of robuſt health, and in inflammatory diſeaſes.

That this ſtimulus of the momentum of the particles of the circu⯑lating fluid is of the greateſt conſequence to the arterial action, appears from the experiment of injecting air into the blood veſſels, which ſeems to deſtroy animal life from the want of this ſtimulus of momentum; for the diſtention of the arteries is not diminiſhed by it, it poſſeſſes no corroſive acrimony, and is leſs liable to repaſs the valves than the blood itſelf; ſince air-valves in all machinery require much leſs accuracy of conſtruction than thoſe which are oppoſed to water.

3. One method of increaſing the velocity of the blood, and in con⯑ſequence the momentum of its particles, is by the exerciſe of the body, or by the friction of its ſurface: ſo, on the contrary, too great [375] indolence contributes to decreaſe this ſtimulus of the momentum of the particles of the circulating blood, and thus tends to induce quieſ⯑cence; as is ſeen in hyſteric caſes, and chloroſis, and the other diſeaſes of ſedentary people.

4. The velocity of the particles of the blood in certain circum⯑ſtances is increaſed by veneſection, which, by removing a part of it, diminiſhes the reſiſtance to the motion of the other part, and hence the momentum of the particles of it is increaſed. This may be eaſily un⯑derſtood by conſidering it in the extreme, ſince, if the reſiſtance was greatly increaſed, ſo as to overcome the propelling power, there could be no velocity, and in conſequence no momentum at all. From this circumſtance ariſes that curious phaenomenon, the truth of which I have been more than once witneſs to, that veneſection will often in⯑ſtantaneouſly relieve thoſe nervous pains, which attend the cold pe⯑riods of hyſteric, aſthmatic, or epileptic diſeaſes; and that even where large doſes of opium have been in vain exhibited. In theſe caſes the pulſe becomes ſtronger after the bleeding, and the extremities regain their natural warmth; and an opiate then given acts with much more certain effect.

VI. There is another cauſe, which ſeems occaſionally to induce quieſcence into ſome part of our ſyſtem, I mean the influence of the ſun and moon; the attraction of theſe luminaries, by decreaſing the gravity of the particles of the blood, cannot affect their momentum, as their vis inertiae remains the ſame; but it may nevertheleſs pro⯑duce ſome chemical change in them, becauſe whatever affects the ge⯑neral attractions of the particles of matter may be ſuppoſed from ana⯑logy to affect their ſpecific attractions or affinities: and thus the ſtimu⯑lus of the particles of blood may be diminiſhed, though not their mo⯑mentum. As the tides of the ſea obey the ſouthing and northing of the moon (allowing for the time neceſſary for their motion, and the obſtructions of the ſhores), it is probable, that there are alſo atmoſphe⯑ric tides on both ſides of the earth, which to the inhabitants of ano⯑ther [376] planet might ſo deflect the light as to reſemble the ring of Sa⯑turn. Now as theſe tides of water, or of air, are raiſed by the dimi⯑nution of their gravity, it follows, that their preſſure on the ſurface of the earth is no greater than the preſſure of the other parts of the ocean, or of the atmoſphere, where no ſuch tides exiſt; and there⯑fore that they cannot affect the mercury in the barometer. In the ſame manner, the gravity of all other terreſtrial bodies is diminiſhed at the times of the ſouthing and northing of the moon, and that in a greater degree when this coincides with the ſouthing and northing of the ſun, and this in a ſtill greater degree about the times of the equi⯑noxes. This decreaſe of the gravity of all bodies during the time the moon paſſes our zenith or nadir might poſſibly be ſhewn by the ſlower vibrations of a pendulum, compared with a ſpring clock, or with aſtronomical obſervation. Since a pendulum of a certain length moves ſlower at the line than near the poles, becauſe the gravity be⯑ing diminiſhed and the vis inertiae continuing the ſame, the motive power is leſs, but the reſiſtance to be overcome continues the ſame. The combined powers of the lunar and ſolar attraction is eſtimated by Sir Iſaac Newton not to exceed one 7,868,850th part of the power of gravitation, which ſeems indeed but a ſmall circumſtance to produce any conſiderable effect on the weight of ſublunary bodies, and yet this is ſufficient to raiſe the tides at the equator above ten feet high; and if it be conſidered, what ſmall impulſes of other bodies produce their effects on the organs of ſenſe adapted to the perception of them, as of vibration on the auditory nerves, we ſhall ceaſe to be ſurpriſed, that ſo minute a diminution in the gravity of the particles of blood ſhould ſo far affect their chemical changes, or their ſtimulating quality, as, joined with other cauſes, ſometimes to produce the beginnings of diſeaſes.

Add to this, that if the lunar influence produces a very ſmall degree of quieſcence at firſt, and if that recurs at certain periods even with leſs power to produce quieſcence than at firſt, yet the quieſcence will [377] daily increaſe by the acquired habit acting at the ſame time, till at length ſo great a degree of quieſcence is induced as to produce phrenſy, canine madneſs, epilepſy, hyſteric pains, or cold fits of fever, inſtances of many of which are to be found in Dr. Mead's work on this ſubject. The ſolar influence alſo appears daily in ſeveral diſ⯑eaſes; but as darkneſs, ſilence, ſleep, and our periodical meals mark the parts of the ſolar circle of actions, it is ſometimes dubious to which of theſe the periodical returns of theſe diſeaſes are to be aſcribed.

As far as I have been able to obſerve, the periods of inflammatory diſeaſes obſerve the ſolar day; as the gout and rheumatiſm have their greateſt quieſcence about noon and midnight, and their exacerbations ſome hours after; as they have more frequently their immediate cauſe from cold air, inanition, or fatigue, than from the effects of lunations: whilſt the cold fits of hyſteric patients, and thoſe in nervous fevers, more frequently occur twice a day, later by near half an hour each time, according to the lunar day; whilſt ſome fits of intermittents, which are undiſturbed by medicines, return at regular ſolar periods, and others at lunar ones; which may, probably, be owing to the dif⯑ference of the periods of thoſe external circumſtances of cold, inani⯑tion, or lunation, which immediately cauſed them.

We muſt, however, obſerve, that the periods of quieſcence and ex⯑acerbation in diſeaſes do not always commence at the times of the ſy⯑zygies or quadratures of the moon and ſun, or at the times of their paſſing the zenith or nadir; but as it is probable, that the ſtimulus of the particles of the circumfluent blood is gradually diminiſhed from the time of the quadratures to that of the ſyzygies, the quieſcence may commence at any hour, when, co-operating with other cauſes of quieſcence, it becomes great enough to produce a diſeaſe: afterwards it will continue to recur at the ſame period of the lunar or ſolar influ⯑ence; the ſame cauſe operating conjointly with the acquired habit, that is with the catenation of this new motion with the diſſevered links of the lunar or ſolar circles of animal action.

[378] In this manner the periods of menſtruation obey the lunar month with great exactneſs in healthy patients (and perhaps the vene⯑real orgaſm in brute animals does the ſame), yet theſe periods do not commence either at the ſyzygies or quadratures of the lunations, but at whatever time of the lunar periods they begin, they obſerve the ſame in their returns till ſome greater cauſe diſturbs them.

Hence, though the beſt way to calculate the time of the expected returns of the paroxyſms of periodical diſeaſes is to count the number of hours between the commencement of the two preceding fits, yet the following obſervations may be worth attending to, when we en⯑deavour to prevent the returns of maniacal or epileptic diſeaſes; whoſe periods (at the beginning of them eſpecially) frequently obſerve the ſyzygies of the moon and ſun, and particularly about the equinox.

The greateſt of the two tides happening in every revolution of the moon, is that when the moon approaches neareſt to the zenith or na⯑dir; for this reaſon, while the ſun is in the northern ſigns, that is during the vernal and ſummer months, the greater of the two diurnal tides in our latitude is that, when the moon is above the horizon; and when the ſun is in the ſouthern ſigns, or during the autumnal and winter months, the greater tide is that, which ariſes when the moon is below the horizon: and as the ſun approaches ſomewhat nearer the earth in winter than in ſummer, the greateſt equinoctial tides are obſerved to be a little before the vernal equinox, and a little after the autumnal one.

Do not the cold periods of lunar diſeaſes commence a few hours be⯑fore the ſouthing of the moon during the vernal and ſummer months, and before the northing of the moon during the autumnal and winter months? Do not palſies and apoplexies, which occur about the equi⯑noxes, happen a few days before the vernal equinoctial lunation, and after the autumnal one? Are not the periods of thoſe diurnal diſ⯑eaſes more obſtinate, that commence many hours before the ſouthing or northing of the moon, than of thoſe which commence at thoſe times? Are not thoſe palſies and apoplexies more dangerous which [379] commence many days before the ſyzygies of the moon, than thoſe which happen at thoſe times? See Sect. XXXVI. on the periods of diſeaſes.

VII. Another very frequent cauſe of the cold fit of fever is the quieſcence of ſome of thoſe large congeries of glands, which compoſe the liver, ſpleen, or pancreas; one or more of which are frequently ſo enlarged in the autumnal intermittents as to be perceptible to the touch externally, and are called by the vulgar ague-cakes. As theſe glands are ſtimulated into action by the ſpecific pungency of the fluids, which they abſorb, the general cauſe of their quieſcence ſeems to be the too great inſipidity of the fluids of the body, co-operating perhaps at the ſame time with other general cauſes of quieſcence.

Hence, in marſhy countries at cold ſeaſons, which have ſucceeded hot ones, and amongſt thoſe, who have lived on innutritious and un⯑ſtimulating diet, theſe agues are moſt frequent. The enlargement of theſe quieſcent viſcera, and the ſwelling of the praecordia in many other fevers, is, moſt probably, owing to the ſame cauſe; which may conſiſt in a general deficiency of the production of ſenſorial power, as well as in the diminiſhed ſtimulation of the fluids; and when the quieſcence of ſo great a number of glands, as conſtitute one of thoſe large viſcera, commences, all the other irritative motions are affected by their connection with it, and the cold fit of fever is produced.

VIII. There are many other cauſes, which produce quieſcence of ſome part of the animal ſyſtem, as fatigue, hunger, thirſt, bad diet, diſappointed love, unwholeſome air, exhauſtion from evacuations, and many others; but the laſt cauſe, that we ſhall mention, as frequently productive of cold fits of fever, is fear or anxiety of mind. The pains, which we are firſt and moſt generally acquainted with, have been pro⯑duced by defect of ſome ſtimulus; thus, ſoon after our nativity we become acquainted with the pain from the coldneſs of the air, from the want of reſpiration, and from the want of food. Now all theſe pains occaſioned by defect of ſtimulus are attended with quieſcence [380] of the organ, and at the ſame time with a greater or leſs degree of quieſcence of other parts of the ſyſtem: thus, if we even endure the pain of hunger ſo as to miſs one meal inſtead of our daily habit of re⯑pletion, not only the periſtaltic motions of the ſtomach and bowels are diminiſhed, but we are more liable to coldneſs of our extremities, as of our noſes, and ears, and feet, than at other times.

Now, as fear is originally excited by our having experienced pain, and is itſelf a painful affection, the ſame quieſcence of other fibrous motions accompany it, as have been moſt frequently connected with this kind of pain, as explained in Sect. XVI. 8. 1. as the coldneſs and paleneſs of the ſkin, trembling, difficult reſpiration, indigeſtion, and other ſymptoms, which contribute to form the cold fit of fevers. Anxiety is fear continued through a longer time, and, by producing chronical torpor of the ſyſtem, extinguiſhes life ſlowly, by what is commonly termed a broken heart.

IX. 1. We now ſtep forwards to conſider the other ſymptoms in conſequence of the quieſcence which begins the fits of fever. If by any of the circumſtances before deſcribed, or by two or more of them acting at the ſame time, a great degree of quieſcence is in⯑duced on any conſiderable part of the circle of irritative motions, the whole claſs of them is more or leſs diſturbed by their irritative aſſocia⯑tions. If this torpor be occaſioned by a deficient ſupply of ſenſorial power, and happens to any of thoſe parts of the ſyſtem, which are ac⯑cuſtomed to perpetual activity, as the vital motions, the torpor in⯑creaſes rapidly, becauſe of the great expenditure of ſenſorial power by the inceſſant activity of thoſe parts of the ſyſtem, as ſhewn in No. 3. 2. of this Section. Hence a deficiency of all the ſecretions ſucceeds, and as animal heat is produced in proportion to the quantity of thoſe ſecre⯑tions, the coldneſs of the ſkin is the firſt circumſtance, which is at⯑tended to. Dr. Martin aſſerts, that ſome parts of his body were warmer than natural in the cold fit of fever; but it is certain, that thoſe, which are uncovered, as the ſingers, and noſe, and ears, are [381] much colder to the touch, and paler in appearance. It is poſſible, that his experiments were made at the beginning of the ſubſequent hot fits; which commence with partial diſtributions of heat, owing to ſome parts of the body regaining their natural irritability ſooner than others.

From the quieſcence of the anaſtomoſing capillaries a paleneſs of the ſkin ſucceeds, and a leſs ſecretion of the perſpirable matter; from the quieſcence of the pulmonary capillaries a difficulty of reſpiration ariſes; and from the quieſcence of the other glands leſs bile, leſs gaſ⯑tric and pancreatic juice, are ſecreted into the ſtomach and inteſtines, and leſs mucus and ſaliva are poured into the mouth; whence ariſes the dry tongue, coſtiveneſs, dry ulcers, and paucity of urine. From the quieſcence of the abſorbent ſyſtem ariſes the great thirſt, as leſs moiſture is abſorbed from the atmoſphere. The abſorption from the atmoſphere was obſerved by Dr. Lyſter to amount to eighteen ounces in one night, above what he had at the ſame time inſenſibly perſpired. See Langriſh. On the ſame account the urine is pale, though in ſmall quantity, for the thinner part is not abſorbed from it; and when repeated ague-fits continue long, the legs ſwell from the diminiſhed abſorption of the cellular abſorbents.

From the quieſcence of the inteſtinal canal a loſs of appetite and flatulencies proceed. From the partial quieſcence of the glandular viſcera a ſwelling and tenſion about the praecordia becomes ſenſible to the touch; which is occaſioned by the delay of the fluids from the defect of venous or lymphatic abſorption. The pain of the forehead, and of the limbs, and of the ſmall of the back, ariſes from the quieſ⯑cence of the membranous faſcia, or muſcles of thoſe parts, in the ſame manner as the ſkin becomes painful, when the veſſels, of which it is compoſed, become quieſcent from cold. The trembling in conſe⯑quence of the pain of coldneſs, the reſtleſſneſs, and the yawning, and ſtretching of the limbs, together with the ſhuddering, or rigours, are [382] convulſive motions; and will be explained amongſt the diſeaſes of vo⯑lition, Sect. XXXIV.

Sickneſs and vomiting is a frequent ſymptom in the beginnings of fever-fits, the muſcular fibres of the ſtomach ſhare the general torpor and debility of the ſyſtem; their motions become firſt leſſened, and then ſtop, and then become retrograde; for the act of vomiting, like the globus hyſtericus and the borborigmi of hypocondriaſis, is always a ſymptom of debility, either from want of ſtimulus, as in hunger; or from want of ſenſorial power, as after intoxication; or from ſym⯑pathy with ſome other torpid irritative motions, as in the cold fits of ague. See Sect. XII. 5. 5. XXIX. 11. and XXXV. 1. 3. where this act of vomiting is further explained.

The ſmall pulſe, which is ſaid by ſome writers to be ſlow at the commencement of ague-fits, and which is frequently trembling and intermittent, is owing to the quieſcence of the heart and arterial ſyſ⯑tem, and to the reſiſtance oppoſed to the circulating fluid from the inactivity of all the glands and capillaries. The great weakneſs and inability to voluntary motions, with the inſenſibility of the extremi⯑ties, are owing to the general quieſcence of the whole moving ſyſtem; or, perhaps, ſimply to the deficient production of ſenſorial power.

If all theſe ſymptoms are further increaſed, the quieſcence of all the muſcles, including the heart and arteries, becomes complete, and death enſues. This is, moſt probably, the caſe of thoſe who are ſtarved to death with cold, and of thoſe who are ſaid to die in Holland from long ſkaiting on their frozen canals.

2. As ſoon as this general quieſcence of the ſyſtem ceaſes, either by the diminution of the cauſe, or by the accumulation of ſenſorial power, (as in ſyncope, Sect. XII. 7. 1.) which is the natural conſequence of previous quieſcence, the hot fit commences. Every gland of the body is now ſtimulated into ſtronger action than is natural, as its irri⯑tability is increaſed by accumulation of ſenſorial power during its late [383] quieſcence, a ſuperabundance of all the ſecretions is produced, and an increaſe of heat in conſequence of the increaſe of theſe ſecretions. The ſkin becomes red, and the perſpiration great, owing to the in⯑creaſed action of the capillaries during the hot part of the paroxyſm. The ſecretion of perſpirable matter is perhaps greater during the hot fit than in the ſweating fit which follows; but as the abſorption of it alſo is greater, it does not ſtand on the ſkin in viſible drops: add to this, that the evaporation of it alſo is greater, from the increaſed heat of the ſkin. But at the decline of the hot fit, as the mouths of the ab⯑ſorbents of the ſkin are expoſed to the cooler air, or bed-clothes, theſe veſſels ſooner loſe their increaſed activity, and ceaſe to abſorb more than their natural quantity: but the ſecerning veſſels for ſome time longer, being kept warm by the circulating blood, continue to pour out an in⯑creaſed quantity of perſpirable matter, which now ſtands on the ſkin in large viſible drops; the exhalation of it alſo being leſſened by the greater coolneſs of the ſkin, as well as its abſorption by the diminiſhed action of the lymphatics. See Claſs I. 1. 2. 3.

The increaſed ſecretion of bile and of other fluids poured into the inteſtines frequently induce a purging at the decline of the hot fit; for as the external abſorbent veſſels have their mouths expoſed to the cold air, as above mentioned, they ceaſe to be excited into unnatural activity ſooner than the ſecretory veſſels, whoſe mouths are expoſed to the warmth of the blood: now, as the internal abſorbents ſympa⯑thize with the external ones, theſe alſo, which during the hot fit drank up the thinner part of the bile, or of other ſecreted fluids, loſe their increaſed activity before the gland loſes its increaſed activity, at the decline of the hot fit; and the looſe dejections are produced from the ſame cauſe, that the increaſed perſpiration ſtands on the ſur⯑face of the ſkin, from the increaſed abſorption ceaſing ſooner than the increaſed ſecretion.

The urine during the cold fit is in ſmall quantity and pale, both from a deficiency of the ſecretion and a deficiency of the abſorption. [384] During the hot fit it is in its uſual quantity, but very high coloured and turbid, becauſe a greater quantity had been ſecreted by the in⯑creaſed action of the kidnies, and alſo a greater quantity of its more aqueous part had been abſorbed from it in the bladder by the increaſed action of the abſorbents; and laſtly, at the decline of the hot fit it is in large quantity and leſs coloured, or turbid, becauſe the abſorbent veſſels of the bladder, as obſerved above, loſe their increaſed action by ſympathy with the cutaneous ones ſooner than the ſecretory veſſels of the kidnies loſe their increaſed activity. Hence the quantity of the ſediment, and the colour of the urine, in fevers, depend much on the quantity ſecreted by the kidnies, and the quantity abſorbed from it again in the bladder: the kinds of ſediment, as the lateri⯑tious, purulent, mucous, or bloody ſediments, depend on other cauſes. It ſhould be obſerved, that if the ſweating be increaſed by the heat of the room, or of the bed-clothes, that a paucity of turbid urine will continue to be produced, as the abſorbents of the bladder will have their activity increaſed by their ſympathy with the veſſels of the ſkin, for the purpoſe of ſupplying the fluid expended in per⯑ſpiration.

The pulſe becomes ſtrong and full owing to the increaſed irritabi⯑lity of the heart and arteries, from the accumulation of ſenſorial power during their quieſcence, and to the quickneſs of the return of the blood from the various glands and capillaries. This increaſed ac⯑tion of all the ſecretory veſſels does not occur very ſuddenly, nor uni⯑verſally at the ſame time. The heat ſeems to begin about the center, and to be diffuſed from thence irregularly to the other parts of the ſyſtem. This may be owing to the ſituation of the parts which firſt became quieſcent and cauſed the fever-fit, eſpecially when a hardneſs or tumour about the praecordia can be felt by the hand; and hence this part, in whatever viſcus it is ſeated, might be the firſt to regain its natural or increaſed irritability.

[385] 3. It muſt be here noted, that, by the increaſed quantity of heat, and of the impulſe of the blood at the commencement of the hot ſit, a great increaſe of ſtimulus is induced, and is now added to the in⯑creaſed irritability of the ſyſtem, which was occaſioned by its previ⯑ous quieſcence. This additional ſtimulus of heat and momentum of the blood augments the violence of the movements of the arterial and glandular ſyſtem in an increaſing ratio. Theſe violent exertions ſtill producing more heat and greater momentum of the moving fluids, till at length the ſenſorial power becomes waſted by this great ſtimu⯑lus beneath its natural quantity, and prediſpoſes the ſyſtem to a ſe⯑cond cold fit.

At length all theſe unnatural exertions ſpontaneouſly ſubſide with the increaſed irritability that produced them; and which was itſelf produced by the preceding quieſcence, in the ſame manner as the eye, on coming from darkneſs into day-light, in a little time ceaſes to be dazzled and pained, and gradually recovers its natural degree of irri⯑tability.

4. But if the increaſe of irritability, and the conſequent increaſe of the ſtimulus of heat and momentum, produce more violent exer⯑tions than thoſe above deſcribed; great pain ariſes in ſome part of the moving ſyſtem, as in the membranes of the brain, pleura, or joints; and new motions of the veſſels are produced in conſequence of this pain, which are called inflammation; or delirium or ſtupor ariſes; as explained in Sect. XXI. and XXXIII.: for the immediate effect is the ſame, whether the great energy of the moving organs ariſes from an increaſe of ſtimulus or an increaſe of irritability; though in the former caſe the waſte of ſenſorial power leads to debility, and in the latter to health.

Recapitulation.

X. Thoſe muſcles, which are leſs frequently exerted, and whoſe actions are interrupted by ſleep, acquire leſs accumulation of ſenſo⯑rial power during their quieſcent ſtate, as the muſcles of locomotion. In theſe muſcles after great exertion, that is, after great exhauſtion of ſenſorial power, the pain of fatigue enſues; and during reſt there is a renovation of the natural quantity of ſenſorial power; but where the reſt, or quieſcence of the muſcle, is long continued, a quantity of ſen⯑ſorial power becomes accumulated beyond what is neceſſary; as ap⯑pears by the uneaſineſs occaſioned by want of exerciſe; and which in young animals is one cauſe exciting them into action, as is ſeen in the play of puppies and kittens.

But when thoſe muſcles, which are habituated to perpetual action, as thoſe of the ſtomach by the ſtimulus of food, thoſe of the veſſels of the ſkin by the ſtimulus of heat, and thoſe which conſtitute the arteries and glands by the ſtimulus of the blood, become for a time quieſcent, from the want of their appropriated ſtimuli, or by their aſ⯑ſociations with other quieſcent parts of the ſyſtem; a greater accu⯑mulation of ſenſorial power is acquired during their quieſcence, and a greater or quicker exhauſtion of it is produced during their increaſed action.

This accumulation of ſenſorial power from deficient action, if it happens to the ſtomach from want of food, occaſions the pain of hun⯑ger; if it happens to the veſſels of the ſkin from want of heat, it oc⯑caſions the pain of cold; and if to the arterial ſyſtem from the want of its adapted ſtimuli, many diſagreeable ſenſations are occaſioned, ſuch as are experienced in the cold ſits of intermittent fevers, and are [387] as various, as there are glands or membranes in the ſyſtem, and are generally termed univerſal uneaſineſs.

When the quieſcence of the arterial ſyſtem is not owing to defect of ſtimulus as above, but to the defective quantity of ſenſorial power, as in the commencement of nervous fever, or irritative fever with weak pulſe, a great torpor of this ſyſtem is quickly induced; becauſe both the irritation from the ſtimulus of the blood, and the aſſociation of the vaſcular motions with each other, continue to excite the arteries into action, and thence quickly exhauſt the ill-ſupplied vaſcular muſcles; for to reſt is death; and therefore thoſe vaſcular muſcles continue to proceed, though with feebler action, to the extreme of wearineſs or faintneſs: while nothing ſimilar to this affects the locomotive muſcles, whoſe actions are generally cauſed by volition, and not much ſubject either to irritation or to other kinds of aſſociations beſides the vo⯑luntary ones, except indeed when they are excited by the laſh of ſlavery.

In theſe vaſcular muſcles, which are ſubject to perpetual action, and thence liable to great accumulation of ſenſorial power during their quieſcence from want of ſtimulus, a great increaſe of activity occurs, either from the renewal of their accuſtomed ſtimulus, or even from much leſs quantities of ſtimulus than uſual. This increaſe of action conſtitutes the hot fit of fever, which is attended with various in⯑creaſed ſecretions, with great concomitant heat, and general uneaſi⯑neſs. The uneaſineſs attending this hot paroxyſm of fever, or fit of exertion, is very different from that, which attends the previous cold fit, or fit of quieſcence, and is frequently the cauſe of inflamma⯑tion, as in pleuriſy, which is treated of in the next ſection.

A ſimilar effect occurs after the quieſcence of our organs of ſenſe; thoſe which are not ſubject to perpetual action, as the taſte and ſmell, are leſs liable to an exuberant accumulation of ſenſorial power after their having for a time been inactive; but the eye, which is in per⯑petual [388] action during the day, becomes dazzled, and liable to inflam⯑mation after a temporary quieſcence.

Where the previous quieſcence has been owing to a defect of ſen⯑ſorial power, and not to a defect of ſtimulus, as in the irritative fever with weak pulſe, a ſimilar increaſe of activity of the arterial ſyſtem ſucceeds, either from the uſual ſtimulus of the blood, or from a ſti⯑mulus leſs than uſual; but as there is in general in theſe caſes of fe⯑ver with weak pulſe a deficiency of the quantity of the blood, the pulſe in the hot fit is weaker than in health, though it is ſtronger than in the cold fit, as explained in No. 2. of this ſection. But at the ſame time in thoſe fevers, where the defect of irritation is owing to the defect of the quantity of ſenſorial power, as well as to the defect of ſtimulus, another circumſtance occurs; which conſiſts in the partial diſtribution of it, as appears in partial fluſhings, as of the face or bo⯑ſom, while the extremities are cold; and in the increaſe of particular ſecretions, as of bile, ſaliva, inſenſible perſpiration, with great heat of the ſkin, or with partial ſweats, or diarrhoea.

There are alſo many uneaſy ſenſations attending theſe increaſed ac⯑tions, which, like thoſe belonging to the hot fit of fever with ſtrong pulſe, are frequently followed by inflammation, as in ſcarlet fever; which inflammation is nevertheleſs accompanied with a pulſe weaker, though quicker, than the pulſe during the remiſſion or intermiſſion of the paroxyſms, though ſtronger than that of the previous cold fit.

From hence I conclude, that both the cold and hot fits of fever are neceſſary conſequences of the perpetual and inceſſant action of the ar⯑terial and glandular ſyſtem; ſince thoſe muſcular fibres and thoſe or⯑gans of ſenſe, which are moſt frequently exerted, become neceſſarily moſt affected both with defect and accumulation of ſenſorial power: and that hence fever-fits are not an effort of nature to relieve herſelf, and that therefore they ſhould always be prevented or diminiſhed as much as poſſible, by any means which decreaſe the general or partial [389] vaſcular actions, when they are greater, or by increaſing them when they are leſs than in health, as deſcribed in Sect. XII. 6. 1.

Thus have I endeavoured to explain, and I hope to the ſatisfaction of the candid and patient reader, the principal ſymptoms or circum⯑ſtances of fever without the introduction of the ſupernatural power of ſpaſm. To the arguments in favour of the doctrine of ſpaſm it may be ſufficient to reply, that in the evolution of medical as well as of dramatic cataſtrophe,

I. 1. AS many motions of the body are excited and continued by irritations, ſo others require, either conjunctly with theſe, or ſepa⯑rately, the pleaſurable or painful ſenſations, for the purpoſe of pro⯑ducing them with due energy. Amongſt theſe the buſineſs of di⯑geſtion ſupplies us with an inſtance: if the food, which we ſwallow, is not attended with agreeable ſenſation, it digeſts leſs perfectly; and if very diſagreeable ſenſation accompanies it, ſuch as a nauſeous idea, [391] or very diſguſtful taſte, the digeſtion becomes impeded; or retrograde motions of the ſtomach and oeſophagus ſucceed, and the food is ejected.

The buſineſs of generation depends ſo much on agreeable ſenſation, that, where the object is diſguſtful, neither voluntary exertion nor irri⯑tation can effect the purpoſe; which is alſo liable to be interrupted by the pain of fear or baſhfulneſs.

Beſides the pleaſure, which attends the irritations produced by the objects of luſt and hunger, there ſeems to be a ſum of pleaſurable af⯑fection accompanying the various ſecretions of the numerous glands, which conſtitutes the pleaſure of life, in contradiſtinction to the te⯑dium vitae. This quantity or ſum of pleaſurable affection ſeems to contribute to the due or energetic performance of the whole move⯑able ſyſtem, as well that of the heart and arteries, as of digeſtion and of abſorption; ſince without the due quantity of pleaſurable ſenſa⯑tion, flatulency and hypochondriaciſm affect the inteſtines, and a lan⯑guor ſeizes the arterial pulſations and ſecretions; as occurs in great and continued anxiety of the mind.

2. Beſides the febrile motions occaſioned by irritation, deſcribed in Sect. XXXII. and termed irritative fever, it frequently happens that pain is excited by the violence of the fibrous contractions; and other new motions are then ſuperadded, in conſequence of ſenſation, which we ſhall term febris ſenſitiva, or ſenſitive fever. It muſt be obſerved, that moſt irritative fevers begin with a decreaſed exertion of irritation, owing to defect of ſtimulus; but that on the contrary the ſenſitive fevers, or inflammations, generally begin with the increaſed exertion of ſenſation, as mentioned in Sect. XXXI. on temperaments: for though the cold fit, which introduces inflammation, commences with decreaſed irritation, yet the inflammation itſelf commences in the hot ſit during the increaſe of ſenſation. Thus a common puſtule, or phlegmon, in a part of little ſenſibility does not excite an inflamma⯑tory fever; but if the ſtomach, inteſtines, or the tender ſubſtance [392] beneath the nails, be injured, great ſenſation is produced, and the whole ſyſtem is thrown into that kind of exertion, which conſtitutes inflammation.

Theſe ſenſitive fevers, like the irritative ones, reſolve themſelves into thoſe with arterial ſtrength, and thoſe with arterial debility, that is with exceſs or defect of ſenſorial power; theſe may be termed the febris ſenſitiva pulſu forti, ſenſitive fever with ſtrong pulſe, which is the ſynocha, or inflammatory fever; and the febris ſenſitiva pulſu de⯑bili, ſenſitive fever with weak pulſe, which is the typhus gravior, or putrid fever of ſome writers.

3. The inflammatory fevers, which are here termed ſenſitive fe⯑vers with ſtrong pulſe, are generally attended with ſome topical in⯑flammation, as pleuriſy, peripneumony, or rheumatiſm, which diſ⯑tinguiſhes them from irritative fevers with ſtrong pulſe. The pulſe is ſtrong, quick, and full; for in this fever there is great irri⯑tation, as well as great ſenſation, employed in moving the arterial ſyſtem. The ſize, or coagulable lymph, which appears on the blood, is probably an increaſed ſecretion from the inflamed internal lining of the whole arterial ſyſtem, the thinner part being taken away by the increaſed abſorption of the inflamed lymphatics.

The ſenſitive fevers with weak pulſe, which are termed putrid or malignant fevers, are diſtinguiſhed from irritative fevers with weak pulſe, called nervous fevers, deſcribed in the laſt ſection, as the for⯑mer conſiſt of inflammation joined with debility, and the latter of de⯑bility alone. Hence there is greater heat and more florid colour of the ſkin in the former, with petechiae, or purple ſpots, and aphthae, or ſloughs in the throat, and generally with previous contagion.

When animal matter dies, as a ſlough in the throat, or the morti⯑fied part of a carbuncle, if it be kept moiſt and warm, as during its adheſion to a living body, it will ſoon putriſy. This, and the origin of contagion from putrid animal ſubſtances, ſeem to have given riſe to the ſeptic and antiſeptic theory of theſe fevers.

[393] The matter in puſtules and ulcers is thus liable to become putrid, and to produce microſcopic animalcula; the urine, if too long re⯑tained, may alſo gain a putreſcent ſmell, as well as the alvine feces; but ſome writers have gone ſo far as to believe, that the blood itſelf in theſe fevers has ſmelt putrid, when drawn from the arm of the pa⯑tient: but this ſeems not well founded; ſince a ſingle particle of putrid matter taken into the blood can produce fever, how can we conceive that the whole maſs could continue a minute in a putrid ſtate without deſtroying life? Add to this, that putrid animal ſubſtances give up air, as in gangrenes; and that hence if the blood was putrid, air ſhould be given out, which in the blood-veſſels is known to occaſion immediate death.

In theſe ſenſitive fevers with ſtrong pulſe (or inflammations) there are two ſenſorial faculties concerned in producing the diſeaſe, viz. ir⯑ritation and ſenſation; and hence, as their combined action is more violent, the general quantity of ſenſorial power becomes further ex⯑hauſted during the exacerbation, and the ſyſtem more rapidly weak⯑ened than in irritative fever with ſtrong pulſe; where the ſpirit of animation is weakened by but one mode of its exertion: ſo that this febris ſenſitiva pulſu forti (or inflammatory fever,) may be conſidered as the febris irritativa pulſu forti, with the addition of inflammation: and the febris ſenſitiva pulſu debili (or malignant fever) may be con⯑ſidered as the febris irritativa pulſu debili (or nervous fever), with the addition of inflammation.

4. In theſe putrid or malignant fevers a deficiency of irritability ac⯑companies the increaſe of ſenſibility; and by this waſte of ſenſorial power by the exceſs of ſenſation, which was already too ſmall, ariſes the delirium and ſtupor which ſo perpetually attend theſe inflamma⯑tory fevers with arterial debility. In theſe caſes the voluntary power firſt ceaſes to act from deficiency of ſenſorial ſpirit; and the ſtimuli from external bodies have no effect on the exhauſted ſenſorial power, and a delirium like a dream is the conſequence. At length the in⯑ternal [394] ſtimuli ceaſe to excite ſufficient irritation, and the ſecretions are either not produced at all, or too parſimonious in quantity. Amongſt theſe the ſecretion of the brain, or production of the ſenſorial power, becomes deficient, till at laſt all ſenſorial power ceaſes, except what is juſt neceſſary to perform the vital motions, and a ſtupor ſucceeds; which is thus owing to the ſame cauſe as the preceding delirium ex⯑erted in a greater degree.

This kind of delirium is owing to a ſuſpenſion of volition, and to the diſobedience of the ſenſes to external ſtimuli, and is always occa⯑ſioned by great debility, or paucity of ſenſorial power; it is therefore a bad ſign at the end of inflammatory fevers, which had previous ar⯑terial ſtrength, as rheumatiſm, or pleuriſy, as it ſhews the preſence of great exhauſtion of ſenſorial power in a ſyſtem, which having lately been expoſed to great excitement, is not ſo liable to be ſtimulated into its healthy action, either by additional ſtimulus of food and medicines, or by the accumulation of ſenſorial power during its preſent torpor. In inflammatory fevers with debility, as thoſe termed putrid fevers, delirium is ſometimes, as well as ſtupor, rather a favourable ſign; as leſs ſenſorial power is waſted during its continuance (ſee Claſs II. 1. 6. 8.), and the conſtitution not having been previouſly expoſed to exceſs of ſtimulation, is more liable to be excited after previous quieſcence.

When the ſum of general pleaſurable ſenſation becomes too great, another kind of delirium ſupervenes, and the ideas thus excited are miſtaken for the irritations of external objects: ſuch a delirium is pro⯑duced for a time by intoxicating drugs, as fermented liquors, or opium: a permanent delirium of this kind is ſometimes induced by the pleaſures of inordinate vanity, or by the enthuſiaſtic hopes of heaven. In theſe caſes the power of volition is incapable of exertion, and in a great degree the external ſenſes become incapable of perceiv⯑ing their adapted ſtimuli, becauſe the whole ſenſorial power is em⯑ployed or expended on the ideas excited by pleaſurable ſenſation.

[395] This kind of delirium is diſtinguiſhed from that which attends the fevers above mentioned from its not being accompanied with general debility, but ſimply with exceſs of pleaſureable ſenſation; and is therefore in ſome meaſure allied to madneſs or to reverie; it differs from the delirium of dreams, as in this the power of volition is not totally ſuſpended, nor are the ſenſes precluded from external ſtimula⯑tion; there is therefore a degree of conſiſtency, in this kind of deli⯑rium, and a degree of attention to external objects, neither of which exiſt in the delirium of fevers or in dreams.

5. It would appear, that the vaſcular ſyſtem of other animals are leſs liable to be put into action by their general ſum of pleaſureable or painful ſenſation; and that the trains of their ideas, and the muſcular motions uſually aſſociated with them, are leſs powerfully connected than in the human ſyſtem. For other animals neither weep, nor ſmile, nor laugh; and are hence ſeldom ſubject to delirium, as treated of in Sect. XVI. on Inſtinct. Now as our epidemic and contagious diſeaſes are probably produced by diſagreeable ſenſation, and not ſimply by irritation; there appears a reaſon, why brute animals are leſs liable to epidemic or contagious diſeaſes; and ſecondly, why none of our contagions, as the ſmall-pox or meaſles, can be communicated to them, though one of theirs, viz. the hydrophobia, as well as many of their poiſons, as thoſe of ſnakes and of inſects, communicate their deleterious or painful effects to mankind.

Where the quantity of general painful ſenſation is too great in the ſyſtem, inordinate voluntary exertions are produced either of our ideas, as in melancholy and madneſs, or of our muſcles, as in con⯑vulſion. From theſe maladies alſo brute animals are much more ex⯑empt than mankind, owing to their greater inaptitude to voluntary exertion, as mentioned in Sect. XVI. on Inſtinct.

II. 1. When any moving organ is excited into ſuch violent mo⯑tions, that a quantity of pleaſureable or painful ſenſation is produced, it frequently happens (but not always) that new motions of the af⯑fected [396] organ are generated in conſequence of the pain or pleaſure, which are termed inflammation.

Theſe new motions are of a peculiar kind, tending to diſtend the old, and to produce new fibres, and thence to elongate the ſtraight muſcles, which ſerve locomotion, and to form new veſſels at the ex⯑tremities or ſides of the vaſcular muſcles.

2. Thus the pleaſureable ſenſations produce an enlargement of the nipples of nurſes, of the papillae of the tongue, of the penis, and probably produce the growth of the body from its embryon ſtate to its maturity; whilſt the new motions in conſequence of painful ſenſa⯑tion, with the growth of the fibres or veſſels, which they occaſion, are termed inflammation.

Hence when the ſtraight muſcles are inflamed, part of their ten⯑dons at each extremity gain new life and ſenſibility, and thus the muſcle is for a time elongated; and inflamed bones become ſoft, vaſ⯑cular, and ſenſible. Thus new veſſels ſhoot over the cornea of in⯑flamed eyes, and into ſchirrous tumours, when they become inflamed; and hence all inflamed parts grow together by intermixture, and in⯑oſculation of the new and old veſſels.

The heat is occaſioned from the increaſed ſecretions either of mu⯑cus, or of the fibres, which produce or elongate the veſſels. The red colour is owing to the pellucidity of the newly formed veſſels, and as the arterial parts of them are probably formed before their corre⯑ſpondent venous parts.

3. Theſe new motions are excited either from the increaſed quan⯑tity of ſenſation in conſequence of greater fibrous contractions, or from increaſed ſenſibility, that is, from the increaſed quantity of ſen⯑ſorial power in the moving organ. Hence they are induced by great external ſtimuli, as by wounds, broken bones; and by acrid or in⯑ſectious materials; or by common ſtimuli on thoſe organs, which have been ſome time quieſcent; as the uſual light of the day inflames the eyes of thoſe, who have been conſined in dungeons; and the [397] warmth of a common fire inflames thoſe, who have been previouſly expoſed to much cold.

But theſe new motions are never generated by that pain, which ariſes from defect of ſtimulus, as from hunger, thirſt, cold, or in⯑anition, with all thoſe pains, which are termed nervous. Where theſe pains exiſt, the motions of the affected part are leſſened; and if inflammation ſucceeds, it is in ſome diſtant parts; as coughs are cauſed by coldneſs and moiſture being long applied to the feet; or it is in conſequence of the renewal of the ſtimulus, as of heat or food, which excites our organs into ſtronger action after their temporary quieſcence; as kibed heels after walking in ſnow.

4. But when theſe new motions of the vaſcular muſcles are ex⯑erted with greater violence, and theſe veſſels are either elongated too much or too haſtily, a new material is ſecreted from their extremi⯑ties, which is of various kinds according to the peculiar animal mo⯑tions of this new kind of gland, which ſecretes it; ſuch is the pus laudabile or common matter, the variolous matter, vencreal matter, catarrhous matter, and many others.

5. Theſe matters are the product of an animal proceſs; they are ſecreted or produced from the blood by certain diſeaſed motions of the extremities of the blood-veſſels, and are on that account all of them contagious; for if a portion of any of theſe matters is tranſmitted into the circulation, or perhaps only inſerted into the ſkin, or beneath the cuticle of an healthy perſon, its ſtimulus in a certain time produces the ſame kind of morbid motions, by which itſelf was produced; and hence a ſimilar matter is generated. See Sect. XXXIX. 6. 1.

6. It is remarkable, that many of theſe contagious matters are capable of producing a ſimilar diſeaſe but once; as the ſmall-pox and meaſles; and I ſuppoſe this is true of all thoſe contagious diſeaſes, which are ſpontaneouſly cured by nature in a certain time; for if the body was capable of receiving the diſeaſe a ſecond time, the patient muſt perpetually infect himſelf by the very matter, which he has [398] himſelf produced, and is lodged about him; and hence he could never become free from the diſeaſe. Something ſimilar to this is ſeen in the ſecondary fever of the confluent ſmall-pox; there is a great abſorption of variolous matter, a very minute part of which would give the genuine ſmall-pox to another perſon; but here it only ſtimulates the ſyſtem into common fever; like that which common pus, or any other acrid material might occaſion.

7. In the pulmonary conſumption, where common matter is daily abſorbed, an irritative fever only, not an inflammatory one, is produced; which is terminated like other irritative fevers by ſweats, or looſe ſtools. Hence it does not appear, that this abſorbed matter always acts as a contagious material producing freſh inflammation or new abſceſſes. Though there is reaſon to believe, that the firſt time any common matter is abſorbed, it has this effect, but not the ſecond time, like the variolous matter above mentioned.

This accounts for the opinion, that the pulmonary conſumption is ſometimes infectious, which opinion was held by the ancients, and continues in Italy at preſent; and I have myſelf ſeen three or four inſtances, where a huſband and wife, who have ſlept together, and have thus much received each other's breath, who have infected each other, and both died in conſequence of the original taint of only one of them. This alſo accounts for the abſceſſes in various parts of the body, that are ſometimes produced after the inoculated ſmall-pox is terminated; for this ſecond abſorption of variolous matter acts like common matter, and produces only irritative fever in thoſe children, whoſe conſtitutions have already experienced the abſorption of com⯑mon matter; and inflammation with a tendency to produce new ab⯑ſceſſes in thoſe, whoſe conſtitutions have not experienced the abſorp⯑tions of common matter.

It is probable, that more certain proofs might have been found to ſhew, that common matter is infectious the firſt time it is abſorbed, [399] tending to produce ſimilar abſceſſes, but not the ſecond time of its abſorption, if this ſubject had been attended to.

8. Theſe contagious diſeaſes are very numerous, as the plague, ſmall-pox, chicken-pox, meaſles, ſcarlet-fever, pemphigus, catarrh, chincough, venereal diſeaſe, itch, trichoma, tinea. The infectious material does not ſeem to be diſſolved by the air, but only mixed with it perhaps in fine powder, which ſoon ſubſides; ſince many of theſe contagions can only be received by actual contact; and others of them only at ſmall diſtances from the infected perſon; as is evident from many perſons having been near patients of the ſmall-pox with⯑out acquiring the diſeaſe.

The reaſon, why many of theſe diſeaſes are received but once, and others repeatedly, is not well underſtood; it appears to me, that the conſtitution becomes ſo accuſtomed to the ſtimuli of theſe infec⯑tious materials, by having once experienced them, that though irri⯑tative motions, as hectic fevers, may again be produced by them, yet no ſenſation, and in conſequence no general inflammation ſucceeds; as diſagreeable ſmells or taſtes by habit ceaſe to be perceived; they continue indeed to excite irritative ideas on the organs of ſenſe, but theſe are not ſucceeded by ſenſation.

There are many irritative motions, which were at firſt ſucceeded by ſenſation, but which by frequent repetition ceaſe to excite ſenſa⯑tion, as explained in Sect. XX. on Vertigo. And, that this circum⯑ſtance exiſts in reſpect to infectious matter appears from a known fact; that nurſes, who have had the ſmall-pox, are liable to experi⯑ence ſmall ulcers on their arms by the contact of variolous matter in liſting their patients; and that when patients, who have formerly had the ſmall-pox have been inoculated in the arm, a phlegmon, or inflamed ſore, has ſucceeded, but no ſubſequent fever. Which ſhews, that the contagious matter of the ſmall-pox has not loſt its power of ſtimulating the part it is applied to, but that the general [400] ſyſtem is not affected in conſequence. See Section XII. 7. 6. XIX. 10.

9. From the accounts of the plague, virulent catarrh, and putrid dyſentery, it ſeems uncertain, whether theſe diſeaſes are experienced more than once; but the venereal diſeaſe and itch are doubtleſs re⯑peatedly infectious; and as theſe diſeaſes are never cured ſpontaneouſly, but require medicines, which act without apparent operation, ſome have ſuſpected, that the contagious material produces ſimilar matter rather by a chemical change of the fluids, than by an animal proceſs; and that the ſpecific medicines deſtroy their virus by chemically com⯑bining with it. This opinion is ſucceſsfully combated by Mr. Hun⯑ter, in his Treatiſe on Venereal Diſeaſe, Part I. c. i.

But this opinion wants the ſupport of analogy, as there is no known proceſs in animal bodies, which is purely chemical, not even digeſtion; nor can any of theſe matters be produced by chemical pro⯑ceſſes. Add to this, that it is probable, that the inſects, obſerved in the puſtules of the itch, and in the ſtools of dyſenteric patients, are the conſequences, and not the cauſes of theſe diſeaſes. And that the ſpecific medicines, which cure the itch and lues venerea, as brimſtone and mercury, act only by increaſing the abſorption of the matter in the ulcuſcles of thoſe diſeaſes, and thence diſpoſing them to heal; which would otherwiſe continue to ſpread.

Why the venereal diſeaſe, and itch, and tenia, or ſcald head, are repeatedly contagious, while thoſe contagions attended with fever can be received but once, ſeems to depend on their being rather local diſ⯑eaſes than univerſal ones, and are hence not attended with fever, ex⯑cept the purulent fever in their laſt ſtages, when the patient is deſtroy⯑ed by them. On this account the whole of the ſyſtem does not be⯑come habituated to theſe morbid actions, ſo as to ceaſe to be affected with ſenſation by a repetition of the contagion. Thus the contagious matter of the venereal diſeaſe, and of the tenia, affects the lymphatic [401] glands, as the inquinal glands, and thoſe about the roots of the hair and neck, where it is arreſted, but does not ſeem to affect the blood⯑veſſels, ſince no fever enſues.

Hence it would appear, that theſe kinds of contagion are propa⯑gated not by means of the circulation, but by ſympathy of diſtant parts with each other; ſince if a diſtant part, as the palate, ſhould be excited by ſenſitive aſſociation into the ſame kind of motions, as the parts originally affected by the contact of infectious matter; that diſtant part will produce the ſame kind of infectious matter; for every ſecretion from the blood is formed from it by the peculiar motions of the ſine extremities of the gland, which ſecretes it; the various ſe⯑creted fluids, as the bile, ſaliva, gaſtric juice, not previouſly exiſting, as ſuch, in the blood-veſſels.

And this peculiar ſympathy between the genitals and the throat, owing to ſenſitive aſſociation, appears not only in the production of venereal ulcers in the throat, but in variety of other inſtances, as in the mumps, in the hydrophobia, ſome coughs, ſtrangulation, the production of the beard, change of voice at puberty. Which are fur⯑ther deſcribed in Claſs IV. 2. 1. 7.

To evince that the production of ſuch large quantities of contagi⯑ous matter, as are ſeen in ſome variolous patients, ſo as to cover the whole ſkin almoſt with puſtules, does not ariſe from any chemical fer⯑mentation in the blood, but that it is owing to morbid motions of the fine extremities of the capillaries, or glands, whether theſe be rup⯑tured or not, appears from the quantity of this matter always cor⯑reſponding with the quantity of the fever; that is, with the violent exertions of thoſe glands and capillaries, which are the terminations of the arterial ſyſtem.

The truth of this theory is evinced further by a circumſtance ob⯑ſerved by Mr. J. Hunter, in his Treatiſe on Venereal Diſeaſe; that in a patient, who was inoculated for the ſmall-pox, and who appear⯑ed afterwards to have been previouſly infected with the meaſles, [402] the progreſs of the ſmall-pox was delayed till the meaſles had run their courſe, and that then the ſmall-pox went through its uſual periods.

Two ſimilar caſes fell under my care, which I ſhall here relate, as it confirms that of Mr. Hunter, and contributes to illuſtrate this part of the theory of contagious diſeaſes. I have tranſcribed the particu⯑lars from a letter of Mr. Lightwood of Yoxal, the ſurgeon who daily attended them, and at my requeſt, after I had ſeen them, kept a kind of journal of their caſes.

Miſs H. and Miſs L. two ſiſters, the one about four and the other about three years old, were inoculated Feb. 7, 1791. On the 10th there was a redneſs on both arms diſcernible by a glaſs. On the 11th their arms were ſo much inflamed as to leave no doubt of the infection having taken place. On the 12th leſs appearance of in⯑flammation on their arms. In the evening Miſs L. had an eruption, which reſembled the meaſles. On the 13th the eruption on Miſs L. was very full on the face and breaſt, like the meaſles, with conſider⯑able fever. It was now known, that the meaſles were in a farm houſe in the neighbourhood. Miſs H.'s arm leſs inflamed than yeſterday. On the 14th Miſs L.'s fever great, and the eruption univerſal. The arm appears to be healed. Miſs H.'s arm ſomewhat redder. They were now put into ſeparate rooms. On the 15th Miſs L.'s arms as yeſterday. Eruption continues. Miſs H.'s arms have varied but little. 16th, the eruptions on Miſs L. are dying away, her fever gone. Be⯑gins to have a little redneſs in one arm at the place of inoculation. Miſs H.'s arms get redder, but ſhe has no appearance of complaint. 20th, Miſs L.'s arms have advanced ſlowly till this day, and now a few puſtules appear. Miſs H.'s arm has made little progreſs from the 16th to this day, and now ſhe has ſome fever. 21ſt, Miſs L. as yeſterday. Miſs H. has much inflammation, and an increaſe of the red circle on one arm to the ſize of half a crown, and had much fever at night, with fetid breath. 22d, Miſs L.'s puſtules continue advanc⯑ing. [403] Miſs H.'s inflammation of her arm and red circle increaſes. A few red ſpots appear in different parts with ſome degree of fever this morning. 23d Miſs L. has a larger crop of puſtules. Miſs H. has ſmall puſtules and great inflammation of her arms, with but one puſtule likely to ſuppurate. After this day they gradually got well, and the puſtules diſappeared.

In one of theſe caſes the meaſles went through their common courſe with milder ſymptoms than uſual, and in the other the meaſly conta⯑gion ſeemed juſt ſufficient to ſtop the progreſs of variolous contagion, but without itſelf throwing the conſtitution into any diſorder. At the ſame time both the meaſles and ſmall-pox ſeem to have been rendered milder. Does not this give an idea, that if they were both inoculated at the ſame time, that neither of them might affect the patient?

From theſe caſes I contend, that the contagious matter of theſe diſ⯑eaſes does not affect the conſtitution by a fermentation, or chemical change of the blood, becauſe then they muſt have proceeded together, and have produced a third ſomething, not exactly ſimilar to either of them: but that they produce new motions of the cutaneous termina⯑tions of the blood-veſſels, which for a time proceed daily with increaſ⯑ing activity, like ſome paroxyſms of fever, till they at length ſecrete or form a ſimilar poiſon by theſe unnatural actions.

Now as in the meaſles one kind of unnatural motion takes place, and in the ſmall-pox another kind, it is eaſy to conceive, that theſe different kinds of morbid motions cannot exiſt together; and there⯑fore, that that which has firſt begun will continue till the ſyſtem be⯑comes habituated to the ſtimulus which occaſions it, and has ceaſed to be thrown into action by it; and then the other kind of ſtimulus will in its turn produce fever, and new kinds of motions peculiar to itſelf.

10. On further conſidering the action of contagious matter, ſince the former part of this work was ſent to the preſs; where I have aſ⯑ſerted, in Sect. XII. 3. 6. that it is probable, that the variolous matter is diſfuſed through the blood; I prevailed on my friend Mr. Power, [404] ſurgeon at Boſworth in Leiceſterſhire to try, whether the ſmall-pox could be inoculated by uſing the blood of a variolous patient inſtead of the matter from the puſtules; as I thought ſuch an experiment migh throw ſome light at leaſt on this intereſting ſubject. The fol⯑lowing is an extract from his letter:—

"March 11, 1793. I inoculated two children, who had not had the ſmall-pox, with blood; which was taken from a patient on the ſecond day after the eruption commenced, and before it was com⯑pleted. And at the ſame time I inoculated myſelf with blood from the ſame perſon, in order to compare the appearances, which might ariſe in a perſon liable to receive the infection, and in one not liable to receive it. On the ſame day I inoculated four other children liable to receive the infection with blood taken from another perſon on the fourth day after the commencement of the eruption. The patients from whom the blood was taken had the diſeaſe mildly, but had the moſt puſtules of any I could ſelect from twenty inoculated patients; and as much of the blood was inſinuated under the cuticle as I could introduce by elevating the ſkin without drawing blood; and three or four ſuch punctures were made in each of their arms, and the blood was uſed in its fluid ſtate.

"As the appearances in all theſe patients, as well as in myſelf, were ſimilar, I ſhall only mention them in general terms. March 13. A ſlight ſubcuticular diſcoloration, with rather a livid appearance, without ſoreneſs or pain, was viſible in them all, as well as in my own hand. 15. The diſcoloration ſomewhat leſs, without pain or ſoreneſs. Some patients inoculated on the ſame day with variolous matter have conſiderable inflammation. 17. The diſcoloration is quite gone in them all, and from my own hand, a dry mark only re⯑maining. And they were all inoculated on the 18th, with variolous matter, which produced the diſeaſe in them all."

Mr. Power afterwards obſerves, that, as the patients from whom the blood was taken had the diſeaſe mildly, it may be ſuppoſed, that [405] though the contagious matter might be mixed with the blood, it might ſtill be in too dilute a ſtate to convey the infection; but adds at the ſame time, that he has diluted recent matter with at leaſt five times its quantity of water, and which has ſtill given the infection; though he has ſometimes diluted it ſo far as to fail.

The following experiments were inſtituted at my requeſt by my friend Mr. Hadley, ſurgeon in Derby, to aſcertain whether the blood of a perſon in the ſmall-pox be capable of communicating the diſeaſe. "Experiment 1ſt. October 18th, 1793. I took ſome blood from a vein in the arm of a perſon, who had the ſmall-pox on the ſecond day of the eruption, and introduced a ſmall quantity of it immedi⯑ately with the point of a lancet between the ſcarf and true ſkin of the right arm of a boy nine years old in two or three different places; the other arm was inoculated with variolous matter at the ſame time.

"19th. The punctured parts of the right arm were ſurrounded with ſome degree of ſubcuticular inflammation. 20th. The inflam⯑mation more conſiderable, with a ſlight degree of itching, but no pain upon preſſure. 21ſt. Upon examining the arm this day with a lens I found the inflammation leſs extenſive, and the redneſs changing to a deep yellow or orange-colour. 22d. Inflammation nearly gone. 23d. Nothing remained, except a ſlight diſcoloration and a little ſcurfy appearance on the punctures. At the ſame time the inflammation of the arm inoculated with variolous matter was increaſing faſt, and he had the diſeaſe mildly at the uſual time.

"Experiment 2d. I inoculated another child at the ſame time and in the ſame manner, with blood taken on the firſt day of the erup⯑tion; but as the appearance and effects were ſimilar to thoſe in the preceding experiment, I ſhall not relate them minutely.

Experiment 3d. October 20th. Blood was taken from a perſon who had the ſmall-pox, on the third day of the eruption, and on the ſixth from the commencement of the eruptive fever. I introduced ſome of it in its fluid ſtate into both arms of a boy ſeven years old. [406] 21ſt. There appeared to be ſome inflammation under the cuticle, where the punctures were made. 22d. Inflammation more con⯑ſiderable. 23d. On this day the inflammation was ſomewhat greater, and the cuticle rather elevated.

24th. Inflammation much leſs, and only a brown or orange-colour remained. 25th. Scarcely any diſcoloration left. On this day he was inoculated with variolous matter, the progreſs of the infection went on in the uſual way, and he had the ſmall-pox very favour⯑ably.

"At this time I was requeſted to inoculate a young perſon, who was thought to have had the ſmall-pox, but his parents were not quite certain; in one arm I introduced variolous matter, and in the other blood, taken as in experiment 3d. On the ſecond day after the ope⯑ration, the punctured parts were inflamed, though I think the arm in which I had inſerted variolous matter was rather more ſo than the other. On the third the inflammation was increaſed, and looked much the ſame as in the preceding experiment. 4th. The inflam⯑mation was much diminiſhed, and on the 5th almoſt gone. He was expoſed at the ſame time to the natural infection, but has continued perfectly well.

"I have frequently obſerved (and believe moſt practitioners have done the ſame), that if variolous matter be inſerted in the arm of a perſon who has previouſly had the ſmall-pox, that the inflammation on the ſecond or third days is much greater, than if they had not had the diſeaſe, but on the fourth or fifth it diſappears.

"On the 23d I introduced blood into the arms of three more chil⯑dren, taken on the third and fourth days of the eruption. The ap⯑pearances were much the ſame as mentioned in experiments firſt and third. They were afterwards inoculated with variolous matter, and had the diſeaſe in the regular way.

"The above experiments were made with blood taken from a ſmall vein in the hand or foot of three or four different patients, [407] whom I had at that time under inoculation. They were ſelected from 160, as having the greateſt number of puſtules. The part was waſhed with warm water before the blood was taken, to prevent the poſſibility of any matter being mixed with it from the ſurface."

Shall we conclude from hence, that the variolous matter never en⯑ters the blood-veſſels? but that the morbid motions of the veſſels of the ſkin around the inſertion of it continue to increaſe in a larger and larger circle for ſix or ſeven days; that then their quantity of morbid action becomes great enough to produce a fever-fit, and to affect the ſtomach by aſſociation of motions? and finally, that a ſecond aſſocia⯑tion of motions is produced between the ſtomach and the other parts of the ſkin, inducing them into morbid actions ſimilar to thoſe of the circle round the inſertion of the variolous matter? Many more ex⯑periments and obſervations are required before this important queſtion can be ſatisfactorily anſwered.

It may be adduced, that as the matter inſerted into the ſkin of the arm frequently ſwells the lymphatic in the axilla, that in that cir⯑cumſtance it ſeems to be there arreſted in its progreſs, and cannot be imagined to enter the blood by that lymphatic gland till the ſwelling of it ſubſides. Some other phaenomena of the diſeaſe are more eaſily reconcileable to this theory of ſympathetic motions than to that of abſorption; as the time taken up between the inſertion of the matter, and the operation of it on the ſyſtem, as mentioned above. For the circle around the inſertion is ſeen to increaſe, and to inflame; and I believe, undergoes a kind of diurnal paroxyſm of torpor and paleneſs with a ſucceeding increaſe of action and colour, like a topical fever⯑fit. Whereas if the matter is conceived to circulate for ſix or ſeven days with the blood, without producing diſorder, it ought to be ren⯑dered milder, or the blood-veſſels more familiarized to its acrimony.

It is much eaſier to conceive from this doctrine of aſſociated or ſympathetic motions of diſtant parts of the ſyſtem, how it happens, that the variolous inſection can be received but once, as before ex⯑plained; [408] than by ſuppoſing, that a change is effected in the maſs of blood by any kind of fermentative proceſs.

The curious circumſtance of the two contagions of ſmall-pox and meaſles not acting at the ſame time, but one of them reſting or ſuſ⯑pending its action till that of the other ceaſes, may be much eaſier explained from ſympathetic or aſſociated actions of the infected part with other parts of the ſyſtem, than it can from ſuppoſing the two contagions to enter the circulation.

The ſkin of the face is ſubject to more frequent viciſſitudes of heat and cold, from its expoſure to the open air, and is in conſequence more liable to ſenſitive aſſociation with the ſtomach than any other part of the ſurface of the body, becauſe their actions have been more fre⯑quently thus aſſociated. Thus in a ſurfeit from drinking cold water, when a perſon is very hot and fatigued, an eruption is liable to appear on the face in conſequence of this ſympathy. In the ſame manner the roſy eruption on the faces of drunkards more probably ariſes from the ſympathy of the face with the ſtomach, rather than between the face and the liver, as is generally ſuppoſed.

This ſympathy between the ſtomach and the ſkin of the face is ap⯑parent in the eruption of the ſmall-pox; ſince, where the diſeaſe is in conſiderable quantity, the eruption on the face firſt ſucceeds the ſick⯑neſs of the ſtomach. In the natural diſeaſe the ſtomach ſeems to be frequently primarily affected, either alone or along with the tonſils, as the matter ſeems to be only diffuſed in the air, and by being mixed with the ſaliva, or mucus of the tonſils, to be ſwallowed into the ſtomach.

After ſome days the irritative circles of motions become diſordered by this new ſtimulus, which acts upon the mucous lining of the ſto⯑mach; and ſickneſs, vertigo, and a diurnal fever ſucceed. Theſe diſordered irritative motions become daily increaſed for two or three days, and then by their increaſed action certain ſenſitive motions, or inflammation, is produced▪ and at the next cold ſit of fever, when [409] the ſtomach recovers from its torpor, an inflammation of the exter⯑nal ſkin is formed in points (which afterwards ſuppurate), by ſenſitive aſſociation, in the ſame manner as a cough is produced in conſequence of expoſing the feet to cold, as deſcribed in Sect. XXV. 1. 1. and Claſs IV. 2. 2. 4. If the inoculated ſkin of the arm, as far as it ap⯑pears inflamed, was to be cut out, or deſtroyed by cauſtic, before the fever commenced, as ſuppoſe on the fourth day after inoculation, would this prevent the diſeaſe? as it is ſuppoſed to prevent the hy⯑drophobia.

III. 1. Where the new veſſels, and enlarged old ones, which conſtitute inflammation, are not ſo haſtily diſtended as to burſt, and form a new kind of gland for the ſecretion of matter, as above men⯑tioned; if ſuch circumſtances happen as diminiſh the painful ſenſa⯑tion, the tendency to growth ceaſes, and by and by an abſorption commences, not only of the ſuperabundant quantity of fluids depo⯑ſited in the inflamed part, but of the ſolids likewiſe, and this even of the hardeſt kind.

Thus during the growth of the ſecond ſet of teeth in children, the roots of the firſt ſet are totally abſorbed, till at length nothing of them remains but the crown; though a few weeks before, if they are drawn immaturely, their roots are found complete. Similar to this Mr. Hunter has obſerved, that where a dead piece of bone is to exfo⯑liate, or to ſeparate from a living one, that the dead part does not pu⯑trify, but remains perfectly ſound, while the ſurface of the living part of the bone, which is in contact with the dead part, becomes abſorbed, and thus effects its ſeparation. Med. Comment. Edinb. V. 1. 425. In the ſame manner the calcareous matter of gouty concre⯑tions, the coagulable lymph depoſited on inflamed membranes in rheu⯑matiſm and extravaſated blood become abſorbed; which are all as ſo⯑lid and as indiſſoluble materials as the new veſſels produced in inflam⯑mation.

This abſorption of the new veſſels and depoſited fluids of inflamed [410] parts is called reſolution: it is produced by firſt uſing ſuch internal means as decreaſe the pain of the part, and in conſequence its new motions, as repeated bleeding, cathartics, diluent potations, and warm bath.

After the veſſels are thus emptied, and the abſorption of the new veſſels and depoſited fluids is evidently begun, it is much promoted by ſtimulating the part externally by ſolutions of lead, or other me⯑tals, and internally by the bark, and ſmall doſes of opium. Hence when an ophthalmy begins to become paler, any acrid eye-water, as a ſolution of ſix grains of white vitriol in an ounce of water, haſtens the abſorption, and clears the eye in a very ſhort time. But the ſame application uſed a few days ſooner would have increaſed the inflam⯑mation. Hence after evacuation opium in ſmall doſes may contri⯑bute to promote the abſorption of fluids depoſited on the brain, as ob⯑ſerved by Mr. Bromfield in his treatiſe of ſurgery.

2. Where an abſceſs is formed by the rupture of theſe new veſſels, the violence of inflammation ceaſes, and a new gland ſeparates a ma⯑terial called pus: at the ſame time a leſs degree of inflammation pro⯑duces new veſſels called vulgarly proud fleſh; which, if no bandage confines its growth, nor any other circumſtance promotes abſorption in the wound, would riſe to a great height above the uſual ſize of the part.

Hence the art of healing ulcers conſiſts in producing a tendency to abſorption in the wound greater than the depoſition. Thus when an ill-conditioned ulcer ſeparates a copious and thin diſcharge, by the uſe of any ſtimulus, as of ſalts of lead, or mercury, or copper exter⯑nally applied, the diſcharge becomes diminiſhed in quantity, and be⯑comes thicker, as the thinner parts are firſt abſorbed.

But nothing ſo much contributes to increaſe the abſorption in a wound as covering the whole limb above the ſore with a bandage, which ſhould be ſpread with ſome plaſter, as with emplaſtrum de minio, to prevent it from ſlipping. By this artificial tight⯑neſs [411] of the ſkin, the arterial pulſations act with double their uſual power in promoting the aſcending current of the fluid in the val⯑vular lymphatics.

Internally the abſorption from ulcers ſhould be promoted firſt by evacuation, then by opium, bark, mercury, ſteel.

3. Where the inflammation proceeds with greater violence or ra⯑pidity, that is, when by the painful ſenſation a more inordinate ac⯑tivity of the organ is produced, and by this great activity an additional quantity of painful ſenſation follows in an increaſing ratio, till the whole of the ſenſorial power, or ſpirit of animation, in the part be⯑comes exhauſted, a mortification enſues, as in a carbuncle, in in⯑flammations of the bowels, in the extremities of old people, or in the limbs of thoſe who are brought near a fire after having been much benumbed with cold. And from hence it appears, why weak people are more ſubject to mortification than ſtrong ones, and why in weak perſons leſs pain will produce mortification, namely, becauſe the ſen⯑ſorial power is ſooner exhauſted by any exceſs of activity. I remem⯑ber ſeeing a gentleman who had the preceding day travelled two ſtages in a chaiſe with what he termed a bearable pain in his bowels; which when I ſaw him had ceaſed rather ſuddenly, and without a paſſage through him; his pulſe was then weak, though not very quick; but as nothing which he ſwallowed would continue in his ſtomach many minutes, I concluded that the bowel was mortified; he died on the next day. It is uſual for patients ſinking under the ſmall-pox with mortified puſtules, and with purple ſpots intermixed, to complain of no pain, but to ſay they are pretty well to the laſt moment.

Recapitulation.

IV. When the motions of any part of the ſyſtem, in conſequence of previous torpor, are performed with more energy than in the irri⯑tative fevers, a diſagreeable ſenſation is produced, and new actions of ſome part of the ſyſtem commence in conſequence of this ſenſation conjointly with the irritation; which motions conſtitute inflamma⯑tion. If the fever be attended with a ſtrong pulſe, as in pleuriſy, or rheumatiſm, it is termed ſynocha ſenſitiva, or ſenſitive fever with ſtrong pulſe; which is uſually termed inflammatory fever. If it be attended with weak pulſe, it is termed typhus ſenſitivus, or ſenſitive fever with weak pulſe, or typhus gravior, or putrid malignant fever.

The ſynocha ſenſitiva, or ſenſitive fever with ſtrong pulſe, is ge⯑nerally attended with ſome topical inflammation, as in peripneumony, hepatitis, and is accompanied with much coagulable lymph, or ſize; which riſes to the ſurface of the blood, when taken into a baſon, as it cools; and which is believed to be the increaſed mucous ſecretion from the coats of the arteries, inſpiſſated by a greater abſorption of its aqueous and ſaline part, and perhaps changed by its delay in the circulation.

The typhus ſenſitivus, or ſenſitive fever with weak pulſe, is fre⯑quently attended with delirium, which is cauſed by the deficiency of the quantity of ſenſorial power, and with variety of cutaneous eruptions.

Inflammation is cauſed by the pains occaſioned by exceſs of action, and not by thoſe pains which are occaſioned by defect of action. Theſe morbid actions, which are thus produced by two ſenſorial powers, viz. by irritation and ſenſation, ſecrete new living ſibres, [413] which elongate the old veſſels, or form new ones, and at the ſame time much heat is evolved from theſe combinations. By the rupture of theſe veſſels, or by a new conſtruction of their apertures, purulent matters are ſecreted of various kinds; which are infectious the firſt time they are applied to the ſkin beneath the cuticle, or ſwallowed with the ſaliva into the ſtomach. This contagion acts not by its be⯑ing abſorbed into the circulation, but by the ſympathies, or aſſociated actions, between the part firſt ſtimulated by the contagious matter and the other parts of the ſyſtem. Thus in the natural ſmall-pox the contagion is ſwallowed with the ſaliva, and by its ſtimulus inflames the ſtomach; this variolous inflammation of the ſtomach increaſes every day, like the circle round the puncture of an inoculated arm, till it becomes great enough to diſorder the circles of irritative and ſenſitive motions, and thus produces fever fits, with ſickneſs and vomiting. Laſtly, after the cold paroxyſm, or fit of torpor, of the ſtomach has increaſed for two or three ſucceſſive days, an inflammation of the ſkin commences in points; which generally firſt appear upon the face, as the aſſociated actions between the ſkin of the face and that of the ſto⯑mach have been more frequently exerted together than thoſe of any other parts of the external ſurface.

Contagious matters, as thoſe of the meaſles and ſmall-pox, do not act upon the ſyſtem at the ſame time; but the progreſs of that which was laſt received is delayed, till the action of the former infection ceaſes. All kinds of matter, even that from common ulcers, are pro⯑bably contagious the firſt time they are inſerted beneath the cuticle or ſwallowed into the ſtomach; that is, as they were formed by certain morbid actions of the extremities of the veſſels, they have the power to excite ſimilar morbid actions in the extremities of other veſſels, to which they are applied; and theſe by ſympathy, or aſſociations of motion, excite ſimilar morbid actions in diſtant parts of the ſyſtem, without entering the circulation; and hence the blood of a patient in the ſmall-pox will not give that diſeaſe by inoculation to others.

[414] When the new fibres or veſſels become again abſorbed into the cir⯑culation, the inflammation ceaſes; which is promoted, after ſufficient evacuations, by external ſtimulants and bandages: but where the ac⯑tion of the veſſels is very great, a mortification of the part is liable to enſue, owing to the exhauſtion of ſenſorial power; which however occurs in weak people without much pain, and without very violent previous inflammation; and, like partial paralyſis, may be eſteemed one mode of natural death of old people, a part dying before the whole.

I. 1. BEFORE we commence this Section on Diſeaſed Voluntary Motions, it may be neceſſary to premiſe, that the word volition is not uſed in this work exactly in its common acceptation. Volition is ſaid in Section V. to bear the ſame analogy to deſire and averſion, which ſenſation does to pleaſure and pain. And hence that, when deſire or averſion produces any action of the muſcular fibres, or of the organs of ſenſe, they are termed volition; and the actions produced in conſequence are termed voluntary actions. Whence it appears, that motions of our muſcles or ideas may be produced in conſequence of deſire or averſion without our having the power to prevent them, and yet theſe motions may be termed voluntary, according to our definition of the word; though in common language they would be called involuntary.

The objects of deſire and averſion are generally at a diſtance, where⯑as thoſe of pleaſure and pain are immediately acting upon our organs. Hence, before deſire or averſion are exerted, ſo as to cauſe any ac⯑tions, there is generally time for deliberation; which conſiſts in diſ⯑covering the means to obtain the object of deſire, or to avoid the ob⯑ject of averſion; or in examining the good or bad conſequences, which may reſult from them. In this caſe it is evident, that we have a power to delay the propoſed action, or to perform it; and this power of chooſing, whether we ſhall act or not, is in common language ex⯑preſſed by the word volition, or will. Whereas in this work the word volition means ſimply the active ſtate of the ſenſorial faculty in producing motion in conſequence of deſire or averſion; whether we have the power of reſtraining that action, or not; that is, whether we exert any actions in conſequence of oppoſite deſires or averſions, or not.

[417] For if the objects of deſire or averſion are preſent, there is no ne⯑ceſſity to inveſtigate or compare the means of obtaining them, nor do we always deliberate about their conſequences; that is, no delibera⯑tion neceſſarily intervenes, and in conſequence the power of chooſing to act or not is not exerted. It is probable, that this twofold uſe of the word volition in all languages has confounded the metaphyſicians, who have diſputed about free will and neceſſity. Whereas from the above analyſis it would appear, that during our ſleep, we uſe no vo⯑luntary exertions at all; and in our waking hours, that they are the conſequence of deſire or averſion.

To will is to act in conſequence of deſire; but to deſire means to deſire ſomething, even if that ſomething be only to become free from the pain, which cauſes the deſire; for to deſire nothing is not to de⯑ſire; the word deſire, therefore, includes both the action and the ob⯑ject or motive; for the object and motive of deſire are the ſame thing. Hence to deſire without an object, that is, without a motive, is a ſoleciſm in language. As if one ſhould aſk, if you could eat without food, or breathe without air.

From this account of volition it appears, that convulſions of the muſcles, as in epileptic fits, may in the common ſenſe of that word be termed involuntary; becauſe no deliberation is interpoſed between the deſire or averſion and the conſequent action; but in the ſenſe of the word, as above defined, they belong to the claſs of voluntary mo⯑tions, as delivered in Vol. II. Claſs III. If this uſe of the word be diſcordant to the ear of the reader, the term morbid voluntary mo⯑tions, or motions in conſequence of averſion, may be ſubſtituted in its ſtead.

If a perſon has a deſire to be cured of the ague, and has at the ſame time an averſion (or contrary deſire) to ſwallowing an ounce of Pe⯑ruvian bark; he balances deſire againſt deſire, or averſion againſt averſion; and thus he acquires the power of chooſing, which is the common acceptation of the word willing. But in the cold fit of ague, [418] after having diſcovered that the act of ſhuddering, or exerting the ſub⯑cutaneous muſcles, relieves the pain of cold; he immediately exerts this act of volition, and ſhudders, as ſoon as the pain and conſequent averſion return, without any deliberation intervening; yet is this act, as well as that of ſwallowing an ounce of the bark, cauſed by voli⯑tion; and that even though he endeavours in vain to prevent it by a weaker contrary volition. This recalls to our minds the ſtory of the hungry aſs between two hay-ſtacks, where the two deſires are ſup⯑poſed ſo exactly to counteract each other, that he goes to neither of the ſtacks, but periſhes by want. Now as two equal and oppoſite de⯑ſires are thus ſuppoſed to balance each other, and prevent all action, it follows, that if one of theſe hayſtacks was ſuddenly removed, that the aſs would irreſiſtibly be hurried to the other, which in the com⯑mon uſe of the word might be called an involuntary act; but which, in our acceptation of it, would be claſſed amongſt voluntary actions, as above explained.

Hence to deliberate is to compare oppoſing deſires or averſions, and that which is the moſt intereſting at length prevails, and produces ac⯑tion. Similar to this, where two pains oppoſe each other, the ſtronger or more intereſting one produces action; as in pleuriſy the pain from ſuffocation would produce expanſion of the lungs, but the pain occa⯑ſioned by extending the inflamed membrane, which lines the cheſt, oppoſes this expanſion, and one or the other alternately prevails.

When any one moves his hand quickly near another perſon's eyes, the eye-lids inſtantly cloſe; this act in common language is termed involuntary, as we have not time to deliberate or to exert any con⯑trary deſire or averſion, but in this work it would be termed a volun⯑tary act, becauſe it is cauſed by the faculty of volition, and after a few trials the nictitation can be prevented by a contrary or oppoſing volition.

The power of oppoſing volitions is beſt exemplified in the ſtory of Mutius Scaevola, who is ſaid to have thruſt his hand into the fire be⯑fore [419] Porcenna, and to have ſuffered it to be conſumed for having failed him in his attempt on the life of that general. Here the aver⯑ſion for the loſs of fame, or the unſatisfied deſire to ſerve his country, the two prevalent enthuſiaſms at that time, were more powerful than the deſire of withdrawing his hand, which muſt be occaſioned by the pain of combuſtion; of theſe oppoſing volitions

If any one is told not to ſwallow his ſaliva for a minute, he ſoon ſwallows it contrary to his will, in the common ſenſe of that word; but this alſo is a voluntary action, as it is performed by the faculty of volition, and is thus to be underſtood. When the power of volition is exerted on any of our ſenſes, they become more acute, as in our attempts to hear ſmall noiſes in the night. As explained in Section XIX. 6. Hence by our attention to the fauces from our deſire not to ſwallow our ſaliva; the fauces become more ſenſible; and the ſtimu⯑lus of the ſaliva is followed by greater ſenſation, and conſequent de⯑ſire of ſwallowing it. So that the deſire or volition in conſequence of the increaſed ſenſation of the ſaliva is more powerful, than the previ⯑ous deſire not to ſwallow it. See Vol. II. Deglutitio invita. In the ſame manner if a modeſt man wiſhes not to want to make water, when he is confined with ladies in a coach or an aſſembly-room; that very act of volition induces the circumſtance, which he wiſhes to avoid, as above explained; inſomuch that I once ſaw a partial inſa⯑nity, which might be called a voluntary diabetes, which was occa⯑ſioned by the fear (and conſequent averſion) of not being able to make water at all.

It is further neceſſary to obſerve here, to prevent any confuſion of voluntary, with ſenſitive, or aſſociate motions, that in all the inſtances of violent efforts to relieve pain, thoſe efforts are at firſt voluntary exertions; but after they have been frequently repeated for the pur⯑poſe [420] of relieving certain pains, they become aſſociated with thoſe pains, and ceaſe at thoſe times to be ſubſervient to the will; as in coughing, ſneezing, and ſtrangury. Of theſe motions thoſe which contribute to remove or diſlodge the offending cauſe, as the actions of the abdominal muſcles in parturition or in vomiting, though they were originally excited by volition, are in this work termed ſenſitive motions; but thoſe actions of the muſcles or organs of ſenſe, which do not contribute to remove the offending cauſe, as in general convul⯑ſions or in madneſs, are in this work termed voluntary motions, or motions in conſequence of averſion, though in common language they are called involuntary ones. Thoſe ſenſitive unreſtrainable actions, which contribute to remove the cauſe of pain are uniformly and inva⯑riably exerted, as in coughing or ſneezing; but thoſe motions which are exerted in conſequence of averſion without contributing to remove the painful cauſe, but only to prevent the ſenſation of it, as in epi⯑leptic, or cataleptic fits, are not uniformly and invariably exerted, but change from one ſet of muſcles to another, as will be further ex⯑plained; and may by this criterion alſo be diſtinguiſhed from the former.

At the ſame time thoſe motions, which are excited by perpetual ſtimulus, or by aſſociation with each other, or immediately by plea⯑ſureable or painful ſenſation, may properly be termed involuntary mo⯑tions, as thoſe of the heart and arteries; as the faculty of volition ſel⯑dom affects thoſe, except when it exiſts in unnatural quantity, as in maniacal people.

2. It was obſerved in Section XIV. on the Production of Ideas, that thoſe parts of the ſyſtem, which are uſually termed the organs of ſenſe, are liable to be excited into pain by the exceſs of the ſtimu⯑lus of thoſe objects, which are by nature adapted to affect them; as of too great light, ſound, or preſſure. But that theſe organs receive no pain from the defect or abſence of theſe ſtimuli, as in darkneſs or ſilence. But that our other organs of perception, which have gene⯑rally [421] been called appetites, as of hunger, thirſt, want of heat, want of freſh air, are liable to be affected with pain by the defect, as well as by the exceſs of their appropriated ſtimuli.

This exceſs or defect of ſtimulus is however to be conſidered only as the remote cauſe of the pain, the immediate cauſe being the exceſs or defect of the natural action of the affected part, according to Sect. IV. 5. Hence all the pains of the body may be divided into thoſe from exceſs of motion, and thoſe from defect of motion; which diſtinction is of great importance in the knowledge and the cure of many diſeaſes. For as the pains from exceſs of motion either gradually ſubſide, or are in general ſucceeded by inflammation; ſo thoſe from defect of motion either gradually ſubſide, or are in general ſucceeded by convulſion, or madneſs. Theſe pains are eaſily diſtinguiſhable from each other by this circumſtance, that the former are attended with heat of the pain⯑ed part, or of the whole body; whereas the latter exiſts without in⯑creaſe of heat in the pained part, and is generally attended with cold⯑neſs of the extremities of the body; which is the true criterion of what have been called nervous pains.

Thus when any acrid material, as ſnuff or lime, falls into the eye, pain and inflammation and heat are produced from the exceſs of ſti⯑mulus; but violent hunger, hemicrania, or the clavus hyſtericus, are attended with coldneſs of the extremities, and defect of circula⯑tion. When we are expoſed to great cold, the pain we experience from the deficiency of heat is attended with a quieſcence of the mo⯑tions of the vaſcular ſyſtem; ſo that no inflammation is produced, but a great deſire of heat, and a tremulous motion of the ſubcutaneous muſcles, which is properly a convulſion in conſequence of this pain from defect of the ſtimulus of heat.

It was before mentioned, that as ſenſation conſiſts in certain move⯑ments of the ſenſorium, beginning at ſome of the extremities of it, and propagated to the central parts of it; ſo volition conſiſts of cer⯑tain other movements of the ſenſorium, commencing in the central [422] parts of it, and propagated to ſome of its extremities. This idea of theſe two great powers of motion in the animal machine is confirmed from obſerving, that they never exiſt in a great degree or univerſally at the ſame time; for while we ſtrongly exert our voluntary motions, we ceaſe to ſeel the pains or uneaſineſſes, which occaſioned us to exert them.

Hence during the time of fighting with fiſts or ſwords no pain is felt by the combatants, till they ceaſe to exert themſelves. Thus in the beginning of ague-fits the painful ſenſation of cold is diminiſhed, while the patient exerts himſelf in the ſhivering and gnaſhing of his teeth. He then ceaſes to exert himſelf, and the pain of cold returns; and he is thus perpetually induced to reiterate theſe exertions, from which he experiences a temporary relief. The ſame occurs in labour⯑pains, the exertion of the parturient woman relieves the violence of the pains for a time, which recur again ſoon after ſhe has ceaſed to uſe thoſe exertions. The ſame is true in many other painful diſeaſes, as in the ſtrangury, teneſmus, and the efforts of vomiting; all theſe diſagreeable ſenſations are diminiſhed or removed for a time by the various exertions they occaſion, and recur alternately with thoſe ex⯑ertions.

The reſtleſſneſs in ſome fevers is an almoſt perpetual exertion of this kind, excited to relieve ſome diſagreeable ſenſations; the recipro⯑cal oppoſite exertions of a wounded worm, the alternate emproſtho⯑tonos and opiſthotonos of ſome ſpaſmodic diſeaſes, and the intervals of all convulſions, from whatever cauſe, ſeem to be owing to this cir⯑cumſtance of the laws of animation; that great or univerſal exertion cannot exiſt at the ſame time with great or univerſal ſenſation, though they can exiſt reciprocally; which is probably reſolvable into the more general law, that the whole ſenſorial power being expanded in one mode of exertion, there is none to ſpare for any other. Whence ſyncope, or temporary apoplexy, ſucceeds to epileptic convul⯑ſions.

[423] 3. Hence when any violent pain afflicts us, of which we can nei⯑ther avoid nor remove the cauſe, we ſoon learn to endeavour to alle⯑viate it, by exerting ſome violent voluntary effort, as mentioned above; and are naturally induced to uſe thoſe muſcles for this pur⯑poſe, which have been in the early periods of our lives moſt frequent⯑ly or moſt powerfully exerted.

Now the firſt muſcles, which infants uſe moſt frequently, are thoſe of reſpiration; and on this account we gain a habit of holding our breath, at the ſame time that we uſe great efforts to exclude it, for this purpoſe of alleviating unavoidable pain; or we preſs out our breath through a ſmall aperture of the larinx, and ſcream violently, when the pain is greater than is relievable by the former mode of ex⯑ertion. Thus children ſcream to relieve any pain either of body or mind, as from anger, or fear of being beaten.

Hence it is curious to obſerve, that thoſe animals, who have more frequently exerted their muſcles of reſpiration violently, as in talking, barking, or grunting, as children, dogs, hogs, ſcream much more, when they are in pain, than thoſe other animals, who uſe little or no language in their common modes of life; as horſes, ſheep, and cows.

The next moſt frequent or moſt powerful efforts, which infants are firſt tempted to produce, are thoſe with the muſcles in biting hard ſubſtances: indeed the exertion of theſe muſcles is very powerful in common maſtication, as appears from the pain we receive, if a bit of bone is unexpectedly found amongſt our ſofter food; and further ap⯑pears from their acting to ſo great mechanical diſadvantage, particu⯑larly when we bite with the inciſores, or canine teeth; which are firſt formed, and thence are firſt uſed to violent exertion.

Hence when a perſon is in great pain, the cauſe of which he can⯑not remove, he ſets his teeth firmly together, or bites ſome ſubſtance between them with great vehemence, as another mode of violent ex⯑ertion to produce a temporary relief. Thus we have a proverb where [424] no help can be had in pain, "to grin and abide;" and the tortures of hell are ſaid to be attended with "gnaſhing of teeth."

Hence in violent ſpaſmodic pains I have ſeen people bite not only their tongues, but their arms or fingers, or thoſe of the attendants, or any object which was near them; and alſo ſtrike, pinch, or tear, others or themſelves, particularly the part of their own body, which is painful at the time. Soldiers, who die of painful wounds in battle, are ſaid by Homer to bite the ground. Thus alſo in the bellon, or colica ſaturnina, the patients are ſaid to bite their own fleſh, and dogs in this diſeaſe to bite up the ground they lie upon. It is pro⯑bable that the great endeavours to bite in mad dogs, and the violence of other mad animals, is owing to the ſame cauſe.

4. If the efforts of our voluntary motions are exerted with ſtill greater energy for the relief of ſome diſagreeable ſenſation, convul⯑ſions are produced; as the various kinds of epilepſy, and in ſome hyſ⯑teric paroxiſms. In all theſe diſeaſes a pain or diſagreeable ſenſation is produced, frequently by worms, or acidity in the bowels, or by a diſeaſed nerve in the ſide, or head, or by the pain of a diſeaſed liver.

In ſome conſtitutions a more intolerable degree of pain is produced in ſome part at a diſtance from the cauſe by ſenſitive aſſociation, as before explained; theſe pains in ſuch conſtitutions ariſe to ſo great a degree, that I verily believe no artificial tortures could equal ſome, which I have witneſſed; and am confident life would not have long been preſerved, unleſs they had been ſoon diminiſhed or removed by the univerſal convulſion of the voluntary motions, or by temporary madneſs.

In ſome of the unfortunate patients I have obſerved, the pain has riſen to an inexpreſſible degree, as above deſcribed, before the con⯑vulſions have ſupervened; and which were preceded by ſcreaming, and grinning; in others, as in the common epilepſy, the convulſion has immediately ſucceeded the commencement of the diſagreeable [425] ſenſations; and as a ſtupor frequently ſucceeds the convulſions, they only ſeemed to remember that a pain at the ſtomach preceded the fit, or ſome other uneaſy feel; or more frequently retained no memory at all of the immediate cauſe of the paroxyſm. But even in this kind of epilepſy, where the patient does not recollect any preceding pain, the paroxiſms generally are preceded by a quivering motion of the under jaw, with a biting of the tongue; the teeth afterwards become preſſed together with vehemence, and the eyes are then convulſed, before the commencement of the univerſal convulſion; which are all efforts to relieve pain.

The reaſon why theſe convulſive motions are alternately exerted and remitted was mentioned above, and in Sect. XII. 1. 3. when the exertions are ſuch as give a temporary relief to the pain, which ex⯑cites them, they ceaſe for a time, till the pain is again perceived; and then new exertions are produced for its relief. We ſee daily examples of this in the loud reiterated laughter of ſome people; the pleaſure⯑able ſenſation, which excites this laughter, ariſes for a time ſo high as to change its name and become painful: the convulſive motions of the reſpiratory muſcles relieve the pain for a time; we are, however, unwilling to loſe the pleaſure, and preſently put a ſtop to this ex⯑ertion, and immediately the pleaſure recurs, and again as inſtantly riſes into pain. All of us have felt the pain of immoderate laughter; children have been tickled into convulſions of the whole body; and others have died in the act of laughing; probably from a paralyſis ſucceeding the long continued actions of the muſcles of reſpira⯑tion.

Hence we learn the reaſon, why children, who are ſo eaſily ex⯑cited to laugh by the tickling of other people's fingers, cannot tickle themſelves into laughter. The exertion of their hands in the en⯑deavour to tickle themſelves prevents the neceſſity of any exertion of the reſpiratory muſcles to relieve the exceſs of pleaſureable affection. See Sect. XVII. 3. 5.

[426] Chryſippus is recorded to have died laughing, when an aſs was in⯑vited to ſup with him. The ſame is related of one of the popes, who, when he was ill, ſaw a tame monkey at his bedſide put on the holy thiara. Hall. Phyſ. T. III. p. 306.

There are inſtances of epilepſy being produced by laughing recorded by Van Swieton, T. III. 402 and 308. And it is well known, that many people have died inſtantaneouſly from the painful exceſs of joy, which probably might have been prevented by the exertions of laughter.

Every combination of ideas, which we attend to, occaſions pain or pleaſure; thoſe which occaſion pleaſure, furniſh either ſocial or ſelfiſh pleaſure, either malicious or friendly, or laſcivious, or ſublime plea⯑ſure; that is, they give us pleaſure mixed with other emotions, or they give us unmixed pleaſure, without occaſioning any other emo⯑tions or exertions at the ſame time. This unmixed pleaſure, if it be great, becomes painful, like all other animal motions from ſtimuli of every kind; and if no other exertions are occaſioned at the ſame time, we uſe the exertion of laughter to relieve this pain. Hence laughter is occaſioned by ſuch wit as excites ſimple pleaſure without any other emotion, ſuch as pity, love, reverence. For ſublime ideas are mixed with admiration, beautiful ones with love, new ones with ſurpriſe; and theſe exertions of our ideas prevent the action of laugh⯑ter from being neceſſary to relieve the painful pleaſure above deſcribed. Whence laughable wit conſiſts of ſrivolous ideas, without connections of any conſequence, ſuch as puns on words, or on phraſes, incon⯑gruous junctions of ideas; on which account laughter is ſo frequent in children.

Unmixed pleaſure leſs than that, which cauſes laughter, cauſes ſleep, as in ſinging children to ſleep, or in ſlight intoxication from wine or food. See Sect. XVIII. 12.

5. If the pains, or diſagreeable ſenſations, above deſcribed do not obtain a temporary relief from theſe convulſive exertions of the muſ⯑cles, [427] thoſe convulſive exertions continue without remiſſion, and one kind of catalepſy is produced. Thus when a nerve or tendon pro⯑duces great pain by its being inflamed or wounded, the patient ſets his teeth firmly together, and grins violently, to diminiſh the pain; and if the pain is not relieved by this exertion, no relaxation of the maxillary muſcles takes place, as in the convulſions above deſcribed, but the jaws remain firmly fixed together. This locked jaw is the moſt frequent inſtance of cataleptic ſpaſm, becauſe we are more in⯑clined to exert the muſcles ſubſervient to maſtication from their early obedience to violent efforts of volition.

But in the caſe related in Sect. XIX. on Reverie, the cataleptic lady had pain in her upper teeth; and preſſing one of her hands ve⯑hemently againſt her cheek-bone to diminiſh this pain, it remained in that attitude for about half an hour twice a day, till the painful pa⯑roxyſm was over.

I have this very day ſeen a young lady in this diſeaſe, (with which ſhe has frequently been afflicted,) ſhe began to-day with violent pain ſhooting from one ſide of the forehead to the occiput, and after various ſtruggles lay on the bed with her fingers and wriſts bent and ſtiff for about two hours; in other reſpects ſhe ſeemed in a ſyncope with a natural pulſe. She then had intervals of pain and of ſpaſm, and took three grains of opium every hour till ſhe had taken nine grains, before the pains and ſpaſm ceaſed.

There is, however, another ſpecies of fixed ſpaſm, which differs from the former, as the pain exiſts in the contracted muſcle, and would ſeem rather to be the conſequence than the cauſe of the con⯑traction, as in the cramp in the calf of the leg, and in many other parts of the body.

In theſe ſpaſms it ſhould ſeem, that the muſcle itſelf is firſt thrown into contraction by ſome diſagreeable ſenſation, as of cold; and that then the violent pain is produced by the great contraction of the muſ⯑cular [428] fibres extending its own tendons, which are ſaid to be ſenſible to extenſion only; and is further explained in Sect. XVIII. 15.

6. Many inſtances have been given in this work, where after vio⯑lent motions excited by irritation, the organ has become quieſcent to leſs, and even to the great irritation, which induced it into violent motion; as after looking long at the ſun or any bright colour, they ceaſe to be ſeen; and after removing from bright day-light into a gloomy room, the eye cannot at firſt perceive the objects, which ſti⯑mulate it leſs. Similar to this is the ſyncope, which ſucceeds after the violent exertions of our voluntary motions, as after epileptic fits, for the power of volition acts in this caſe as the ſtimulus in the other. This ſyncope is a temporary palſy, or apoplexy, which ceaſes after a time, the muſcles recovering their power of being excited into action by the efforts of volition; as the eye in the circumſtance above men⯑tioned recovers in a little time its power of ſeeing objects in a gloomy room; which were inviſible immediately after coming out of a ſtronger light. This is owing to an accumulation of ſenſorial power during the inaction of thoſe fibres, which were before accuſtomed to per⯑petual exertions, as explained in Sect. XII. 7. 1. A ſlighter degree of this diſeaſe is experienced by every one after great fatigue, when the muſcles gain ſuch inability to further action, that we are obliged to reſt them for a while, or to ſummon a greater power of volition to continue their motions.

In all the ſyncopes, which I have ſeen induced after convulſive fits, the pulſe has continued natural, though the organs of ſenſe as well as the locomotive muſcles, have ceaſed to perform their functions; for it is neceſſary for the perception of objects, that the external organs of ſenſe ſhould be properly excited by the voluntary power, as the eye-lids muſt be open, and perhaps the muſcles of the eye put into action to diſtend, and thence give greater pellucidity to the cornea, which in ſyncope, as in death, appears flat and leſs tranſparent. [429] The tympanum of the ear alſo ſeems to require a voluntary exertion of its muſcles, to gain its due tenſion, and it is probable the other external organs of ſenſe require a ſimilar voluntary exertion to adapt them to the diſtinct perception of objects. Hence in ſyncope as in ſleep, as the power of volition is ſuſpended, no external objects are perceived. See Sect. XVIII. 5. During the time which the patient lies in a fainting fit, the ſpirit of animation becomes accumulated; and hence the muſcles in a while become irritable by their uſual ſti⯑mulation, and the fainting fit ceaſes. See Sect. XII. 7. 1.

7. If the exertion of the voluntary motions has been ſtill more energetic, the quieſcence, which ſucceeds, is ſo complete, that they cannot again be excited into action by the efforts of the will. In this manner the palſy, and apoplexy (which is an univerſal palſy) are fre⯑quently produced after convulſions, or other violent exertions; of this I ſhall add a few inſtances.

Platernus mentions ſome, who have died apoplectic from violent exertions in dancing; and Dr. Mead, in his Eſſay on Poiſons, records a patient in the hydrophobia, who at one effort broke the cords which bound him, and at the ſame inſtant expired. And it is pro⯑bable, that thoſe, who have expired from immoderate laughter, have died from this paralyſis conſequent to violent exertion. Mrs. Scott of Stafford was walking in her garden in perfect health with her neigh⯑bour Mrs. [...]; the latter accidentally fell into a muddy rivulet, and tried in vain to diſengage herſelf by the aſſiſtance of Mrs. Scott's hand. Mrs. Scott exerted her utmoſt power for many minutes, firſt to aſſiſt her friend, and next to prevent herſelf from being pulled into the moraſs, as her diſtreſſed companion would not diſengage her hand. After other aſſiſtance was procured by their united ſcreams, Mrs. Scott walked to a chair about twenty yards from the brook, and was ſeized with an apoplectic ſtroke; which continued many days, and terminated in a total loſs of her right arm, and her ſpeech; neither of which ſhe ever after perfectly recovered.

[430] It is ſaid, that many people in Holland have died after ſkating too long or too violently on their frozen canals; it is probable the death of theſe, and of others, who have died ſuddenly in ſwimming, has been owing to this great quieſcence or paralyſis; which has ſucceeded very violent exertions, added to the concomitant cold, which has had greater effect after the ſufferers had been heated and exhauſted by pre⯑vious exerciſe.

I remember a young man of the name of Nairne at Cambridge, who walking on the edge of a barge fell into the river. His couſin and fellow-ſtudent of the ſame name, knowing the other could not ſwim, plunged into the water after him, caught him by his clothes, and approaching the bank by a vehement exertion propelled him ſafe to the land, but that inſtant, ſeized, as was ſuppoſed, by the cramp, or paralyſis, ſunk to riſe no more. The reaſon why the cramp of the muſcles, which compoſe the calf of the leg, is ſo liable to affect ſwimmers, is, becauſe theſe muſcles have very weak antagoniſts, and are in walking generally elongated again after their contraction by the weight of the body on the ball of the toe, which is very much greater than the reſiſtance of the water in ſwimming. See Section XVIII. 15.

It does not follow that every apoplectic or paralytic attack is im⯑mediately preceded by vehement exertion; the quieſcence, which ſucceeds exertion, and which is not ſo great as to be termed paralyſis, frequently recurs afterwards at certain periods; and by other cauſes of quieſcence, occurring with thoſe periods, as was explained in treat⯑ing of the paroxyſms of intermitting fevers; the quieſcence at length becomes ſo great as to be incapable of again being removed by the efforts of volition, and complete paralyſis is formed. See Section XXXII. 3. 2.

Many of the paralytic patients, whom I have ſeen, have evidently had diſeaſed livers from the too frequent potation of ſpirituous liquors; ſome of them have had the gutta roſea on their faces and breaſts; [431] which has in ſome degree receded either ſpontaneouſly, or by the uſe of external remedies, and the paralytic ſtroke has ſucceeded; and as in ſeveral perſons, who have drank much vinous ſpirits, I have ob⯑ſerved epileptic fits to commence at about forty or fifty years of age, without any hereditary taint, from the ſtimulus, as I believed, of a diſeaſed liver; I was induced to aſcribe many paralytic caſes to the ſame ſource; which were not evidently the effect of age, or of un⯑acquired debility. And the account given before of dropſies, which very frequently are owing to a paralyſis of the abſorbent ſyſtem, and are generally attendant on free drinkers of ſpirituous liquors, con⯑firmed me in this opinion.

The diſagreeable irritation of a diſeaſed liver produces exertions and conſequent quieſcence; theſe by the accidental concurrence of other cauſes of quieſcence, as cold, ſolar or lunar periods, inanition, the want of their uſual portion of ſpirit of wine, at length produces pa⯑ralyſis.

This is further confirmed by obſerving, that the muſcles, we moſt frequently, or moſt powerfully exert, are moſt liable to palſy; as thoſe of the voice and of articulation, and of thoſe paralytics which I have ſeen, a much greater proportion have loſt the uſe of their right arm; which is ſo much more generally exerted than the left.

I cannot diſmiſs this ſubject without obſerving, that after a para⯑lytic ſtroke, if the vital powers are not much injured, that the pa⯑tient has all the movements of the affected limb to learn over again, juſt as in early infancy; the limb is firſt moved by the irritation of its muſcles, as in ſtretching, (of which a caſe was related in Section VII. 1. 3.) or by the electric concuſſion; afterwards it becomes obedient to ſenſation, as in violent danger or fear; and laſtly, the muſcles become again aſſociated with volition, and gradually acquire their uſual habits of acting together.

Another phaenomenon in palſies is, that when the limbs of one ſide [432] are diſabled, thoſe of the other are in perpetual motion. This can only be explained from conceiving that the power of motion, what⯑ever it is, or wherever it reſides, and which is capable of being ex⯑hauſted by fatigue, and accumulated in reſt, is now leſs expended, whilſt one half of the body is incapable of receiving its uſual propor⯑tion of it, and is hence derived with greater eaſe or in greater abundance into the limbs, which remain unaffected.

II. 1. The exceſs or defect of voluntary exertion produces ſimilar effects upon the ſenſual motions, or ideas of the mind, as thoſe al⯑ready mentioned upon the muſcular fibres. Thus when any violent pain, ariſing from the defect of ſome peculiar ſtimulus, exiſts either in the muſcular or ſenſual ſyſtems of fibres, and which cannot be removed by acquiring the defective ſtimulus; as in ſome conſtitutions convulſions of the muſcles are produced to procure a temporary relief, ſo in other conſtitutions vehement voluntary exertions of the ideas of the mind are produced for the ſame purpoſe; for during this exertion, like that of the muſcles, the pain either vaniſhes or is diminiſhed: this violent exertion conſtitutes madneſs; and in many caſes I have ſeen the madneſs take place, and the convulſions ceaſe, and recipro⯑cally the madneſs ceaſe, and the convulſions ſupervene. See Section III. 5. 8.

2. Madneſs is diſtinguiſhable from delirium, as in the latter the patient knows not the place where he reſides, nor the perſons of his friends or attendants, nor is conſcious of any external objects, except when ſpoken to with a louder voice, or ſtimulated with unuſual force, and even then he ſoon relapſes into a ſtate of inattention to every thing about him. Whilſt in the former he is perfectly ſenſible to every thing external, but has the voluntary powers of his mind in⯑tenſely exerted on ſome particular object of his deſire or averſion, he harbours in his thoughts a ſuſpicion of all mankind, leſt they ſhould counteract his deſigns; and while he keeps his intentions, and the [433] motives of his actions profoundly ſecret; he is perpetually ſtudying the means of acquiring the object of his wiſh, or of preventing or re⯑venging the injuries he ſuſpects.

3. A late French philoſopher, Mr. Helvetius, has deduced almoſt all our actions from this principle of their relieving us from the ennui or taedium vitae; and true it is, that our deſires or averſions are the motives of all our voluntary actions; and human nature ſeems to ex⯑cel other animals in the more facil uſe of this voluntary power, and on that account is more liable to inſanity than other animals. But in mania this violent exertion of volition is expended on miſtaken ob⯑jects, and would not be relieved, though we were to gain or eſcape the objects, that excite it. Thus I have ſeen two inſtances of mad⯑men, who conceived that they had the itch, and ſeveral have believed they had the venereal infection, without in reality having a ſymptom of either of them. They have been perpetually thinking upon this ſubject, and ſome of them were in vain ſalivated with deſign of con⯑vincing them to the contrary.

4. In the minds of mad people thoſe volitions alone exiſt, which are unmixed with ſenſation; immoderate ſuſpicion is generally the firſt ſymptom, and want of ſhame, and want of delicacy about clean⯑lineſs. Suſpicion is a voluntary exertion of the mind ariſing from the pain of fear, which it is exerted to relieve: ſhame is the name of a peculiar diſagreeable ſenſation, ſee Fable of the Bees, and delicacy about cleanlineſs ariſes from another diſagreeable ſenſation. And therefore are not found in the minds of maniacs, which are employed ſolely in voluntary exertions. Hence the moſt modeſt women in this diſeaſe walk naked amongſt men without any kind of concern, uſe obſcene diſcourſe, and have no delicacy about their natural eva⯑cuations.

5. Nor are maniacal people more attentive to their natural appe⯑tites, or to the irritations which ſurround them, except as far as may reſpect their ſuſpicions or deſigns; for the violent and perpetual ex⯑ertions [434] of their voluntary powers of mind prevents their perception of almoſt every other object, either of irritation or of ſenſation. Hence it is that they bear cold, hunger, and fatigue, with much greater pertinacity than in their ſober hours, and are leſs injured by them in reſpect to their general health. Thus it is aſſerted by hiſtorians, that Charles the Twelfth of Sweden ſlept on the ſnow, wrapped only in his cloak, at the ſiege of Frederickſtad, and bore extremes of cold, and hunger, and fatigue, under which numbers of his ſoldiers periſh⯑ed; becauſe the king was inſane with ambition, but the ſoldier had no ſuch powerful ſtimulus to preſerve his ſyſtem from debility and death.

6. Beſides the inſanities ariſing from exertions in conſequence of pain, there is alſo a pleaſureable inſanity, as well as a pleaſureable delirium; as the inſanity of perſonal vanity, and that of religious fa⯑naticiſm. When agreeable ideas excite into motion the ſenſorial power of ſenſation, and this again cauſes other trains of agreeable ideas, a conſtant ſtream of pleaſureable ideas ſucceeds, and produces pleaſureable delirium. So when the ſenſorial power of volition ex⯑cites agreeable ideas, and the pleaſure thus produced excites more vo⯑lition in its turn, a conſtant flow of agreeable voluntary ideas ſuc⯑ceeds; which when thus exerted in the extreme conſtitutes in⯑ſanity.

Thus when our muſcular actions are excited by our ſenſations of pleaſure, it is termed play; when they are excited by our volition, it is termed work; and the former of theſe is attended with leſs fa⯑tigue, becauſe the muſcular actions in play produce in their turn more pleaſureable ſenſation; which again has the property of pro⯑ducing more muſcular action. An agreeable inſtance of this I ſaw this morning. A little boy, who was tired with walking, begged of his papa to carry him. "Here," ſays the reverend doctor, "ride upon my gold-headed cane;" and the pleaſed child, putting it be⯑tween his legs, gallopped away with delight, and complained no [435] more of his fatigue. Here the aid of another ſenſorial power, that of pleaſureable ſenſation, ſuperadded vigour to the exertion of exhauſted volition. Which could otherwiſe only have been excited by additional pain, as by the laſh of ſlavery. On this account where the whole ſenſorial power has been exerted on the contemplation of the promiſed joys of heaven, the ſaints of all perſecuted religions have borne the tortures of martyrdom with otherwiſe unaccountable firmneſs.

7. There are ſome diſeaſes, which obtain at leaſt a temporary relief from the exertions of inſanity; many inſtances of dropſies being thus for a time cured are recorded. An elderly woman labouring with aſcites I twice ſaw relieved for ſome weeks by inſanity, the dropſy ceaſed for ſeveral weeks, and recurred again alternating with the in⯑ſanity. A man afflicted with difficult reſpiration on lying down, with very irregular pulſe, and oedematous legs, whom I ſaw this day, has for above a week been much relieved in reſpect to all thoſe ſymptoms by the acceſſion of inſanity, which is ſhewn by inordinate ſuſpicion, and great anger.

In caſes of common temporary anger the increaſed action of the ar⯑terial ſyſtem is ſeen by the red ſkin, and increaſed pulſe, with the immediate increaſe of muſcular activity. A friend of mine, when he was painfully fatigued by riding on horſeback, was accuſtomed to call up ideas into his mind, which uſed to excite his anger or indignation, and thus for a time at leaſt relieved the pain of fatigue. By this tempo⯑rary inſanity, the effect of the voluntary power upon the whole of his ſyſtem was increaſed; as in the caſes of dropſy above mentioned, it would appear, that the increaſed action of the voluntary faculty of the ſenſorium affected the abſorbent ſyſtem, as well as the ſecerning one.

8. In reſpect to relieving inflammatory pains, and removing fever, I have ſeen many inſtances, as mentioned in Sect. XII. 2. 4. One lady, whom I attended, had twice at ſome years interval a locked [436] jaw, which relieved a pain on her ſternum with peripneumony. Two other ladies I ſaw, who towards the end of violent peripneumony, in which they frequently loſt blood, were at length cured by inſanity ſupervening. In the former the increaſed voluntary exertion of the muſcles of the jaw, in the latter that of the organs of ſenſe, removed the diſeaſe; that is, the diſagreeable ſenſation, which had produced the inflammation, now excited the voluntary power, and theſe new voluntary exertions employed or expended the ſuperabundant ſenſorial power, which had previouſly been exerted on the arterial ſyſtem, and cauſed inflammation.

Another caſe, which I think worth relating, was of a young man about twenty; he had laboured under an irritative fever with debility for three or four weeks, with very quick and very feeble pulſe, and other uſual ſymptoms of that ſpecies of typhus, but at this time com⯑plained much and frequently of pain of his legs and feet. When thoſe who attended him were nearly in deſpair of his recovery, I obſerved with pleaſure an inſanity of mind ſupervene: which was totally dif⯑ferent from delirium, as he knew his friends, calling them by their names, and the room in which he lay, but became violently ſuſpici⯑ous of his attendants, and calumniated with vehement oaths his ten⯑der mother, who ſat weeping by his bed. On this his pulſe became flower and ſirmer, but the quickneſs did not for ſome time intirely ceaſe, and he gradually recovered. In this caſe the introduction of an increaſed quantity of the power of volition gave vigour to thoſe move⯑ments of the ſyſtem, which are generally only actuated by the power of irritation, and of aſſociation.

Another caſe I recollect of a young man, about twenty-five, who had the fcarlet-fever, with very quick pulſe, and an univerſal erup⯑tion on his ſkin, and was not without reaſon eſteemed to be in great danger of his life. After a few days an inſanity ſupervened, which his friends miſtook for delirium, and he gradually recovered, and the [437] cuticle peeled off. From theſe and a few other caſes I have always eſteemed inſanity to be a favourable ſign in fevers, and have cautiouſly diſtinguiſhed it from delirium.

III. Another mode of mental exertion to relieve pain, is by pro⯑ducing a train of ideas not only by the efforts of volition, as in inſa⯑nity; but by thoſe of ſenſation likewiſe, as in delirium and ſleep. This mental effort is termed reverie, or ſomnambulation, and is de⯑ſcribed more at large in Sect. XIX. on that ſubject. But I ſhall here relate another caſe of that wonderful diſeaſe, which fell yeſterday under my eye, and to which I have ſeen many analogous alienations of mind, though not exactly ſimilar in all circumſtances. But as all of them either began or terminated with pain or convulſion, there can be no doubt but that they are of epileptic origin, and conſtitute another mode of mental exertion to relieve ſome painful ſen⯑ſation.

1. Maſter A. about nine years old, had been ſeized at ſeven every morning for ten days with uncommon fits, and had had ſlight re⯑turns in the afternoon. They were ſuppoſed to originate from worms, and had been in vain attempted to be removed by vermi⯑fuge purges. As his fit was expected at ſeven yeſterday morning, I ſaw him before that hour; he was aſleep, ſeemed free from pain, and his pulſe natural. About ſeven he began to complain of pain about his navel, or more to the left ſide, and in a few minutes had exertions of his arms and legs like ſwimming. He then for half an hour hunted a pack of hounds; as appeared by his hallooing, and calling the dogs by their names, and diſcourſing with the at⯑tendants of the chaſe, deſcribing exactly a day of hunting, which (I was informed) he had witneſſed a year before, going through all the moſt minute circumſtances of it; calling to people, who were then preſent, and lamenting the abſence of others, who were then alſo abſent. After this [...] he imitated, as he lay in bed, [438] ſome of the plays of boys, as ſwimming and jumping. He then ſung an Engliſh and then an Italian ſong; part of which with his eyes open, and part with them cloſed, but could not be awakened or excited by any violence, which it was proper to uſe.

After about an hour he came ſuddenly to himſelf with ap⯑parent ſurpriſe, and ſeemed quite ignorant of any part of what had paſſed, and after being apparently well for half an hour, he ſuddenly fell into a great ſtupor, with ſlower pulſe than natural, and a ſlow moaning reſpiration, in which he continued about another half hour, and then recovered.

The ſequel of this diſeaſe was favourable; he was directed one grain of opium at ſix every morning, and then to riſe out of bed; at half paſt ſix he was directed fifteen drops of laudanum in a glaſs of wine and water. The firſt day the paroxyſm became ſhorter, and leſs violent. The doſe of opium was increaſed to one⯑half more, and in three or four days the fits left him. The bark and filings of iron were alſo exhibited twice a day; and I believe the com⯑plaint returned no more.

2. In this paroxyſm it muſt be obſerved, that he began with pain, and ended with ſtupor, in both circumſtances reſembling a fit of epilepſy. And that therefore the exertions both of mind and body, both the vo⯑luntary ones, and thoſe immediately excited by pleaſureable ſenſation, were exertions to relieve pain.

The hunting ſcene appeared to be rather an act of memory than of imagination, and was therefore rather a voluntary exer⯑tion, though attended with the pleaſureable eagerneſs, which was the conſequence of thoſe ideas recalled by recollection, and not the cauſe of them.

Theſe ideas thus voluntarily recollected were ſucceeded by ſen⯑ſations of pleaſure, though his ſenſes were unaffected by the ſti⯑muli [439] of viſible or audible objects; or ſo weakly excited by them as not to produce ſenſation or attention. And the pleaſure thus ex⯑cited by volition produced other ideas and other motions in con⯑ſequence of the ſenſorial power of ſenſation. Whence the mixed catenations of voluntary and ſenſitive ideas and muſcular motions in reverie; which, like every other kind of vehement exertion, contribute to relieve pain, by expending a large quantity of ſenſorial power.

Thoſe fits generally commence during ſleep, from whence I ſup⯑poſe they have been thought to have ſome connection with ſleep, and have thence been termed Somnambaliſm; but their commence⯑ment during ſleep is owing to our increaſed excitability by internal ſenſations at that time, as explained in Sect. XVIII. 14 and 15, and not to any ſimilitude between reverie and ſleep.

3. I was once concerned for a very elegant and ingenious young lady, who had a reverie on alternate days, which continued nearly the whole day; and as in her days of diſeaſe ſhe took up the ſame kind of ideas, which ſhe had converſed about on the alternate day before, and could recollect nothing of them on her well-day; ſhe appeared to her friends to poſſeſs two minds. This caſe alſo was of epileptic kind, and was cured, with ſome relapſes, by opium admi⯑niſtered before the commencement of the paroxyſm.

4 Whence it appears, that the methods of relieving inflammatory pains, is by removing all ſtimulus, as by veneſection, cool air, mu⯑cilaginous diet, aqueous potation, ſilence, darkneſs.

The methods of relieving pains from defect of ſtimulus is by ſup⯑plying the peculiar ſtimulus required, as of food, or warmth.

And the general method of relieving pain is by exciting into action ſome great part of the ſyſtem for the purpoſe of expending a part of the ſenſorial power. This is done either by exertion of the vo⯑luntary ideas and muſcles, as in inſanity and convulſion; or by [440] exerting both voluntary and ſenſitive motions, as in reverie; or by exciting the irritative motions by wine or opium internally, and by the warm bath or bliſters externally; or laſtly, by exciting the ſenſitive ideas by good news, affecting ſtories, or agreeable paſſions.

I. 1. MANY ſynchronous and ſucceſſive motions of our muſcular fibres, and of our organs of ſenſe, or ideas, become aſſociated ſo as to form indiſſoluble tribes or trains of action, as ſhewn in Section X. on Aſſociate Motions. Some conſtitutions more eaſily eſtabliſh theſe aſ⯑ſociations, whether by voluntary, ſenſitive, or irritative repetitions, and ſome more eaſily loſe them again, as ſhewn in Section XXXI. on Temperaments.

When the beginning of ſuch a train of actions becomes by any means diſordered, the ſucceeding part is liable to become diſturbed [442] in conſequence, and this is commonly termed ſympathy or conſent of parts by the writers of medicine. For the more clear underſtanding of theſe ſympathies we muſt conſider a tribe or train of actions as di⯑vided into two parts, and call one of them the primary or original motions, and the other the ſecondary or ſympathetic ones.

The primary and ſecondary parts of a train of irritative actions may reciprocally affect each other in four different manners. 1. They may both be exerted with greater energy than natural. 2. The former may act with greater, and the latter with leſs energy. 3. The former may act with leſs, and the latter with greater energy. 4. They may both act with leſs energy than natural. I ſhall now give an example of each kind of theſe modes of action, and endeavour to ſhew, that though the primary and ſecondary parts of theſe trains or tribes of motion are connected by irritative aſſociation, or their previous habits of acting together, as deſcribed in Sect. XX. on Ver⯑tigo. Yet that their acting with ſimilar or diſſimilar degrees of ener⯑gy, depends on the greater or leſs quantity of ſenſorial power, which the primary part of the train expends in its exertions.

The actions of the ſtomach conſtitute ſo important a part of the aſſociations of both irritative and ſenſitive motions, that it is ſaid to ſympathize with almoſt every part of the body; the firſt example, which I ſhall adduce to ſhew that both the primary and ſecondary parts of a train of irritative aſſociations of motion act with increaſed energy, is taken from the conſent of the ſkin with this organ. When the action of the fibres of the ſtomach is increaſed, as by the ſtimulus of a full meal, the exertions of the cutaneous arteries of the face be⯑come increaſed by their irritative aſſociations with thoſe of the ſto⯑mach, and a glow or fluſhing of the face ſucceeds. For the ſmall veſſels of the ſkin of the face having been more accuſtomed to the va⯑rieties of action, from their frequent expoſure to various degrees of cold and heat become more eaſily excited into increaſed action, than thoſe of the covered parts of our bodies, and thus act with more [443] energy from their irritative or ſenſitive aſſociations with the ſtomach. On this account in ſmall-pox the eruption in conſequence of the pre⯑vious affection of the ſtomach breaks out a day ſooner on the face than on the hands, and two days ſooner than on the trunk, and recedes in ſimilar times after maturation.

But ſecondly, in weaker conſtitutions, that is, in thoſe who poſ⯑ſeſs leſs ſenſorial power, ſo much of it is expended in the increaſed actions of the fibres of the ſtomach excited by the ſtimulus of a meal, that a ſenſe of chilneſs ſucceeds inſtead of the univerſal glow above mentioned; and thus the ſecondary part of the aſſociated train of mo⯑tions is diminiſhed in energy, in conſequence of the increaſed activity of the primary part of it.

2. Another inſtance of a ſimilar kind, where the ſecondary part of the train acts with leſs energy in conſequence of the greater exertions of the primary part, is the vertigo attending intoxication; in this circumſtance ſo much ſenſorial power is expended on the ſtomach, and on its neareſt or more ſtrongly aſſociated motions, as thoſe of the ſubcutaneous veſſels, and probably of the membranes of ſome inter⯑nal viſcera, that the irritative motions of the retina become imper⯑fectly exerted from deficiency of ſenſorial power, as explained in Sect. XX. and XXI. on Vertigo and on Drunkenneſs, and hence the ſtaggering inebriate cannot completely balance himſelf by ſuch in⯑diſtinct viſion.

3. An inſtance of the third circumſtance, where the primary part of a train of irritative motions acts with leſs, and the ſecondary part with greater energy, may be obſerved by making the following ex⯑periment. If a perſon lies with his arms and ſhoulders out of bed, till they become cold, a temporary coryza or catarrh is produced; ſo that the paſſage of the noſtrils becomes totally obſtructed; at leaſt this happens to many people; and then on covering the arms and ſhoulders, till they become warm, the paſſage of the noſtrils ceaſes [444] again to be obſtructed, and a quantity of mucus is diſcharged from them. In this caſe the quieſcence of the veſſels of the ſkin of the arms and ſhoulders, occaſioned by expoſure to cold air, produces by irritative aſſociation an increaſed action of the veſſels of the membrane of the noſtrils; and the accumulation of ſenſorial power during the torpor of the arms and ſhoulders is thus expended in producing a temporary coryza or catarrh.

Another inſtance may be adduced from the ſympathy or conſent of the motions of the ſtomach with other more diſtant links of the very extenſive tribes or trains of irritative motions aſſociated with them, deſcribed in Sect. XX. on Vertigo. When the actions of the fibres of the ſtomach are diminiſhed or inverted, the actions of the abſorbent veſſels, which take up the mucus from the lungs, pericardium, and other cells of the body, become increaſed, and abſorb the fluids accu⯑mulated in them with greater avidity, as appears from the exhibition of foxglove, antimony, or other emetics in caſes of anaſarca, attend⯑ed with unequal pulſe and difficult reſpiration.

That the act of nauſea and vomiting is a decreaſed exertion of the fibres of the ſtomach may be thus deduced; when an emetic medi⯑cine is adminiſtered, it produces the pain of ſickneſs, as a diſagreeable taſte in the mouth produces the pain of nauſea; theſe pains, like that of hunger, or of cold, or like thoſe, which are uſually termed ner⯑vous, as the head-ach or hemicrania, do not excite the organ into greater action; but in this caſe I imagine the pains of ſickneſs or of nauſea counteract or deſtroy the pleaſureable ſenſation, which ſeems neceſſary to digeſtion, as ſhewn in Sect. XXXIII. 1. 1. The peri⯑ſtaltic motions of the fibres of the ſtomach become enfeebled by the want of this ſtimulus of pleaſureable ſenſation, and in conſequence ſtop for a time, and then become inverted; for they cannot become inverted without being previouſly ſtopped. Now that this inverſion of the trains of motion of the fibres of the ſtomach is owing to the de⯑ficiency [445] of pleaſureable ſenſation is evinced from this circumſtance, that a nauſeous idea excited by words will produce vomiting as effec⯑tually as a nauſeous drug.

Hence it appears, that the act of nauſea or vomiting expends leſs ſenſorial power than the uſual periſtaltic motions of the ſtomach in the digeſtion of our aliment; and that hence there is a greater quan⯑tity of ſenſorial power becomes accumulated in the fibres of the ſto⯑mach, and more of it in conſequence to ſpare for the action of thoſe parts of the ſyſtem, which are thus aſſociated with the ſtomach, as of the whole abſorbent ſeries of veſſels, and which are at the ſame time excited by their uſual ſtimuli.

From this we can underſtand, how after the operation of an emetic the ſtomach becomes more irritable and ſenſible to the ſtimulus, and the pleaſure of food; ſince as the ſenſorial power becomes accumu⯑lated during the nauſea and vomiting, the digeſtive power is after⯑wards exerted more forceably for a time. It ſhould, however, be here remarked, that though vomiting is in general produced by the defect of this ſtimulus of pleaſureable ſenſation, as when a nauſeous drug is adminiſtered; yet in long continued vomiting, as in ſea-ſick⯑neſs, or from habitual dram-drinking, it ariſes from deficiency of ſen⯑ſorial power, which in the former caſe is exhauſted by the increaſed exertion of the irritative ideas of viſion, and in the latter by the fre⯑quent application of an unnatural ſtimulus.

4. An example of the fourth circumſtance above mentioned, where both the primary and ſecondary parts of a train of motions proceed with energy leſs than natural, may be obſerved in the dyſpnoea, which occurs in going into a very cold bath, and which has been de⯑ſcribed and explained in Sect. XXXII. 3. 2.

And by the increaſed debility of the pulſations of the heart and ar⯑teries during the operation of an emetic. Secondly, from the ſlow⯑neſs and intermiſſion of the pulſations of the heart from the inceſſant efforts to vomit occaſioned by an overdoſe of digitalis. And thirdly, [446] from the total ſtoppage of the motions of the heart, or death, in con⯑ſequence of the torpor of the ſtomach, when affected with the com⯑mencement or cold paroxyſm of the gout. See Sect. XXV. 17.

II. 1. The primary and ſecondary parts of the trains of ſenſitive aſſociation reciprocally affect each other in different manners. 1. The increaſed ſenſation of the primary part may ceaſe, when that of the ſecondary part commences. 2. The increaſed action of the primary part may ceaſe, when that of the ſecondary part commences. 3. The primary part may have increaſed ſenſation, and the ſecondary part increaſed action. 4. The primary part may have increaſed ac⯑tion, and the ſecondary part increaſed ſenſation.

Examples of the firſt mode, where the increaſed ſenſation of the primary part of a train of ſenſitive aſſociation ceaſes, when that of the ſecondary part commences, are not unfrequent; as this is the general origin of thoſe pains, which continue ſome time without being attended with inflammation, ſuch as the pain at the pit of the ſtomach from a ſtone at the neck of the gall-bladder, and the pain of ſtrangury in the glans penis from a ſtone at the neck of the urinary bladder. In both theſe caſes the part, which is affected ſecondarily, is believed to be much more ſenſible than the part primarily affected, as deſcribed in the catalogue of diſeaſes, Claſs II. 1. 1. 10. and IV. 2. 1. 1. and IV. 2. 1. 2.

The hemicrania, or nervous headach, as it is called, when it ori⯑ginates from a decaying tooth, is another diſeaſe of this kind; as the pain of the carious tooth always ceaſes, when the pain over one eye and temple commences. And it is probable, that the violent pains, which induce convulſions in painful epilepſies, are produced in the ſame manner, from a more ſenſible part ſympathizing with a diſeaſed one of leſs ſenſibility. See Catalogue of Diſeaſes, Claſs IV. 2. 1. 5. and III. 1. 1. 7.

The laſt tooth, or dens ſapientiae, of the upper jaw moſt frequent⯑ly decays firſt, and is liable to produce pain over the eye and temple [447] of that ſide. The laſt tooth of the under-jaw is alſo liable to produce a ſimilar hemicrania, when it begins to decay. When a tooth in the upper-jaw is the cauſe of the headach, a ſlighter pain is ſometimes perceived on the cheek-bone. And when a tooth in the lower-jaw is the cauſe of headach, a pain ſometimes affects the tendons of the muſcles of the neck, which are attached near the jaws. But the clavus hyſtericus, or pain about the middle of the parietal bone on one ſide of the head, I have ſeen produced by the ſecond of the mo⯑lares, or grinders, of the under-jaw; of which I ſhall relate the fol⯑lowing caſe. See Claſs II. 1. 1. 4. and IV. 2. 1. 5.

Mrs. [...], about 30 years of age, was ſeized with great pain about the middle of the right parietal bone, which had continued a whole day before I ſaw her, and was ſo violent as to threaten to oc⯑caſion convulſions. Not being able to detect a decaying tooth, or a tender one, by examination with my eye, or by ſtriking them with a tea-ſpoon, and fearing bad conſequences from her tendency to con⯑vulſion, I adviſed her to extract the laſt tooth of the under-jaw on the affected ſide; which was done without any good effect. She was then directed to loſe blood, and to take a briſk cathartic; and after that had operated, about 60 drops of laudanum were given her, with large doſes of bark; by which the pain was removed. In about a fortnight ſhe took a cathartic medicine by ill advice, and the pain re⯑turned with greater violence in the ſame place; and, before I could arrive, as ſhe lived 30 miles from me, ſhe ſuffered a paralytic ſtroke; which affected her limbs and her face on one ſide, and relieved the pain of her head.

About a year afterwards I was again called to her on account of a pain as violent as before exactly on the ſame part of the other parietal bone. On examining her mouth I found the ſecond molaris of the under-jaw on the ſide before affected was now decayed, and con⯑cluded, that this tooth had occaſioned the ſtroke of the palſy by the pain and conſequent exertion it had cauſed. On this account I ear⯑neſtly [448] entreated her to allow the ſound molaris of the ſame jaw op⯑poſite to the decayed one to be extracted; which was forthwith done, and the pain of her head immediately ceaſed, to the aſtoniſhment of her attendants.

In the caſes above related of the pain exiſting in a part diſtant from the ſeat of the diſeaſe, the pain is owing to defect of the uſual mo⯑tions of the painful part. This appears from the coldneſs, paleneſs, and emptineſs of the affected veſſels, or of the extremities of the body in general, and from there being no tendency to inflammation. The increaſed action of the primary part of theſe aſſociated motions, as of the hepatic termination of the bile-duct from the ſtimulus of a gall⯑ſtone, or of the interior termination of the urethra from the ſtimulus of a ſtone in the bladder, or laſtly, of a decaying tooth in hemicrania, deprives the ſecondary part of theſe aſſociated motions, namely, the exterior terminations of the bile-duct or urethra, or the pained mem⯑branes of the head in hemicrania, of their natural ſhare of ſenſorial power: and hence the ſecondary parts of theſe ſenſitive trains of aſſo⯑ciation become pained from the deficiency of their uſual motions, which is accompanied with deficiency of ſecretions and of heat. See Sect. IV. 5. XII. 5. 3. XXXIV. 1.

Why does the pain of the primary part of the aſſociation ceaſe, when that of the ſecondary part commences? This is a queſtion of intricacy, but perhaps not inexplicable. The pain of the primary part of theſe aſſociated trains of motion was owing to too great ſtimu⯑lus, as of the ſtone at the neck of the bladder, and was conſequently cauſed by too great action of the pained part. This greater action than natural of the primary part of theſe aſſociated motions, by em⯑ploying or expending the ſenſorial power of irritation belonging to the whole aſſociated train of motions, occaſioned torpor, and conſequent pain, in the ſecondary part of the aſſociated train; which was poſ⯑ſeſſed of greater ſenſibility than the primary part of it. Now the great pain of the ſecondary part of the train, as ſoon as it commences, [449] employs or expends the ſenſorial power of ſenſation belonging to the whole aſſociated train of motions; and in conſequence the motions of the primary part, though increaſed by the ſtimulus of an extrane⯑ous body, ceaſe to be accompanied with pain or ſenſation.

If this mode of reaſoning be juſt it explains a curious fact, why when two parts of the body are ſtrongly ſtimulated, the pain is felt only in one of them, though it is poſſible by voluntary attention it may be alternately perceived in them both. In the ſame manner, when two new ideas are preſented to us from the ſtimulus of external bodies, we attend to but one of them at a time. In other words, when one ſet of fibres, whether of the muſcles or organs of ſenſe, contract ſo ſtrongly as to excite much ſenſation; another ſet of fibres contracting more weakly do not excite ſenſation at all, becauſe the ſenſorial power of ſenſation is pre-occupied by the firſt ſet of fibres. So we cannot will more then one effect at once, though by aſſocia⯑tions previouſly formed we can move many fibres in combination.

Thus in the inſtances above related, the termination of the bile duct in the duodenum, and the exterior extremity of the urethra, are more ſenſible than their other terminations. When theſe parts are deprived of their uſual motions by deficiency of the ſenſorial power of irritation, they become painful according to law the fifth in Section IV. and the leſs pain originally excited by the ſtimulus of concreted bile, or of a ſtone at their other extremities ceaſes to be perceived. Afterwards, however, when the concretions of bile, or the ſtone on the urinary bladder, become more numerous or larger, the pain from their increaſed ſtimulus becomes greater than the aſſociated pain; and is then felt at the neck of the gall bladder or urinary bladder; and the pain of the glans penis, or at the pit of the ſtomach, ceaſes to be perceived.

2. Examples of the ſecond mode, where the increaſed action of the primary part of a train of ſenſitive aſſociation ceaſes, when that of the ſecondary part commences, are alſo not unfrequent; as this is [450] the uſual manner of the tranſlation of inflammations from internal to external parts of the ſyſtem, ſuch as when an inflammation of the liver or ſtomach is tranſlated to the membranes of the foot, and forms the gout; or to the ſkin of the face, and forms the roſy drop, or when an inflammation of the membranes of the kidneys is tranſ⯑lated to the ſkin of the loins, and forms one kind of herpes, called ſhingles; in theſe caſes by whatever cauſe the original inflammation may have been produced, as the ſecondary part of the train of ſenſi⯑tive aſſociation is more ſenſible, it becomes exerted with greater vio⯑lence than the firſt part of it; and by both its increaſed pain, and the increaſed motion of its fibres, ſo far diminiſhes or exhauſts the ſenſorial power of ſenſation; that the primary part of the train being leſs ſenſible ceaſes both to feel pain, and to act with unnatural energy.

3. Examples of the third mode, where the primary part of a train of ſenſitive aſſociation of motions may experience increaſed ſenſation, and the ſecondary part increaſed action, are likewiſe not unfrequent; as it is in this manner that moſt inflammations commence. Thus, after ſtanding ſome time in ſnow, the feet become affected with the pain of cold, and a common coryza, or inflammation of the mem⯑brane of the noſtrils, ſucceeds. It is probable that the internal in⯑flammations, as pleuriſies, or hepatitis, which are produced after the cold paroxyſm of fever, originate in the ſame manner from the ſym⯑pathy of thoſe parts with ſome others, which were previouſly pained from quieſcence; as happens to various parts of the ſyſtem during the cold fits of fevers. In theſe caſes it would ſeem, that the ſen⯑ſorial power of ſenſation becomes accumulated during the pain of cold, as the torpor of the veſſels occaſioned by the defect of heat con⯑tributes to the increaſe or accumulation of the ſenſorial power of irritation, and that both theſe become exerted on ſome internal part, which was not rendered torpid by the cold which affected the external [451] parts, nor by its aſſociation with them; or which ſooner recovered its ſenſibility.

4. An example of the fourth mode, or where the primary part of a ſenſitive aſſociation of motions may have increaſed action, and the ſecondary part increaſed ſenſation, may be taken from the pain of the ſhoulder, which attends inflammation of the membranes of the liver, ſee Hepatitis, Claſs IV. 2. 1. 6. in this circumſtance ſo much ſenſorial power ſeems to be expended in the violent actions and ſenſations of the inflamed membranes of the liver, that the membranes aſſociated with them become quieſcent to their uſual ſtimuli, and painful in conſequence.

There may be other modes in which the primary and ſecondary parts of the trains of aſſociated ſenſitive motions may reciprocally affect each other, as may be ſeen by looking over Claſs IV. in the catalogue of diſeaſes; all which may probably be reſolved into the plus and minus of ſenſorial power, but we have not yet had ſufficient obſervations made upon them with a view to this doctrine.

III. The aſſociated trains of our ideas may have ſympathies, and their primary and ſecondary parts affect each other in ſome manner ſimilar to thoſe above deſcribed; and may thus occaſion various curi⯑ous phenomena not yet adverted to, beſides thoſe explained in the Sections on Dreams, Reveries, Vertigo, and Drunkenneſs; and may thus diſturb the deductions of our reaſonings, as well as the ſtreams of our imaginations; preſent us with falſe degrees of fear, attach unfounded value to trivial circumſtances; give occaſion to our early prejudices and antipathies; and thus embarraſs the happineſs of our lives. A copious and curious harveſt might be reaped from this pro⯑vince of ſcience, in which, however, I ſhall not at preſent wield my ſickle.

I. IF any of our muſcles be made to contract violently by the power of volition, as thoſe of the fingers, when any one hangs by his hands on a ſwing, fatigue ſoon enſues; and the muſcles ceaſe to act owing to the temporary exhauſtion of the ſpirit of animation; as ſoon as this is again accumulated in the muſcles, they are ready to contract again by the efforts of volition.

Thoſe violent muſcular actions induced by pain become in the ſame manner intermitted and recurrent; as in labour-pains, vomit⯑ing, teneſmus, ſtrangury; owing likewiſe to the temporary exhauſ⯑tion of the ſpirit of animation, as above mentioned.

[453] When any ſtimulus continues long to act with unnatural violence, ſo as to produce too energetic action of any of our moving organs, thoſe motions ſoon ceaſe, though the ſtimulus continues to act; as in looking long on a bright object, as on an inch-ſquare of red ſilk laid on white paper in the ſunſhine. See Plate I. in Sect. III. 1.

On the contrary, where leſs of the ſtimulus of volition, ſenſation, or irritation, have been applied to a muſcle than uſual; there ap⯑pears to be an accumulation of the ſpirit of animation in the moving organ; by which it is liable to act with greater energy from leſs quantity of ſtimulus, than was previouſly neceſſary to excite it into ſo great action; as after having been immerſed in ſnow the cutaneous veſſels of our hands are excited into ſtronger action by the ſtimulus of a leſs degree of heat, than would previouſly have produced that effect.

From hence the periods of ſome fever-fits may take their origin, either ſimply, or by their accidental coincidence with lunar and ſolar periods, or with the diurnal periods of heat and cold, to be treated of below; for during the cold fit at the commencement of a fever, from whatever cauſe that cold fit may have been induced, it follows, 1. That the ſpirit of animation muſt become accumulated in the parts, which exert during this cold fit leſs than their natural quan⯑tity of action. 2. If the cauſe producing the cold fit does not in⯑creaſe, or becomes diminiſhed; the parts before benumbed or in⯑active become now excitable by ſmaller ſtimulus, and are thence thrown into more violent action than is natural; that is a hot fit ſuc⯑ceeds the cold one. 3. By the energetic action of the ſyſtem during the hot fit, if it continues long, an exhauſtion of the ſpirit of ani⯑mation takes place; and another cold fit is liable to ſucceed, from the moving ſyſtem not being excitable into action from its uſual ſtimulus. This inirritability of the ſyſtem from a too great previous ſtimulus, and conſequent exhauſtion of ſenſorial power, is the cauſe of the general debility, and ſickneſs, and head-ach, ſome hours after in⯑toxication. [454] And hence we ſee one of the cauſes of the periods of fever-fits; which however are frequently combined with the periods of our diurnal habits, or of heat and cold, or of ſolar or lunar periods.

II. 1. The following are natural animal actions, which are fre⯑quently catenated with our daily habits of life, as well as excited by their natural irritations. The periods of hunger and thirſt become catenated with certain portions of time, or degrees of exhauſtion, or other diurnal habits of life. And if the pain of hunger be not re⯑lieved by taking food at the uſual time, it is liable to ceaſe till the next period of time or other habits recur; this is not only true in reſpect to our general deſire of food, but the kinds of it alſo are governed by this periodical habit; inſomuch that beer taken to break⯑faſt will diſturb the digeſtion of thoſe, who have been accuſtomed to tea; and tea taken at dinner will diſagree with thoſe, who have been accuſtomed to beer. Whence it happens, that thoſe, who have weak ſtomachs, will be able to digeſt more food, if they take their meals at regular hours; becauſe they have both the ſtimulus of the aliment they take, and the periodical habit, to aſſiſt their digeſ⯑tion.

The periods of emptying the bladder are not only dependent on the acrimony or diſtention of the water in it, but are frequently [455] catenated with external cold applied to the ſkin, as in cold bathing, or waſhing the hands; or with other habits of life, as many are ac⯑cuſtomed to empty the bladder before going to bed, or into the houſe after a journey, and this whether it be full or not.

Our times of reſpiration are not only governed by the ſtimulus of the blood in the lungs, or our deſire of freſh air, but alſo by our attention to the hourly objects before us. Hence when a perſon is earneſtly contemplating an idea of grief, he forgets to breathe, till the ſenſation in his lungs becomes very urgent; and then a ſigh ſuc⯑ceeds for the purpoſe of more forceably puſhing forwards the blood, which is accumulated in the lungs.

Our times of reſpiration are alſo frequently governed in part by our want of a ſteady ſupport for the actions of our arms, and hands, as in threading a needle, or hewing wood, or in ſwimming; when we are intent upon theſe objects, we breathe at the intervals of the exertion of the pectoral muſcles.

2. The following natural animal actions are influenced by ſolar periods. The periods of ſleep and of waking depends much on the ſolar period, for we are inclined to ſleep at a certain hour, and to awake at a certain hour, whether we have had more or leſs fatigue during the day, if within certain limits; and are liable to wake at a certain hour, whether we went to bed earlier or later within certain limits. Hence it appears, that thoſe who complain of want of ſleep, will be liable to ſleep better or longer, if they accuſtom themſelves to go to reſt, and to riſe, at certain hours.

The periods of evacuating the bowels are generally connected with ſome part of the ſolar day, as well as with the acrimony or diſten⯑tion occaſioned by the feces. Hence one method of correcting coſ⯑tiveneſs is by endeavouring to eſtabliſh a habit of evacuation at a cer⯑tain hour of the day, as recommended by Mr. Locke, which may be accompliſhed by uſing daily voluntary efforts at thoſe times, joined with the uſual ſtimulus of the material to be evacuated.

[456] 3. The following natural animal actions are connected with lunar periods. 1. The periods of female menſtruation are connected with lunar periods to great exactneſs, in ſome inſtances even to a few hours. Theſe do not commence or terminate at the full or change, or at any other particular part of the lunation, but after they have commenced at any part of it, they continue to recur at that part with great regularity, unleſs diſturbed by ſome violent circumſtance, as explained in Sect. XXXII. No. 6. their return is immediately cauſed by deficient venous abſorption, which is owing to the want of the ſtimulus, deſigned by nature, of amatorial copulation, or of the growing fetus. When the catamenia returns ſooner than the period of lunation, it ſhews a tendency of the conſtitution to inirritabi⯑lity; that is to debility, or deficiency of ſenſorial power, and is to be relieved by ſmall doſes of ſteel and opium.

The venereal orgaſm of birds and quadrupeds ſeems to commence, or return about the moſt powerful lunations at the vernal or autumnal equinoxes; but if it be diſappointed of its object, it is ſaid to recur at monthly periods; in this reſpect reſembling the female catamenia. Whence it is believed, that women are more liable to become preg⯑nant at or about the time of their catamenia, than at the intermediate times; and on this account they are ſeldom much miſtaken in their reckoning of nine lunar periods from the laſt menſtruation; the in⯑attention to this may ſometimes have been the cauſe of ſuppoſed bar⯑renneſs, and is therefore worth the obſervation of thoſe, who wiſh to have children.

III. We now come to the periods of diſeaſed animal actions. The periods of fever-fits, which depend on the ſtated returns of noc⯑turnal cold, are diſcuſſed in Sect. XXXII. 3. Thoſe, which origi⯑nate or recur at ſolar or lunar periods, are alſo explained in Section XXXII. 6. Theſe we ſhall here enumerate; obſerving, however, that it is not more ſurpriſing, that the influence of the varying attractions of the ſun and moon, ſhould raiſe the ocean into mountains, than [457] that it ſhould affect the nice ſenſibilities of animal bodies; though the manner of its operation on them is difficult to be underſtood. It is probable however, that as this influence gradually leſſens during the courſe of the day, or of the lunation, or of the year, ſome actions of our ſyſtem become leſs and leſs; till at length a total quieſcence of ſome part is induced; which is the commencement of the pa⯑roxyſms of fever, of menſtruation, of pain with decreaſed action of the affected organ, and of conſequent convulſion.

1. A diurnal fever in ſome weak people is diſtinctly obſerved to come on towards evening, and to ceaſe with a moiſt ſkin early in the morning, obeying the ſolar periods. Perſons of weak conſtitutions are liable to get into better ſpirits at the acceſs of the hot fit of this evening fever; and are thence inclined to fit up late; which by fur⯑ther enfeebling them increaſes the diſeaſe; whence they loſe their ſtrength and their colour.

2. The periods of hectic fever, ſuppoſed to ariſe from abſorption of matter, obeys the diurnal periods like the above, having the exacer⯑beſcence towards evening, and its remiſſion early in the morning, with ſweats, or diarrhoea, or urine with white ſediment.

3. The periods of quotidian fever are either catenated with ſolar time, and return at the intervals of twenty-four hours; or with lunar time, recurring at the intervals of about twenty-five hours. There is great uſe in knowing with what circumſtances the periodical return of new morbid motions are conjoined, as the moſt effectual times of exhibiting the proper medicines are thus determined. So if the tor⯑por, which uſhers in an ague fit, is catenated with the lunar day: it is know, when the bark or opium muſt be given, ſo as to exert its principal effect about the time of the expected return. Solid opium ſhould be given about an hour before the expected cold fit; liquid opium and wine about half an hour; the bark repeatedly for ſix or eight hours previous to the expected return.

[458] 4. The periods of tertian fevers, reckoned from the commence⯑ment of one cold fit to the commencement of the next cold fit, recur with ſolar intervals of forty-eight hours, or with lunar ones of about fifty hours. When theſe times of recurrence begin one or two hours earlier than the ſolar period, it ſhews, that the torpor or cold fit is produced by leſs external influence; and therefore that it is more li⯑able to degenerate into a fever with only remiſſions; ſo when men⯑ſtruation recurs ſooner than the period of lunation, it ſhews a ten⯑dency of the habit to torpor or inirritability.

5. The periods of quartan fevers return at ſolar intervals of ſeventy⯑two hours, or at lunar ones of about ſeventy-four hours and an half. This kind of ague appears moſt in moiſt cold autumns, and in cold countries replete with marſhes. It is attended with greater debility, and its cold acceſs more difficult to prevent. For where there is pre⯑viouſly a deficiency of ſenſorial power, the conſtitution is liable to run into greater torpor from any further diminution of it; two ounces of bark and ſome ſteel ſhould be given on the day before the return of the cold paroxyſm, and a pint of wine by degrees a few hours before its return, and thirty drops of laudanum one hour before the expected cold fit.

6. The periods of the gout generally commence about an hour be⯑fore ſun-riſe, which is uſually the coldeſt part of the twenty-four hours. The greater periods of the gout ſeem alſo to obſerve the ſolar influence, returning about the ſame ſeaſon of the year.

7. The periods of the pleuriſy recur with exacerbation of the pain and fever about ſun-ſet, at which time veneſection is of moſt ſervice. The ſame may be obſerved of the inflammatory rheumatiſm, and other fevers with arterial ſtrength, which ſeem to obey ſolar periods; and thoſe with debility ſeem to obey lunar ones.

8. The periods of fevers with arterial debility ſeem to obey the lunar day, having their acceſs daily nearly an hour later; and have [459] ſometimes two acceſſes in a day, reſembling the lunar effects upon the tides.

9. The periods of rhaphania, or convulſions of the limbs from rheu⯑matic pains, ſeem to be connected with ſolar influence, returning at nearly the ſame hour for weeks together, unleſs diſturbed by the ex⯑hibition of powerful doſes of opium.

So the periods of Tuſſis ferina, or violent cough with ſlow pulſe, called nervous cough, recurs by ſolar periods. Five grains of opium given at the time the cough commenced diſturbed the period, from ſeven in the evening to eleven, at which time it regularly returned for ſome days, during which time the opium was gradually omitted. Then 120 drops of laudanum were given an hour before the acceſs of the cough, and it totally ceaſed. The laudanum was continued a fortnight, and then gradually diſcontinued.

10. The periods of hemicrania, and of painful epilepſy, are liable to obey lunar periods, both in their diurnal returns, and in their greater periods of weeks, but are alſo induced by other exciting cauſes.

11. The periods of arterial haemorrhages ſeem to return at ſolar periods about the ſame hour of the evening or morning. Perhaps the venous haemorrhages obey the lunar periods, as the catamenia, and haemorrhoids.

12. The periods of the haemorrhoids, or piles, in ſome recur monthly, in others only at the greater lunar influence about the equi⯑noxes.

13. The periods of haemoptoe ſometimes obey ſolar influence, re⯑curring early in the morning for ſeveral days; and ſometimes lunar periods, recurring monthly; and ſometimes depend on our hours of ſleep. See Claſs I. 2. 1. 9.

14. Many of the firſt periods of epileptic ſits obey the monthly lu⯑nation with ſome degree of accuracy; others recur only at the moſt powerful lunations before the vernal equinox, and after the autumnal [460] one; but when the conſtitution has gained a habit of relieving diſ⯑agreeable ſenſations by this kind of exertion, the fit recurs from any ſlight cauſe.

15. The attack of palſy and apoplexy are known to recur with great frequency about the equinoxes.

16. There are numerous inſtances of the effect of the lunations upon the periods of inſanity, whence the name of lunatic has been given to thoſe afflicted with this diſeaſe.

IV. The critical days, in which fevers are ſuppoſed to terminate, have employed the attention of medical philoſophers from the days of Hippocrates to the preſent time. In whatever part of a lunation a fever commences, which owes either its whole cauſe to ſolar and lu⯑nar influence, or to this in conjunction with other cauſes; it would ſeem, that the effect would be the greateſt at the full and new moon, as the tides riſe higheſt at thoſe times, and would be the leaſt at the quadratures; thus if a fever-fit ſhould commence at the new or full moon, occaſioned by the ſolar and lunar attraction diminiſhing ſome chemical affinity of the particles of blood, and thence decreaſing their ſtimulus on our ſanguiferous ſyſtem, as mentioned in Sect. XXXII. 6. this effect will daily decreaſe for the firſt ſeven days, and will then increaſe till about the fourteenth day, and will again decreaſe till about the twenty-firſt day, and increaſe again till the end of the lunation. If a fever-fit from the above cauſe ſhould commence on the ſeventh day after either lunation, the reverſe of the above circumſtances would happen. Now it is probable, that thoſe fevers, whoſe criſis or ter⯑minations are influenced by lunations, may begin at one or other of the above times, namely at the changes or quadratures; though ſuf⯑ficient obſervations have not been made to aſcertain this circumſtance. Hence I conclude, that the ſmall-pox and meaſles have their critical days, not governed by the times required for certain chemical changes in the blood, which affect or alter the ſtimulus of the contagious matter, but from the daily increaſing or decreaſing effect of this lunar [461] link of catenation, as explained in Section XVII. 3. 3. And as other fevers terminate moſt frequently about the ſeventh, fourteenth, twenty-firſt, or about the end of four weeks, when no medical aſ⯑ſiſtance has diſturbed their periods, I conclude, that theſe criſes, or terminations, are governed by periods of the lunations, though we are ſtill ignorant of their manner of operation.

In the diſtinct ſmall-pox the veſtiges of lunation are very apparent, after inoculation a quarter of a lunation precedes the commencement of the fever, another quarter terminates with the complete eruption, another quarter with the complete maturation, and another quarter terminates the complete abſorption of a material now rendered inoffen⯑ſive to the conſtitution.

I. THE larger cryſtals of ſaline bodies may be conceived to ariſe from the combination of ſmaller cryſtals of the ſame form, owing to the greater attractions of their ſides than of their angles. Thus if four cubes were floating in a fluid, whoſe friction or reſiſtance is no⯑thing, it is certain the ſides of theſe cubes would attract each other ſtronger than their angles; and hence that theſe four ſmaller cubes would ſo arrange themſelves as to produce one larger one.

There are other means of chemical accretion, ſuch as the depoſi⯑tions of diſſolved calcareous or ſiliceous particles, as are ſeen in the formation of the ſtalactites of limeſtone in Derbyſhire, or of calcedone in Cornwall. Other means of adheſion are produced by heat and preſſure, as in the welding of iron-bars; and other means by ſimple preſſure, as in forcing two pieces of caoutchou, or elaſtic gum, to ad⯑here; [463] and laſtly, by the agglutination of a third ſubſtance penetrating the pores of the other two, as in the agglutination of wood by means of animal gluten. Though the ultimate particles of animal bodies are held together during life, as well as after death, by their ſpecific attraction of coheſion, like all other matter; yet it does not appear, that their original organization was produced by chemical laws, and their production and increaſe muſt therefore only be looked for from the laws of animation.

II. When the pain of hunger requires relief, certain parts of the material world, which ſurround us, when applied to our palates, ex⯑cite into action the muſcles of deglutition; and the material is ſwal⯑lowed into the ſtomach. Here the new aliment becomes mixed with certain animal fluids, and undergoes a chemical proceſs, termed di⯑geſtion; which however chemiſtry has not yet learnt to imitate out of the bodies of living animals or vegetables. This proceſs ſeems very ſimilar to the ſaccharine proceſs in the lobes of farinaceous ſeeds, as of barley, when it begins to germinate; except that, along with the ſugar, oil and mucilage are alſo produced; which form the chyle of animals, which is very ſimilar to their milk.

The reaſon, I imagine, why this chyle-making, or ſaccharine pro⯑ceſs, has not yet been imitated by chemical operations, is owing to the materials being in ſuch a ſituation in reſpect to warmth, moiſture, and motion; that they will immediately change into the vinous or acetous fermentation; except the new ſugar be abſorbed by the nu⯑merous lacteal or lymphatic veſſels, as ſoon as it is produced; which is not eaſy to imitate in the laboratory.

Theſe lacteal veſſels have mouths, which are irritated into action by the ſtimulus of the fluid, which ſurrounds them; and by animal ſelection, or appetency, they abſorb ſuch part of the fluid as is agree⯑able to their palate; thoſe parts, for inſtance, which are already con⯑verted into chyle, before they have time to undergo another change by a vinous or acetous fermentation. This animal abſorption of fluid [464] is almoſt viſible to the naked eye in the action of the puncta lacry⯑malia; which imbibe the tears from the eye, and diſcharge them again into the noſtrils.

III. The arteries conſtitute another reſervoir of a changeful fluid; from which, after its recent oxygenation in the lungs, a further ani⯑mal ſelection of various fluids is abſorbed by the numerous glands; theſe ſelect their reſpective fluids from the blood, which is perpetu⯑ally undergoing a chemical change; but the ſelection by theſe glands, like that of the lacteals, which open their mouths into the digeſting aliment in the ſtomach, is from animal appetency, not from chemical affinity; ſecretion cannot therefore be imitated in the laboratory, as it conſiſts in a ſelection of part of a fluid during the chemical change of that fluid.

The mouths of the lacteals, and lymphatics, and the ultimate ter⯑minations of the glands, are finer than can eaſily be conceived; yet it is probable, that the pores, or interſtices of the parts, or coats, which conſtitute theſe ultimate veſſels, may ſtill have greater tenuity; and that theſe pores from the above analogy muſt poſſeſs a ſimilar power of irritability, and abſorb by their living energy the particles of fluid adapted to their purpoſes, whether to replace the parts abraded or diſſolved, or to elongate and enlarge themſelves. Not only every kind of gland is thus endued with its peculiar appetency, and ſelects the material agreeable to its taſte from the blood, but every individual pore acquires by animal ſelection the material, which it wants; and thus nutrition ſeems to be performed in a manner ſo ſimilar to ſecre⯑tion; that they only differ in the one retaining, and the other parting again with the particles, which they have ſelected from the blood.

This way of accounting for nutrition from ſtimulus, and the con⯑ſequent animal ſelection of particles, is much more analogous to other phenomena of the animal microcoſm, than by having recourſe to the microſcopic animalcula, or organic particles of Buffon and Needham; [465] which being already compounded muſt themſelves require nutritive particles to continue their own exiſtence. And muſt be liable to un⯑dergo a change by our digeſtive or ſecretory organs; otherwiſe man⯑kind would ſoon reſemble by their theory the animals, which they feed upon. He, who is nouriſhed by beef or veniſon, would in time become horned; and he, who feeds on pork or bacon, would gain a noſe proper for rooting into the earth, as well as for the perception of odours.

The whole animal ſyſtem may be conſidered as conſiſting of the extremities of the nerves, or of having been produced from them; if we except perhaps the medullary part of the brain reſiding in the head and ſpine, and in the trunks of the nerves. Theſe extremities of the nerves are either of thoſe of locomotion, which are termed muſcular fibres; or of thoſe of ſenſation, which conſtitute the im⯑mediate organs of ſenſe, and which have alſo their peculiar motions. Now as the fibres, which conſtitute the bones and membranes, poſ⯑ſeſſed originally ſenſation and motion; and are liable again to poſſeſs them, when they become inflamed; it follows, that thoſe were, when firſt formed, appendages to the nerves of ſenſation or locomo⯑tion, or were formed from them. And that hence all theſe ſolid parts of the body, as they have originally conſiſted of extremities of nerves, require an appoſition of nutritive particles of a ſimilar kind, contrary to the opinion of Buffon and Needham above recited.

Laſtly, as all theſe filaments have poſſeſſed, or do poſſeſs, the power of contraction, and of conſequent inertion or elongation; it ſeems probable, that the nutritive particles are applied during their times of elongation; when their original conſtituent particles are re⯑moved to a greater diſtance from each other. For each muſcular or ſenſual fibre may be conſidered as a row or ſtring of beads; which approach, when in contraction, and recede during its reſt or elonga⯑tion; and our daily experience ſhews us, that great action emaciates the ſyſtem, and that it is repaired during reſt.

[466] Something like this is ſeen out of the body; for if a hair, or a ſingle untwiſted fibre of flax or ſilk, be ſoaked in water; it becomes longer and thicker by the water, which is abſorbed into its pores. Now if a hair could be ſuppoſed to be thus immerſed in a ſolution of particles ſimilar to thoſe, which compoſe it; one may imagine, that it might be thus increaſed in weight and magnitude; as the particles of oak-bark increaſe the ſubſtance of the hides of beaſts in the proceſs of making leather. I mention theſe not as philoſophic analogies, but as ſimiles to facilitate our ideas, how an accretion of parts may be ef⯑fected by animal appetences, or ſelections, in a manner ſomewhat ſimilar to mechanical or chemical attractions.

If thoſe new particles of matter, previouſly prepared by digeſtion and ſanguification, only ſupply the places of thoſe, which have been abraded by the actions of the ſyſtem, it is properly termed nutrition. If they are applied to the extremities of the nervous fibrils, or in ſuch quantity as to increaſe the length or craſſitude of them, the body be⯑comes at the ſame time enlarged, and its growth is increaſed, as well as its deficiences repaired.

In this laſt caſe ſomething more than a ſimple appoſition or ſelec⯑tion of particles ſeems to be neceſſary; as many parts of the ſyſtem during its growth are cauſed to recede from thoſe, with which they were before in contact; as the ends of the bones, or cartilages, recede from each other, as their growth advances: this proceſs reſembles inflammation, as appears in ophthalmy, or in the production of new fleſh in ulcers, where old veſſels are enlarged, and new ones pro⯑duced; and like that is attended with ſenſation. In this ſituation the veſſels become diſtended with blood, and acquire greater ſenſibility, and may thus be compared to the erection of the penis, or of the nipples of the breaſts of women; while new particles become added at the ſame time; as in the proceſs of nurition above deſcribed.

When only the natural growth of the various parts of the body are produced, a pleaſureable ſenſation attends it, as in youth, and [467] perhaps in thoſe, who are in the progreſs of becoming fat. When an unnatural growth is the conſequence, as in inflammatory diſeaſes, a painful ſenſation attends the enlargement of the ſyſtem.

IV. This appoſition of new parts, as the old ones diſappear, ſe⯑lected from the aliment we take, firſt enlarges and ſtrengthens our bodies for twenty years, for another twenty years it keeps us in health and vigour, and adds ſtrength and ſolidity to the ſyſtem; and then gradually ceaſes to nouriſh us properly, and for another twenty years we gradually ſink into decay, and finally ceaſe to act, and to exiſt.

On conſidering this ſubject one ſhould have imagined at firſt view, that it might have been eaſier for nature to have ſupported her pro⯑geny for ever in health and life, than to have perpetually reproduced them by the wonderful and myſterious proceſs of generation. But it ſeems our bodies by long habit ceaſe to obey the ſtimulus of the aliment, which ſhould ſupport us. After we have acquired our height and ſolidity we make no more new parts, and the ſyſtem obeys the irritations, ſenſations, volitions, and aſſociations, with leſs and leſs energy, till the whole ſinks into inaction.

Three cauſes may conſpire to render our nerves leſs excitable, which have been already mentioned. 1. If a ſtimulus be greater than natural, it produces too great an exertion of the ſtimulated organ, and in conſequence exhauſts the ſpirit of animation; and the moving organ ceaſes to act, even though the ſtimulus be con⯑tinued. And though reſt will recruit this exhauſtion, yet ſome de⯑gree of permanent injury remains, as is evident after expoſing the eyes long to too ſtrong a light. 2. If excitations weaker than na⯑tural be applied, ſo as not to excite the organ into action, (as when ſmall doſes of aloe or rhubarb are exhibited,) they may be gradually increaſed, without exciting the organ into action; which will thus acquire a habit of diſobedience to the ſtimulus; thus by increaſing the [468] doſe by degrees, great quantities of opium or wine may be taken without intoxication. See Sect. XII. 3. 1.

3. Another mode, by which life is gradually undermined, is when irritative motions continue to be produced in conſequence of ſtimulus, but are not ſucceeded by ſenſation; hence the ſtimulus of contagious matter is not capable of producing fever a ſecond time, becauſe it is not ſucceeded by ſenſation. See Sect. XII. 3. 6. And hence, owing to the want of the general pleaſureable ſenſation, which ought to at⯑tend digeſtion and glandular ſecretion, an irkſomeneſs of life enſues▪ and, where this is in greater exceſs, the melancholy of old age oc⯑curs, with torpor or debility.

From hence I conclude, that it is probable that the fibrillae, or moving filaments at the extremities of the nerves of ſenſe, and the fibres which conſtitute the muſcles (which are perhaps the only parts of the ſyſtem that are endued with contractile life) are not changed, as we advance in years, like the other parts of the body; but only enlarged or elongated with our growth; and in conſequence they be⯑come leſs and leſs excitable into action. Whence, inſtead of gradu⯑ally changing the old animal, the generation of a totally new one becomes neceſſary with undiminiſhed excitability; which many years will continue to acquire new parts, or new ſolidity, and then loſing its excitability in time, periſh like its parent.

V. From this idea the art of preſerving long health and life may be deduced; which muſt conſiſt in uſing no greater ſtimulus, whether of the quantity or kind of our food and drink, or of external circum⯑ſtances, ſuch as heat, and exerciſe, and wakefulneſs, than is ſuf⯑ficient to preſerve us in vigour; and gradually, as we grow old to in⯑creaſe the ſtimulus of our aliment, as the inirritability of our ſyſtem increaſes.

The debilitating effects aſcribed by the poet MARTIAL to the ex⯑ceſſive uſe of warm bathing in Italy, may with equal propriety be ap⯑plied [469] to the warm rooms of England; which, with the general ex⯑ceſſive ſtimulus of ſpirituous or fermented liquors, and in ſome in⯑ſtances of immoderate venery, contribute to ſhorten our lives.

I. FROM the recent diſcoveries of many ingenious philoſophers it appears, that during reſpiration the blood imbibes the vital part of the air, called oxygene, through the membranes of the lungs; and that hence reſpiration may be aptly compared to a ſlow combuſtion. As in combuſtion the oxygene of the atmoſphere unites with ſome phlo⯑giſtic or inflammable body, and forms an acid (as in the production of vitriolic acid from ſulphur, or carbonic acid from charcoal,) giving out at the ſame time a quantity of the matter of heat; ſo in reſpira⯑tion the oxygene of the air unites with the phlogiſtic part of the blood, and probably produces phoſphoric or animal acid, changing the colour [471] of the blood from a dark to a bright red; and probably ſome of the matter of heat is at the ſame time given out according to the theory of Dr. Crawford. But as the evolution of heat attends almoſt all che⯑mical combinations, it is probable, that it alſo attends the ſecretions of the various fluids from the blood; and that the conſtant combina⯑tions or productions of new fluids by means of the glands conſtitute the more general ſource of animal heat; this ſeems evinced by the univerſal evolution of the matter of heat in the bluſh of ſhame or of anger; in which at the ſame time an increaſed ſecretion of the per⯑ſpirable matter occurs; and the partial evolution of it from topical in⯑flammations, as in gout or rheumatiſm, in which there is a ſecretion of new blood-veſſels.

Some medical philoſophers have aſcribed the heat of animal bodies to the friction of the particles of the blood againſt the ſides of the veſ⯑ſels. But no perceptible heat has ever been produced by the agitation of water, or oil, or quickſilver, or other fluids; except thoſe fluids have undergone at the ſame time ſome chemical change, as in agitat⯑ing milk or wine, till they become ſour.

Beſides the ſuppoſed production of phoſphoric acid, and change of colour of the blood, and the production of carbonic acid, there would appear to be ſomething of a more ſubtile nature perpetually acquired from the atmoſphere; which is too fine to be long contained in ani⯑mal veſſels, and therefore requires perpetual renovation; and without which life cannot continue longer than a minute or two; this ethe⯑real fluid is probably ſecreted from the blood by the brain, and perpetually diſſipated in the actions of the muſcles and organs of ſenſe.

That the blood acquires ſomething from the air, which is imme⯑diately neceſſary to life, appears from an experiment of Dr. Hare (Philoſ. Tranſact. abridged, Vol. III. p. 239.) who found, "that birds, mice, &c. would live as long again in a veſſel, where he had crowded in double the quantity of air by a condenſing engine, than [472] they did when confined in air of the common denſity." Whereas if ſome kind of deleterious vapour only was exhaled from the blood in reſpiration; the air, when condenſed into half its compaſs, could not be ſuppoſed to receive ſo much of it.

II. Sir Edward Hulſe, a phyſician of reputation at the beginning of the preſent century, was of opinion, that the placenta was a reſpira⯑tory organ, like the gills of fiſh; and not an organ to ſupply nutri⯑ment to the foetus; as mentioned in Derham's Phyſico-theology. Many other phyſicians ſeem to have eſpouſed the ſame opinion, as noticed by Haller. Elem. Phyſiologiae, T. 1. Dr. Gipſon publiſhed a defence of this theory in the Medical Eſſays of Edinburgh, Vol. I. and II. which doctrine is there controverted at large by the late Alex⯑ander Monro; and ſince that time the general opinion has been, that the placenta is an organ of nutrition only, owing perhaps rather to the authority of ſo great a name, than to the validity of the arguments adduced in its ſupport. The ſubject has lately been reſumed by Dr. James Jeffray, and by Dr. Foreſter French, in their inaugural diſſer⯑tations at Edinburgh and at Cambridge; who have defended the con⯑trary opinion in an able and ingenious manner; and from whoſe Theſes I have extracted many of the following remarks.

Firſt, by the late diſcoveries of Dr. Prieſtley, M. Lavoiſier, and other philoſophers, it appears, that the baſis of atmoſpherical air, called oxygene, is received by the blood through the membranes of the lungs; and that by this addition the colour of the blood is changed from a dark to a light red. Secondly, that water poſſeſſes oxygene alſo as a part of its compoſition, and contains air likewiſe in its pores; whence the blood of fiſh receives oxygene from the water, or from the air it contains, by means of their gills, in the ſame manner as the blood is oxygenated in the lungs of air-breathing animals; it changes its colour at the ſame time from a dark to a light red in the veſſels of their gills, which conſtitute a pulmonary organ adapted to the me⯑dium in which they live. Thirdly, that the placenta conſiſts of ar⯑teries [473] carrying the blood to its extremities, and a vein bringing it back, reſembling exactly in ſtructure the lungs and gills above men⯑tioned; and that the blood changes its colour from a dark to a light red in paſſing through theſe veſſels.

This analogy between the lungs and gills of animals, and the pla⯑centa of the fetus, extends through a great variety of other circum⯑ſtances; thus air-breathing creatures and fiſh can live but a few mi⯑nutes without air or water; or when they are confined in ſuch air or water, as has been ſpoiled by their own reſpiration; the ſame hap⯑pens to the fetus, which, as ſoon as the placenta is ſeparated from the uterus, muſt either expand its lungs, and receive air, or die. Hence from the ſtructure, as well as the uſe of the placenta, it appears to be a reſpiratory organ, like the gills of fiſh, by which the blood in the fetus becomes oxygenated.

From the terminations of the placental veſſels not being obſerved to bleed after being torn from the uterus, while thoſe of the uterus effuſe a great quantity of florid arterial blood, the terminations of the placental veſſels would ſeem to be inſerted into the arterial ones of the mother; and to receive oxygenation from the paſſing currents of her blood through their coats or membranes; which oxygenation is proved by the change of the colour of the blood from dark to light red in its paſſage from the placental arteries to the placental vein.

The curious ſtructure of the cavities or lacunae of the placenta, demonſtrated by Mr. J. Hunter, explain this circumſtance. That in⯑genious philoſopher has ſhewn, that there are numerous cavities or lacunae formed on that ſide of the placenta, which is in contact with the uterus; thoſe cavities or cells are filled with blood from the ma⯑ternal arteries, which open into them; which blood is again taken up by the maternal veins, and is thus perpetually changed. While the terminations of the placental arteries and veins are ſpread in fine reti⯑culation on the ſides of theſe cells. And thus, as the growing fetus [474] requires greater oxygenation, an apparatus is produced reſembling exactly the air-cells of the lungs.

In cows, and other ruminating animals, the internal ſurface of the uterus is unequal like hollow cups, which have been called coty⯑ledons; and into theſe cavities the prominencies of the numerous pla⯑centas, with which the fetus of thoſe animals is furniſhed, are in⯑ſerted, and ſtrictly adhere; though they may be extracted without effuſion of blood. Theſe inequalities of the uterus, and the nu⯑merous placentas in conſequence, ſeem to be deſigned for the purpoſe of expanding a greater ſurface for the terminations of the placental veſſels for the purpoſe of receiving oxygenation from the uterine ones; as the progeny of this claſs of animals are more completely formed before their nativity, than that of the carnivorous claſſes, and muſt thence in the latter weeks of pregnancy require greater oxygenation. Thus calves and lambs can walk about in a few minutes after their birth; while puppies and kittens remain many days without opening their eyes. And though on the ſeparation of the cotyledons of ru⯑minating animals no blood is effuſed, yet this is owing clearly to the greater power of contraction of their uterine lacunae or alveoli. See Medical Eſſays, Vol. V. page 144. And from the ſame cauſe they are not liable to a ſanguiferous menſtruation.

The neceſſity of the oxygenation of the blood in the fetus is farther illuſtrated by the analogy of the chick in the egg; which appears to have its blood oxygenated at the extremities of the veſſels ſurround⯑ing the yolk; which are ſpread on the air-bag at the broad end of the egg, and may abſorb oxygene through that moiſt membrane from the air conſined behind it; and which is ſhewn by experiments in the ex⯑hauſted receiver to be changeable through the ſhell.

This analogy may even be extended to the growing ſeeds of vege⯑tables; which were ſhewn by Mr. Scheele to require a renovation of the air over the water, in which they were confined. Many vege⯑table [475] ſeeds are ſurrounded with air in their pods or receptacles, as peas, the fruit of ſtaphylea, and lichnis veſicaria; but it is probable, that thoſe ſeeds, after they are ſhed, as well as the ſpawn of fiſh, by the ſituation of the former on or near the moiſt and aerated ſurface of the earth, and of the latter in the ever-changing and ventilated water, may not be in need of an apparatus for the oxygenation of their firſt blood, before the leaves of one, and the gills of the other, are produced for this purpoſe.

III. 1. There are many arguments, beſides the ſtrict analogy be⯑tween the liquor amnii and the albumen ovi, which ſhew the former to be a nutritive fluid; and that the fetus in the latter months of pregnancy takes it into its ſtomach; and that in conſequence the pla⯑centa is produced for ſome other important purpoſe.

Firſt, that the liquor amnii is not an excrementitious fluid is evinced, becauſe it is found in greater quantity, when the fetus is young, decreaſing after a certain period till birth. Haller aſſerts, "that in ſome animals but a ſmall quantity of this fluid remains at the birth. In the eggs of hens it is conſumed on the eighteenth day, ſo that at the excluſion of the chick ſcarcely any remains. In rabbits before birth there is none." Elem. Phyſiol. Had this been an ex⯑crementitious fluid, the contrary would probably have occurred. Secondly, the ſkin of the fetus is covered with a whitiſh cruſt or pel⯑licle, which would ſeem to preclude any idea of the liquor amnii being produced by any exſudation of perſpirable matter. And it can⯑not conſiſt of urine, becauſe in brute animals the urachus paſſes from the bladder to the alantois for the expreſs purpoſe of carrying off that fluid; which however in the human fetus ſeems to be retained in the diſtended bladder, as the feces are accumulated in the bowels of all animals.

2. The nutritious quality of the liquid, which ſurrounds the fetus, appears from the following conſiderations. 1. It is coagulable by heat, by nitrous acid, and by ſpirit of wine, like milk, ſerum of blood, and [476] other fluids, which daily experience evinces to be nutritious. 2. It has a ſaltiſh taſte, according to the accurate Baron Haller, not unlike the whey of milk, which it even reſembles in ſmell. 3. The white of the egg which conſtitutes the food of the chick, is ſhewn to be nu⯑tritious by our daily experience; beſides the experiment of its nutri⯑tious effects mentioned by Dr. Fordyce in his late Treatiſe on Digeſ⯑tion, p. 178; who adds, that it much reſembles the eſſential parts of the ſerum of blood.

3. A fluid ſimilar to the fluid, with which the fetus is ſurrounded, except what little change may be produced by a beginning digeſtion, is found in the ſtomach of the fetus; and the white of the egg is found in the ſame manner in the ſtomach of the chick.

Numerous hairs, ſimilar to thoſe of its ſkin, are perpetually found among the contents of the ſtomach in new-born calves; which muſt therefore have licked themſelves before their nativity. Blaſii Anatom. See Sect. XVI. 2. on Inſtinct.

The chick in the egg is ſeen gently to move in its ſurrounding fluid, and to open and ſhut its mouth alternately. The ſame has been ob⯑ſerved in puppies. Haller's El. Phyſ. I. 8. p. 201.

A column of ice has been ſeen to reach down the oeſophagus from the mouth to the ſtomach in a frozen fetus; and this ice was the liquor amnii frozen.

The meconium, or firſt faeces, in the bowels of new-born infants evince, that ſomething has been digeſted; and what could this be but the liquor amnii together with the recrements of the gaſtric juice and gall, which were neceſſary for its digeſtion?

There have been recorded ſome monſtrous births of animals with⯑out heads, and conſequently without mouths, which ſeem to have been delivered on doubtful authority, or from inaccurate obſervation. There are two of ſuch monſtrous productions however better atteſted; one of a human fetus, mentioned by Gipſon in the Scots Medical Eſſays; which having the gula impervious was furniſhed with an aperture into [477] the wind-pipe, which communicated below into the gullet; by means of which the liquor amnii might be taken into the ſtomach before na⯑tivity without danger of ſuffocation, while the fetus had no occaſion to breathe. The other monſtrous fetus is deſcribed by Vander Wiel, who aſſerts, that he ſaw a monſtrous lamb, which had no mouth; but inſtead of it was furniſhed with an opening in the lower part of the neck into the ſtomach. Both theſe inſtances evidently favour the doc⯑trine of the fetus being nouriſhed by the mouth; as otherwiſe there had been no neceſſity for new or unnatural apertures into the ſtomach, when the natural ones were deficient?

From theſe facts and obſervations we may ſafely infer, that the fetus in the womb is nouriſhed by the fluid which ſurrounds it; which during the firſt period of geſtation is abſorbed by the naked lacteals; and is afterwards ſwallowed into the ſtomach and bowels, when theſe organs are perfected; and laſtly that the placenta is an organ for the purpoſe of giving due oxygenation to the blood of the fetus; which is more neceſſary, or at leaſt more frequently neceſſary, than even the ſupply of food.

The queſtion of the great Harvey becomes thus eaſily anſwered. "Why is not the fetus in the womb ſuffocated for want of air, when it remains there even to the tenth month without reſpiration: yet if it be born in the ſeventh or eighth month, and has once reſpired, it becomes immediately ſuffocated for want of air, if its reſpiration be obſtructed?"

For further information on this ſubject, the reader is referred to the Tentamen Medicum of Dr. Jeffray, printed at Edinburgh in 1786. And it is hoped that Dr. French will ſome time give his theſes on this ſubject to the public.

I. THE ingenious Dr. Hartley in his work on man, and ſome other philoſophers, have been of opinion, that our immortal part ac⯑quires during this life certain habits of action or of ſentiment, which become for ever indiſſoluble, continuing after death in a future ſtate of exiſtence; and add, that if theſe habits are of the malevolent kind, they muſt render the poſſeſſor miſerable even in heaven. I would apply this ingenious idea to the generation or production of the em⯑bryon, or new animal, which partakes ſo much of the form and pro⯑penſities of the parent.

Owing to the imperfection of language the offspring is termed a new animal, but is in truth a branch or elongation of the parent; ſince a part of the embryon-animal is, or was, a part of the parent; and therefore in ſtrict language it cannot be ſaid to be entirely new at the time of its production; and therefore it may retain ſome of the habits of the parent-ſyſtem.

At the earlieſt period of its exiſtence the embryon, as ſecreted from the blood of the male, would ſeem to conſiſt of a living filament with certain capabilities of irritation, ſenſation, volition, and aſſociation; and alſo with ſome acquired habits or propenſities peculiar to the pa⯑rent: the former of theſe are in common with other animals; the latter ſeem to diſtinguiſh or produce the kind of animal, whether man or quadruped, with the ſimilarity of feature or form to the pa⯑rent. It is difficult to be conceived, that a living entity can be ſepa⯑rated or produced from the blood by the action of a gland; and which ſhall afterwards become an animal ſimilar to that in whoſe veſſels it is formed; even though we ſhould ſuppoſe with ſome modern theo⯑riſts, that the blood is alive; yet every other hypotheſis concerning generation reſts on principles ſtill more difficult to our comprehenſion.

At the time of procreation this ſpeck of entity is received into an appropriated nidus, in which it muſt acquire two circumſtances ne⯑ceſſary to its life and growth; one of theſe is food or ſuſtenance, which is to be received by the abſorbent mouths of its veſſels; and [481] the other is that part of atmoſpherical air, or of water, which by the new chemiſtry is termed oxygene, and which affects the blood by paſſing through the coats of the veſſels which contain it. The fluid ſurrounding the embryon in its new habitation, which is called liquor amnii, ſupplies it with nouriſhment; and as ſome air cannot but be in⯑troduced into the uterus along with the new embryon, it would ſeem that this ſame fluid would for a ſhort time, ſuppoſe for a few hours, ſupply likewiſe a ſufficient quantity of the oxygene for its immediate exiſtence.

On this account the vegetable impregnation of aquatic plants is performed in the air; and it is probable that the honey-cup or nectary of vegetables requires to be open to the air, that the anthers and ſtigmas of the flower may have food of a more oxygenated kind than the common vegetable ſap-juice.

On the introduction of this primordium of entity into the uterus the irritation of the liquor amnii, which ſurrounds it, excites the ab⯑ſorbent mouths of the new veſſels into action; they drink up a part of it, and a pleaſurable ſenſation accompanies this new action; at the ſame time the chemical affinity of the oxygene acts through the veſ⯑ſels of the rubeſcent blood; and a previous want, or diſagreeable ſen⯑ſation, is relieved by this proceſs.

As the want of this oxygenation of the blood is perpetual, (as ap⯑pears from the inceſſant neceſſity of breathing by lungs or gills,) the veſſels become extended by the efforts of pain or deſire to ſeek this neceſſary object of oxygenation, and to remove the diſagreeable ſen⯑ſation, which that want occaſions. At the ſame time new particles of matter are abſorbed, or applied to theſe extended veſſels, and they become permanently elongated, as the fluid in contact with them ſoon looſes the oxygenous part, which it at firſt poſſeſſed, which was ow⯑ing to the introduction of air along with the embryon. Theſe new blood-veſſels approach the ſides of the uterus, and penetrate with their fine terminations into the veſſels of the mother; or adhere to them, [482] acquiring oxygene through their coats from the paſſing currents of the arterial blood of the mother. See Sect. XXXVIII. 2.

This attachment of the placental veſſels to the internal ſide of the uterus by their own proper efforts appears further illuſtrated by the many inſtances of extra-uterine fetuſes, which have thus attached or inſerted their veſſels into the peritoneum; or on the viſcera, exactly in the ſame manner as they naturally inſert or attach them to the uterus.

The abſorbent veſſels of the embryon continue to drink up nou⯑riſhment from the fluid in which they ſwim, or liquor amnii; and which at firſt needs no previous digeſtive preparation; but which, when the whole apparatus of digeſtion becomes complete, is ſwal⯑lowed by the mouth into the ſtomach, and being mixed with ſaliva, gaſtric juice, bile, pancreatic juice, and mucus of the inteſtines, be⯑comes digeſted, and leaves a recrement, which produces the firſt feces of the infant, called meconium.

The liquor amnii is ſecreted into the uterus, as the fetus requires it, and may probably be produced by the irritation of the fetus as an extraneous body; ſince a ſimilar fluid is acquired from the peritoneum in caſes of extra-uterine geſtation. The young caterpillars of the gadfly placed in the ſkins of cows, and the young of the ichneumon-fly placed in the backs of the caterpillars on cabbages, ſeem to pro⯑duce their nouriſhment by their irritating the ſides of their nidus. A vegetable ſecretion and concretion is thus produced on oak-leaves by the gall-inſect, and by the cynips in the bedeguar of the roſe; and by the young graſshopper on many plants, by which the animal ſur⯑rounds itſelf with froth. But in no circumſtance is extra-uterine geſ⯑tation ſo exactly reſembled as by the eggs of a fly, which are de⯑poſited in the frontal ſinus of ſheep and calves. Theſe eggs float in ſome ounces of fluid collected in a thin pellicle or hydatide. This bag of fluid compreſſes the optic nerve on one ſide, by which the viſion being leſs diſtinct in that eye, the animal turns in perpetual [483] circles towards the ſide affected, in order to get a more accurate view of objects; for the ſame reaſon as in ſquinting the affected eye is turned away from the object contemplated. Sheep in the warm months keep their noſes cloſe to the ground to prevent this fly from ſo readily getting into their noſtrils.

The liquor amnii is ſecreted into the womb as it is required, not only in reſpect to quantity, but, as the digeſtive powers of the fetus become formed, this fluid becomes of a different conſiſtence and qua⯑lity, till it is exchanged for milk after nativity. Haller. Phyſiol. V. 1. In the egg the white part, which is analogous to the liquor amnii of quadrupeds, conſiſts of two diſtinct parts; one of which is more viſcid, and probably more difficult of digeſtion, and more nutritive than the other; and this latter is uſed in the laſt week of incubation. The yolk of the egg is a ſtill ſtronger or more nutritive fluid, which is drawn up into the bowels of the chick juſt at its excluſion from the ſhell, and ſerves it for nouriſhment for a day or two, till it is able to digeſt, and has learnt to chuſe the harder ſeeds or grains, which are to afford it ſuſtenance. Nothing analogous to this yolk is found in the fetus of lactiferous animals, as the milk is another nutritive fluid ready prepared for the young progeny.

The yolk therefore is not neceſſary to the ſpawn of fiſh, the eggs of inſects, or for the ſeeds of vegetables; as their embryons have probably their food preſented to them as ſoon as they are excluded from their ſhells, or have extended their roots. Whence it happens that ſome inſects produce a living progeny in the ſpring and ſummer, and eggs in the autumn; and ſome vegetables have living roots or buds produced in the place of ſeeds, as the polygonum viviparum, and magical onions. See Botanic Garden, p. ii. art. anthoxanthum.

There ſeems however to be a reſervoir of nutriment prepared for ſome ſeeds beſides their cotyledons or ſeed-leaves, which may be ſup⯑poſed in ſome meaſure analogous to the yolk of the egg. Such are the ſaccharine juices of apples, grapes and other fruits, which ſupply nu⯑trition [484] to the ſeeds after they fall on the ground. And ſuch is the milky juice in the centre of the coco-nut, and part of the kernel of it; the ſame I ſuppoſe of all other monocotyledon ſeeds, as of the palms, graſſes, and lilies.

II. 1. The proceſs of generation is ſtill involved in impenetrable obſcurity, conjectures may nevertheleſs be formed concerning ſome of its circumſtances. Firſt, the eggs of fiſh and frogs are impregnated, after they leave the body of the female; becauſe they are depoſited in a fluid, and are not therefore covered with a hard ſhell. It is how⯑ever remarkable, that neither frogs nor fiſh will part with their ſpawn without the preſence of the male; on which account female carp and gold-fiſh in ſmall ponds, where there are no males, frequently die from the diſtention of their growing ſpawn. 2. The eggs of fowls, which are laid without being impregnated, are ſeen to contain only the yolk and white, which are evidently the food or ſuſtenance for the future chick. 3. As the cicatricula of theſe eggs is given by the cock, and is evidently the rudiment of the new animal; we may con⯑clude, that the embryon is produced by the male, and the proper food and nidus by the female. For if the female be ſuppoſed to form an equal part of the embryon, why ſhould ſhe form the whole of the apparatus for nutriment and for oxygenation? the male in many ani⯑mals is larger, ſtronger, and digeſts more food than the female, and therefore ſhould contribute as much or more towards the reproduc⯑tion of the ſpecies; but if he contributes only half the embryon, and none of the apparatus for ſuſtenance and oxygenation, the diviſion is unequal; the ſtrength of the male, and his conſumption of food are too great for the effect, compared with that of the female, which is contrary to the uſual courſe of nature.

In objection to this theory of generation it may be ſaid, if the ani⯑malcula in ſemine, as ſeen by the microſcope, be all of them rudi⯑ments of homunculi, when but one of them can find a nidus, what a waſte nature has made of her productions? I do not aſſert [485] that theſe moving particles, viſible by the microſcope, are homun⯑ciones; perhaps they may be the creatures of ſtagnation or putridity, or perhaps no creatures at all; but if they are ſuppoſed to be rudi⯑ments of homunculi, or embryons, ſuch a profuſion of them correſ⯑ponds with the general efforts of nature to provide for the continu⯑ance of her ſpecies of animals. Every individual tree produces innu⯑merable ſeeds, and every individual fiſh innumerable ſpawn, in ſuch inconceivable abundance as would in a ſhort ſpace of time crowd the earth and ocean with inhabitants; and theſe are much more per⯑fect animals than the animalcula in ſemine can be ſuppoſed to be, and periſh in uncounted millions. This argument only ſhews, that the productions of nature are governed by general laws; and that by a wiſe ſuperfluity of proviſion ſhe has enſured their continuance.

2. That the embryon is ſecreted or produced by the male, and not by the conjunction of fluids from both male and female, appears from the analogy of vegetable ſeeds. In the large flowers, as the tulip, there is no ſimilarity of apparatus between the anthers and the ſtigma: the ſeed is produced according to the obſervations of Spallanzani long before the flowers open, and in conſequence long before it can be im⯑pregnated, like the egg in the pullet. And after the prolific duſt is ſhed on the ſtigma, the ſeed becomes coagulated in one point firſt, like the cicatricula of the impregnated egg. See Botanic Garden, Part I. additional note 38. Now in theſe ſimple products of nature, if the female contributed to produce the new embryon equally with the male, there would probably have been ſome viſible ſimilarity of parts for this purpoſe, beſides thoſe neceſſary for the nidus and ſuſtenance of the new progeny. Beſides in many flowers the males are more numerous than the females, or than the ſeparate uterine cells in their germs, which would ſhew, that the office of the male was at leaſt as impor⯑tant as that of the female; whereas if the female, beſides producing the egg or ſeed, was to produce an equal part of the embryon, the of⯑fice of reproduction would be unequally divided between them.

[486] Add to this, that in the moſt ſimple kind of vegetable reproduction, I mean the buds of trees, which are their viviparous offspring, the leaf is evidently the parent of the bud, which riſes in its boſom, ac⯑cording to the obſervation of Linnaeus. This leaf conſiſts of abſor⯑bent veſſels, and pulmonary ones, to obtain its nutriment, and to im⯑pregnate it with oxygene. This ſimple piece of living organization is alſo furniſhed with a power of reproduction; and as the new offspring is thus ſupported adhering to its father, it needs no mother to ſupply it with a nidus, and nutriment, and oxygenation; and hence no fe⯑male leaf has exiſtence.

I conceive that the veſſels between the bud and the leaf communi⯑cate or inoſculate; and that the bud is thus ſerved with vegetable blood, that is, with both nutriment and oxygenation, till the death of the parent-leaf in autumn. And in this reſpect it differs from the fetus of viviparous animals. Secondly, that then the bark-veſſels belonging to the dead-leaf, and in which I ſuppoſe a kind of manna to have been depoſited, become now the placental veſſels, if they may be ſo called, of the new bud. From the vernal ſap thus produced of one ſugar-maple-tree in New-York and in Pennſylvania, five or ſix pounds of good ſugar may be made annually without deſtroying the tree. Account of maple-ſugar by B. Ruſh. London, Phillips. (See Botanic Garden, Part I. additional note on vegetable placentation.)

Theſe veſſels, when the warmth of the vernal ſun hatches the young bud, ſerve it with a ſaccharine nutriment, till it acquires leaves of its own, and ſhoots a new ſyſtem of abſorbents down the bark and root of the tree, juſt as the farinaceous or oily matter in ſeeds, and the ſaccharine matter in fruits, ſerve their embryons with nutri⯑ment, till they acquire leaves and roots. This analogy is as force⯑able in ſo obſcure a ſubject, as it is curious, and may in large buds, as of the horſe-cheſnut, be almoſt ſeen by the naked eye; if with a penknife the remaining rudiment of the laſt year's leaf, and of the new bud in its boſom, be cut away ſlice by ſlice. The ſeven ribs of [487] the laſt year's leaf will be ſeen to have ariſen from the pith in ſeven diſtinct points making a curve; and the new bud to have been pro⯑duced in their centre, and to have pierced the alburnum and cortex, and grown without the aſſiſtance of a mother. A ſimilar proceſs may be ſeen on diſſecting a tulip-root in winter; the leaves, which incloſed the laſt year's flower-ſtalk, were not neceſſary for the flower; but each of theſe was the father of a new bud, which may be now found at its baſe; and which, as it adheres to the parent, required no mother.

This paternal offspring of vegetables, I mean their buds and bulbs, is attended with a very curious circumſtance; and that is, that they exactly reſemble their parents, as is obſervable in grafting fruit-trees, and in propagating flower-roots; whereas the ſeminal offspring of plants, being ſupplied with nutriment by the mother, is liable to per⯑petual variation. Thus alſo in the vegetable claſs dioicia, where the male flowers are produced on one tree, and the female ones on ano⯑ther; the buds of the male trees uniformly produce either male flow⯑ers, or other buds ſimilar to themſelves; and the buds of the female trees produce either female flowers, or other buds ſimilar to them⯑ſelves; whereas the ſeeds of theſe trees produce either male or fe⯑male plants. From this analogy of the production of vegetable buds without a mother, I contend that the mother does not contribute to the formation of the living ens in animal generation, but is neceſſary only for ſupplying its nutriment and oxygenation.

There is another vegetable fact publiſhed by M. Koelreuter, which he calls "a complete metamorphoſis of one natural ſpecies of plants into another," which ſhews, that in ſeeds as well as in buds, the em⯑bryon proceeds from the male parent, though the form of the ſubſe⯑quent mature plant is in part dependant on the female. M. Koelreuter impregnated a ſtigma of the nicotiana ruſtica with the farina of the ni⯑cotiana paniculata, and obtained prolific ſeeds from it. With the plants which ſprung from theſe ſeeds, he repeated the experiment, [488] impregnating them with the farina of the nicotiana paniculata. As the mule plants which he thus produced were prolific, he continued to impregnate them for many generations with the farina of the ni⯑cotiana paniculata, and they became more and more like the male pa⯑rent, till he at length obtained ſix plants in every reſpect perfectly ſi⯑milar to the nicotiana paniculata; and in no reſpect reſembling their female parent the nicotiana ruſtica. Blumenbach on Generation.

3. It is probable that the inſects, which are ſaid to require but one impregnation for ſix generations, as the aphis (ſee Amenit. Academ.) produce their progeny in the manner above deſcribed, that is, without a mother, and not without a father; and thus experience a lucina ſine concubitu. Thoſe who have attended to the habits of the polypus, which is found in the ſtagnant water of our ditches in July, affirm, that the young ones branch out from the ſide of the parent like the buds of trees, and after a time ſeparate themſelves from them. This is ſo analogous to the manner in which the buds of trees appear to be produced, that theſe polypi may be conſidered as all male animals, pro⯑ducing embryons, which require no mother to ſupply them with a nidus, or with nutriment, and oxygenation.

This lateral or lineal generation of plants, not only obtains in the buds of trees, which continue to adhere to them, but is beautifully ſeen in the wires of knot-graſs, polygonum aviculare, and in thoſe of ſtrawberries, fragaria veſca. In theſe an elongated creeping bud is protruded, and, where it touches the ground, takes root, and pro⯑duces a new plant derived from its father, from which it acquires both nutriment and oxygenation; and in conſequence needs no maternal apparatus for theſe purpoſes. In viviparous flowers, as thoſe of al⯑lium magicum, and polygonum viviparum, the anthers and the ſtig⯑mas become effete and periſh; and the lateral or paternal offspring ſucceeds inſtead of ſeeds, which adhere till they are ſufficiently ma⯑ture, and then fall upon the ground, and take root like other bulbs.

The lateral production of plants by wires, while each new plant is [489] thus chained to its parent, and continues to put forth another and ano⯑ther, as the wire creeps onward on the ground, is exactly reſembled by the tape-worm, or taenia, ſo often found in the bowels, ſtretch⯑ing itſelf in a chain quite from the ſtomach to the rectum. Linnaeus aſſerts, "that it grows old at one extremity, while it continues to ge⯑nerate young ones at the other, proceeding ad infinitum, like a root of graſs. The ſeparate joints are called gourd-worms, and propagate new joints like the parent without end, each joint being furniſhed with its proper mouth, and organs of digeſtion." Syſtema naturae. Vermes tenia. In this animal there evidently appears a power of re⯑production without any maternal apparatus for the purpoſe of ſupply⯑ing nutriment and oxygenation to the embryon, as it remains at⯑tached to its father till its maturity. The volvox globator, which is a tranſparent animal, is ſaid by Linnaeus to bear within it ſons and grand-ſons to the fifth generation. Theſe are probably living fetuſes, produced by the father, of different degrees of maturity, to be de⯑truded at different periods of time, like the unimpregnated eggs of various ſizes, which are found in poultry; and as they are produced without any known copulation, contribute to evince, that the living embryon in other orders of animals is formed by the male-parent, and not by the mother, as one parent has the power to produce it.

This idea of the reproduction of animals from a ſingle living fila⯑ment of their fathers, appears to have been ſhadowed or allegorized in the curious account in ſacred writ of the formation of Eve from a rib of Adam.

From all theſe analogies I conclude, that the embryon is produced ſolely by the male, and that the female ſupplies it with a proper nidus, with ſuſtenance, and with oxygenation; and that the idea of the ſe⯑men of the male conſtituting only a ſtimulus to the egg of the fe⯑male, exciting it into life, (as held by ſome philoſophers) has no ſup⯑port from experiment or analogy.

III. 1. Many ingenious philoſophers have found ſo great difficulty [490] in conceiving the manner of the reproduction of animals, that they have ſuppoſed all the numerous progeny to have exiſted in miniature in the animal originally created; and that theſe infinitely minute forms are only evolved or diſtended, as the embryon increaſes in the womb. This idea, beſides its being unſupported by any analogy we are ac⯑quainted with, aſcribes a greater tenuity to organized matter, than we can readily admit; as theſe included embryons are ſuppoſed each of them to conſiſt of the various and complicate parts of animal bodies: they muſt poſſeſs a much greater degree of minuteneſs, than that which was aſcribed to the devils that tempted St. Anthony; of whom 20,000 were ſaid to have been able to dance a ſaraband on the point of the fineſt needle without incommoding each other.

2. Others have ſuppoſed, that all the parts of the embryon are formed in the male, previous to its being depoſited in the egg or ute⯑rus; and that it is then only to have its parts evolved or diſtended as mentioned above; but this is only to get rid of one difficulty by pro⯑poſing another equally incomprehenſible: they found it difficult to conceive, how the embryon could be formed in the uterus or egg, and therefore wiſhed it to be formed before it came thither. In anſwer to both theſe doctrines it may be obſerved, 1ſt, that ſome ani⯑mals, as the crab-fiſh, can reproduce a whole limb, as a leg which has been broken off; others, as worms and ſnails, can reproduce a head, or a tail, when either of them has been cut away; and that hence in theſe animals at leaſt a part can be formed anew, which can⯑not be ſuppoſed to have exiſted previouſly in miniature.

Secondly, there are new parts or new veſſels produced in many diſ⯑caſes, as on the cornea of the eye in ophthalmy, in wens and cancers, which cannot be ſuppoſed to have had a prototype or original minia⯑ture in the embryon.

Thirdly, how could mule-animals be produced, which partake of the forms of both the parents, if the original embryon was a minia⯑ture exiſting in the ſemen of the male parent? if an embryon of the [491] male aſs was only expanded, no reſemblance to the mare could exiſt in the mule.

This miſtaken idea of the extenſion of parts ſeems to have had its riſe from the mature man reſembling the general form of the fetus; and from thence it was believed, that the parts of the fetus were diſtended into the man; whereas they have increaſed 100 times in weight, as well as 100 times in ſize; now no one will call the ad⯑ditional 99 parts a diſtention of the original one part in reſpect to weight. Thus the uterus during pregnancy is greatly enlarged in thickneſs and ſolidity as well as in capacity, and hence muſt have ac⯑quired this additional ſize by accretion of new parts, not by an ex⯑tenſion of the old ones; the familiar act of blowing up the bladder of an animal recently ſlaughtered has led our imaginations to apply this idea of diſtention to the increaſe of ſize from natural growth; which however muſt be owing to the appoſition of new parts; as it is evinc⯑ed from the increaſe of weight along with the increaſe of dimenſion; and is even viſible to our eyes in the elongation of our hair from the colour of its ends; or when it has been dyed on the head; and in the growth of our nails from the ſpecks ſometimes obſervable on them; and in the increaſe of the white creſcent at their roots, and in the growth of new fleſh in wounds, which conſiſts of new nerves as well as of new blood-veſſels.

3. Laſtly, Mr. Buffon has with great ingenuity imagined the ex⯑iſtence of certain organic particles, which are ſuppoſed to be partly alive, and partly mechanic ſprings. The latter of theſe were diſco⯑vered by Mr. Needham in the milt or male organ of a ſpecies of cuttle fiſh, called calmar; the former, or living animalcula, are found in both male and female ſecretions, in the infuſions of ſeeds, as of pep⯑per, in the jelly of roaſted veal, and in all other animal and vegetable ſubſtances. Theſe organic particles he ſuppoſes to exiſt in the ſper⯑matic fluids of both ſexes, and that they are derived thither from every part of the body, and muſt therefore reſemble, as he ſuppoſes, [492] the parts from whence they are derived. Theſe organic particles he believes to be in conſtant activity, till they become mixed in the womb, and then they inſtantly join and produce an embryon or fetus ſimilar to the two parents.

Many objections might be adduced to this fanciful theory, I ſhall only mention two. Firſt, that it is analogous to no known animal laws. And ſecondly, that as theſe fluids, replete with organic par⯑ticles derived both from the male and female organs, are ſuppoſed to be ſimilar; there is no reaſon why the mother ſhould not produce a female embryon without the aſſiſtance of the male, and realize the lucina ſine concubitu.

IV. 1. I conceive the primordium, or rudiment of the embryon, as ſecreted from the blood of the parent, to conſiſt of a ſimple living filament as a muſcular fibre; which I ſuppoſe to be an extremity of a nerve of loco-motion, as a fibre of the retina is an extremity of a nerve of ſenſation; as for inſtance one of the fibrils, which compoſe the mouth of an abſorbent veſſel; I ſuppoſe this living filament, of whatever form it may be, whether ſphere, cube, or cylinder, to be endued with the capability of being excited into action by certain kinds of ſtimulus. By the ſtimulus of the ſurrounding fluid, in which it is received from the male, it may bend into a ring; and thus form the beginning of a tube. Such moving filaments, and ſuch rings, are deſcribed by thoſe, who have attended to microſcopic animalcula. This living ring may now embrace or abſorb a nutritive particle of the fluid, in which it ſwims; and by drawing it into its pores, or joining it by compreſſion to its extremities, may increaſe its own length or craſſitude; and by degrees the living ring may become a living tube.

2. With this new organization, or accretion of parts, new kinds of irritability may commence; for ſo long as there was but one living organ, it could only be ſuppoſed to poſſeſs irritability; ſince ſenſibility may be conceived to be an extenſion of the effect of irritability over [493] the reſt of the ſyſtem. Theſe new kinds of irritability and of ſenſi⯑bility in conſequence of new organization, appear from variety of facts in the more mature animal; thus the formation of the teſtes, and conſequent ſecretion of the ſemen, occaſion the paſſion of luſt; the lungs muſt be previouſly formed before their exertions to obtain freſh air can exiſt; the throat or oeſophagus muſt be formed previous to the ſenſation or appetites of hunger and thirſt; one of which ſeems to reſide at the upper end, and the other at the lower end of that canal.

Thus alſo the glans penis, when it is diſtended with blood, ac⯑quires a new ſenſibility, and a new appetency. The ſame occurs to the nipples of the breaſts of female animals, when they are diſtended with blood, they acquire the new appetency of giving milk. So in⯑flamed tendons and membranes, and even bones, acquire new ſenſa⯑tions; and the parts of mutilated animals, as of wounded ſnails, and polypi, and crabs, are reproduced; and at the ſame time acquire ſen⯑ſations adapted to their ſituations. Thus when the head of a ſnail is reproduced after decollation with a ſharp raſor, thoſe curious teleſco⯑pic eyes are alſo reproduced, and acquire their ſenſibility to light, as well as their adapted muſcles for retraction on the approach of injury.

With every new change, therefore, of organic form, or addition of organic parts, I ſuppoſe a new kind of irritability or of ſenſibility to be produced; ſuch varieties of irritability or of ſenſibility exiſt in our adult ſtate in the glands; every one of which is furniſhed with an irritability, or a taſte, or appetency, and a conſequent mode of action peculiar to itſelf.

In this manner I conceive the veſſels of the jaws to produce thoſe of the teeth, thoſe of the fingers to produce the nails, thoſe of the ſkin to produce the hair; in the ſame manner as afterwards about the age of puberty the beard and other great changes in the form of the body, and diſpoſition of the mind, are produced in conſequence of the [494] new ſecretion of ſemen; for if the animal is deprived of this ſecretion thoſe changes do not take place. Theſe changes I conceive to be formed not by elongation or diſtention of primeval ſtamina, but by appoſition of parts; as the mature crab-fiſh, when deprived of a limb, in a certain ſpace of time has power to regenerate it; and the tadpole puts forth its feet long after its excluſion from the ſpawn; and the caterpillar in changing into a butterfly acquires a new form, with new powers, new ſenſations, and new deſires.

The natural hiſtory of butterflies, and moths, and beetles, and gnats, is full of curioſity; ſome of them paſs many months, and others even years, in their caterpillar or grub ſtate; they then reſt many weeks without food, ſuſpended in the air, buried in the earth, or ſubmerſed in water; and change themſelves during this time into an animal apparently of a different nature; the ſtomachs of ſome of them, which before digeſted vegetable leaves or roots, now only di⯑geſt honey; they have acquired wings for the purpoſe of ſeeking this new food, and a long proboſcis to collect it from flowers, and I ſup⯑poſe a ſenſe of ſmell to detect the ſecret places in flowers, where it is formed. The moths, which fly by night, have a much longer pro⯑boſcis rolled up under their chins like a watch ſpring; which they extend to collect the honey from flowers in their ſleeping ſtate; when they are cloſed, and the nectaries in conſequence more difficult to be plundered. The beetle kind are furniſhed with an external covering of a hard material to their wings, that they may occaſionally again make holes in the earth, in which they paſſed the former ſtate of their exiſtence.

But what moſt of all diſtinguiſhes theſe new animals is, that they are new furniſhed with the powers of reproduction; and that they now differ from each other in ſex, which does not appear in their caterpillar or grub ſtate. In ſome of them the change from a cater⯑pillar into a butterfly or moth ſeems to be accompliſhed for the ſole purpoſe of their propagation; ſince they immediately die after this is [495] finiſhed, and take no food in the interim, as the ſilk-worm in this climate; though it is poſſible, it might take honey as food, if it was preſented to it. For in general it would ſeem, that food of a more ſtimulating kind, the honey of vegetables inſtead of their leaves, was neceſſary for the purpoſe of the ſeminal reproduction of theſe animals, exactly ſimilar to what happens in vegetables; in theſe the juices of the earth are ſufficient for their purpoſe of reproduction by buds or bulbs; in which the new plant ſeems to be formed by irritative mo⯑tions, like the growth of their other parts, as their leaves or roots; but for the purpoſe of ſeminal or amatorial reproduction, where ſenſa⯑tion is required, a more ſtimulating food becomes neceſſary for the anther, and ſtigma; and this food is honey; as explained in Sect. XIII. on Vegetable Animation.

The gnat and the tadpole reſemble each other in their change from natant animals with gills into aerial animals with lungs; and in their change of the element in which they live; and probably of the food, with which they are ſupported; and laſtly, with their acquiring in their new ſtate the difference of ſex, and the organs of ſeminal or ama⯑torial reproduction. While the polypus, who is their companion in their former ſtate of life, not being allowed to change his form and element, can only propagate like vegetable buds by the ſame kind of irritative motions, which produces the growth of his own body, with⯑out the ſeminal or amatorial propagation, which requires ſenſation; and which in gnats and tadpoles ſeems to require a change both of food and of reſpiration.

From hence I conclude, that with the acquiſition of new parts, new ſenſations, and new deſires, as well as new powers, are pro⯑duced; and this by accretion to the old ones, and not by diſtention of them. And finally, that the moſt eſſential parts of the ſyſtem, as the brain for the purpoſe of diſtributing the power of life, and the placenta for the purpoſe of oxygenating the blood, and the additional abſorbent veſſels for the purpoſe of acquiring aliment, are firſt formed [496] by the irritations above mentioned, and by the pleaſureable ſenſations attending thoſe irritations, and by the exertions in conſequence of painful ſenſations, ſimilar to thoſe of hunger and ſuffocation. After theſe an apparatus of limbs for future uſes, or for the purpoſe of moving the body in its preſent natant ſtate, and of lungs for future reſpiration, and of teſtes for future reproduction, are formed by the irritations and ſenſations, and conſequent exertions of the parts pre⯑viouſly exiſting, and to which the new parts are to be attached.

3. In confirmation of theſe ideas it may be obſerved, that all the parts of the body endeavour to grow, or to make additional parts to themſelves throughout our lives; but are reſtrained by the parts im⯑mediately containing them; thus, if the ſkin be taken away, the fleſhy parts beneath ſoon ſhoot out new granulations, called by the vulgar proud fleſh. If the perioſteum be removed, a ſimilar growth commences from the bone. Now in the caſe of the imperfect embryon, the containing or confining parts are not yet ſuppoſed to be formed, and hence there is nothing to reſtrain its growth.

4. By the parts of the embryon being thus produced by new ap⯑poſitions, many phenomena both of animal and vegetable productions receive an eaſier explanation; ſuch as that many fetuſes are deficient at the extremities, as in a finger or a toe, or in the end of the tongue, or in what is called a hare-lip with deficiency of the palate. For if there ſhould be a deficiency in the quantity of the firſt nutritive par⯑ticles laid up in the egg for the reception of the firſt living filament, the extreme parts, as being laſt formed, muſt ſhew this deficiency by their being imperfect.

This idea of the growth of the embryon accords alſo with the pro⯑duction of ſome monſtrous births, which conſiſt of a duplicature of the limbs, as chickens with four legs; which could not occur, if the fetus was formed by the diſtention of an original ſtamen, or minia⯑ture. For if there ſhould be a ſuperfluity of the firſt nutritive par⯑ticles laid up in the egg for the firſt living filament; it is eaſy to con⯑ceive, [497] that a duplicature of ſome parts may be formed. And that ſuch ſuperfluous nouriſhment ſometimes exiſts, is evinced by the double yolks in ſome eggs, which I ſuppoſe were thus formed previ⯑ous to their impregnation by the exuberant nutriment of the hen.

This idea is confirmed by the analogy of the monſters in the ve⯑getable world alſo; in which a duplicate or triplicate production of various parts of the flower is obſervable, as a triple nectary in ſome columbines, and a triple petal in ſome primroſes; and which are ſup⯑poſed to be produced by abundant nouriſhment.

5. If the embryon be received into a fluid, whoſe ſtimulus is dif⯑ferent in ſome degree from the natural, as in the production of mule⯑animals, the new irritabilities or ſenſibilities acquired by the increaſing or growing organized parts may differ, and thence produce parts not ſimilar to the father, but of a kind belonging in part to the mother; and thus, though the original ſtamen or living ens was derived totally from the father, yet new irritabilities or ſenſibilities being excited, a change of form correſponding with them will be produced. Nor could the production of mules exiſt, if the ſtamen or miniature of all the parts of the embryon is previouſly formed in the male ſemen, and is only diſtended by nouriſhment in the female uterus. Whereas this difficulty ceaſes, if the embryon be ſuppoſed to conſiſt of a living fila⯑ment, which acquires or makes new parts with new irritabilities, as it advances in its growth.

The form, ſolidity, and colour, of the particles of nutriment laid up for the reception of the firſt living filament, as well as their pecu⯑liar kind of ſtimulus, may contribute to produce a difference in the form, ſolidity, and colour of the fetus, ſo as to reſemble the mother, as it advances in life. This alſo may eſpecially happen during the firſt ſtate of the exiſtence of the embryon, before it has acquired or⯑gans, which can change theſe firſt nutritive particles, as explained in No. 5. 2. of this Section. And as theſe nutritive particles are ſup⯑poſed [498] to be ſimilar to thoſe, which are formed for her own nutrition, it follows that the fetus ſhould ſo far reſemble the mother.

This explains, why hereditary diſeaſes may be derived either from the male or female parent, as well as the peculiar form of either of their bodies. Some of theſe hereditary diſeaſes are ſimply owing to a deficient activity of a part of the ſyſtem, as of the abſorbent veſſels, which open into the cells or cavities of the body, and thus occaſion dropſies. Others are at the ſame time owing to an increaſe of ſenſa⯑tion, as in ſcrophula and conſumption; in theſe the obſtruction of the fluids is firſt cauſed by the inirritability of the veſſels, and the inflam⯑mation and ulcers which ſucceed, are cauſed by the conſequent in⯑creaſe of ſenſation in the obſtructed part. Other hereditary diſeaſes, as the epilepſy, and other convulſions, conſiſt in too great voluntary exertions in conſequence of diſagreeable ſenſation in ſome particular diſeaſed part. Now as the pains, which occaſion theſe convulſions, are owing to defect of the action of the diſeaſed part, as ſhewn in Sect. XXXIV. it is plain, that all theſe hereditary diſeaſes may have their origin either from defective irritability derived from the father, or from deficiency of the ſtimulus of the nutriment derived from the mother. In either caſe the effect would be ſimilar; as a ſcrophulous race is frequently produced among the poor from the deficient ſtimu⯑lus of bad diet, or of hunger; and among the rich, by a deficient ir⯑ritability from their having been long accuſtomed to too great ſtimulus, as of vinous ſpirit.

6. From this account of reproduction it appears, that all animals have a ſimilar origin, viz. from a ſingle living filament; and that the difference of their forms and qualities has ariſen only from the dif⯑ferent irritabilities and ſenſibilities, or voluntarities, or aſſociabilities, of this original living filament; and perhaps in ſome degree from the different forms of the particles of the fluids, by which it has been at firſt ſtimulated into activity. And that from hence, as Linnaeus has [499] conjectured in reſpect to the vegetable world, it is not impoſſible, but the great variety of ſpecies of animals, which now tenant the earth, may have had their origin from the mixture of a few natural orders. And that thoſe animal and vegetable mules, which could continue their ſpecies, have done ſo, and conſtitute the numerous families of animals and vegetables which now exiſt; and that thoſe mules, which were produced with imperfect organs of generation, periſhed without reproduction, according to the obſervation of Ari⯑ſtotle; and are the animals, which we now call mules. See Botanic Garden, Part II. Note on Dianthus.

Such a promiſcuous intercourſe of animals is ſaid to exiſt at this day in New South Wales by Captain Hunter. And that not only amongſt the quadrupeds and birds of different kinds, but even amongſt the fiſh, and, as he believes, amongſt the vegetables. He ſpeaks of an animal between the opoſſum and the kangaroo, from the ſize of a ſheep to that of a rat. Many fiſh ſeemed to partake of the ſhark; ſome with a ſkait's head and ſhoulders, and the hind part of a ſhark; others with a ſhark's head and the body of a mullet; and ſome with a ſhark's head and the flat body of a ſting-ray. Many birds partake of the parrot; ſome have the head, neck, and bill of a parrot, with long ſtraight feet and legs; others with legs and feet of a parrot, with head and neck of a ſea-gull. Voyage to South Wales by Captain John Hunter, p. 68.

7. All animals therefore, I contend, have a ſimilar cauſe of their organization, originating from a ſingle living filament, endued indeed with different kinds of irritabilities and ſenſibilities, or of animal ap⯑petencies; which exiſt in every gland, and in every moving organ of the body, and are as eſſential to living organization as chemical affini⯑ties are to certain combinations of inanimate matter.

If I might be indulged to make a ſimile in a philoſophical work, I ſhould ſay, that the animal appetencies are not only perhaps leſs nu⯑merous originally than the chemical affinities; but that like theſe lat⯑ter, [500] they change with every new combination; thus vital air and azote, when combined, produce nitrous acid; which now acquires the property of diſſolving ſilver; ſo with every new additional part to the embryon, as of the throat or lungs, I ſuppoſe a new animal ap⯑petency to be produced.

In this early formation of the embryon from the irritabilities, ſen⯑ſibilities, and aſſociabilities, and conſequent appetencies, the faculty of volition can ſcarcely be ſuppoſed to have had its birth. For about what can the fetus deliberate, when it has no choice of objects? But in the more advanced ſtate of the fetus, it evidently poſſeſſes volition; as it frequently changes its attitude, though it ſeems to ſleep the greateſt part of its time; and afterwards the power of volition con⯑tributes to change or alter many parts of the body during its growth to manhood, by our early modes of exertion in the various depart⯑ments of life. All theſe faculties then conſtitute the vis fabricatrix, and the vis conſervatrix, as well as the vis medicatrix of nature, ſo much ſpoken of, but ſo little underſtood by philoſophers.

8. When we revolve in our minds, firſt, the great changes, which we ſee naturally produced in animals after their nativity, as in the production of the butterfly with painted wings from the crawling ca⯑terpillar; or of the reſpiring frog from the ſubnatant tadpole; from the feminine boy to the bearded man, and from the infant girl to the lacteſcent woman; both which changes may be prevented by certain mutilations of the glands neceſſary to reproduction.

Secondly, when we think over the great changes introduced into various animals by artificial or accidental cultivation, as in horſes, which we have exerciſed for the different purpoſes of ſtrength or ſwiftneſs, in carrying burthens or in running races; or in dogs, which have been cultivated for ſtrength and courage, as the bull-dog; or for acuteneſs of his ſenſe of ſmell, as the hound and ſpaniel; or for the ſwiftneſs of his foot, as the greyhound; or for his ſwimming in the water, or for drawing ſnow-ſledges, as the rough-haired dogs of [501] the north; or laſtly, as a play-dog for children, as the lap-dog; with the changes of the forms of the cattle, which have been domeſticated from the greateſt antiquity, as camels, and ſheep; which have un⯑dergone ſo total a transformation, that we are now ignorant from what ſpecies of wild animals they had their origin. Add to theſe the great changes of ſhape and colour, which we daily ſee produced in ſmaller animals from our domeſtication of them, as rabbits, or pid⯑geons; or from the difference of climates and even of ſeaſons; thus the ſheep of warm climates are covered with hair inſtead of wool; and the hares and partridges of the latitudes, which are long buried in ſnow, become white during the winter months; add to theſe the various changes produced in the forms of mankind, by their early modes of exertion; or by the diſeaſes occaſioned by their habits of life; both of which became hereditary, and that through many ge⯑nerations. Thoſe who labour at the anvil, the oar, or the loom, as well as thoſe who carry ſedan-chairs, or who have been educated to dance upon the rope, are diſtinguiſhable by the ſhape of their limbs; and the diſeaſes occaſioned by intoxication deform the countenance with leprous eruptions, or the body with tumid viſcera, or the joints with knots and diſtortions.

Thirdly, when we enumerate the great changes produced in the ſpecies of animals before their nativity; theſe are ſuch as reſemble the form or colour of their parents, which have been altered by the cultivation or accidents above related, and are thus continued to their poſterity. Or they are changes produced by the mixture of ſpecies as in mules; or changes produced probably by the exuberance of nou⯑riſhment ſupplied to the fetus, as in monſtrous births with additional limbs; many of theſe enormities of ſhape are propagated, and conti⯑nued as a variety at leaſt, if not as a new ſpecies of animal. I have ſeen a breed of cats with an additional claw on every foot; of poultry alſo with an additional claw, and with wings to their feet; and of others without rumps. Mr. Buffon mentions a breed of dogs without [502] tails, which are common at Rome and at Naples, which he ſuppoſes to have been produced by a cuſtom long eſtabliſhed of cutting their tails cloſe off. There are many kinds of pidgeons, admired for their peculiarities, which are monſters thus produced and propagated. And to theſe muſt be added, the changes produced by the imagination of the male parent, as will be treated of more at large in No. VI. of this Section.

When we conſider all theſe changes of animal form, and innumer⯑able others, which may be collected from the books of natural hiſtory; we cannot but be convinced, that the fetus or embryon is formed by appoſition of new parts, and not by the diſtention of a primordial neſt of germs, included one within another, like the cups of a conjurer.

Fourthly, when we revolve in our minds the great ſimilarity of ſtructure, which obtains in all the warm-blooded animals, as well quadrupeds, birds, and amphibious animals, as in mankind; from the mouſe and bat to the elephant and whale; one is led to conclude, that they have alike been produced from a ſimilar living filament. In ſome this filament in its advance to maturity has acquired hands and fingers, with a fine ſenſe of touch, as in mankind. In others it has acquired claws or talons, as in tygers and eagles. In others, toes with an intervening web, or membrane, as in ſeals and geeſe. In others it has acquired cloven hoofs, as in cows and ſwine; and whole hoofs in others, as in the horſe. While in the bird kind this original living filament has put forth wings inſtead of arms or legs, and feathers inſtead of hair. In ſome it has protruded horns on the forehead inſtead of teeth in the fore part of the upper jaw; in others tuſhes inſtead of horns; and in others beaks inſtead of either. And all this exactly as is daily ſeen in the tranſmutations of the tadpole, which acquires legs and lungs, when he wants them; and loſes his tail, when it is no longer of ſer⯑vice to him.

Fifthly, from their firſt rudiment, or primordium, to the termi⯑nation of their lives, all animals undergo perpetual transformations; [503] which are in part produced by their own exertions in conſequence of their deſires and averſions, of their pleaſures and their pains, or of ir⯑ritations, or of aſſociations; and many of theſe acquired forms or pro⯑penſities are tranſmitted to their poſterity. See Sect. XXXI. 1.

As air and water are ſupplied to animals in ſufficient profuſion, the three great objects of deſire, which have changed the forms of many animals by their exertions to gratify them, are thoſe of luſt, hunger, and ſecurity. A great want of one part of the animal world has con⯑ſiſted in the deſire of the excluſive poſſeſſion of the females; and theſe have acquired weapons to combat each other for this purpoſe, as the very thick, ſhield-like, horny ſkin on the ſhoulder of the boar is a de⯑fence only againſt animals of his own ſpecies, who ſtrike obliquely upwards, nor are his tuſhes for other purpoſes, except to defend him⯑ſelf, as he is not naturally a carnivorous animal. So the horns of the ſtag are ſharp to offend his adverſary, but are branched for the pur⯑poſe of parrying or receiving the thruſts of horns ſimilar to his own, and have therefore been formed for the purpoſe of combating other ſtags for the excluſive poſſeſſion of the females; who are obſerved, like the ladies in the times of chivalry, to attend the car of the victor.

The birds, which do not carry food to their young, and do not therefore marry, are armed with ſpurs for the purpoſe of fighting for the excluſive poſſeſſion of the females, as cocks and quails. It is cer⯑tain that theſe weapons are not provided for their defence againſt other adverſaries, becauſe the females of theſe ſpecies are without this ar⯑mour. The final cauſe of this conteſt amongſt the males ſeems to be, that the ſtrongeſt and moſt active animal ſhould propagate the ſpecies, which ſhould thence become improved.

Another great want conſiſts in the means of procuring food, which has diverſified the forms of all ſpecies of animals. Thus the noſe of the ſwine has become hard for the purpoſe of turning up the ſoil in [504] ſearch of inſects and of roots. The trunk of the elephant is an elonga⯑tion of the noſe for the purpoſe of pulling down the branches of trees for his food, and for taking up water without bending his knees. Beaſts of prey have acquired ſtrong jaws or talons. Cattle have ac⯑quired a rough tongue and a rough palate to pull off the blades of graſs, as cows and ſheep. Some birds have acquired harder beaks to crack nuts, as the parrot. Others have acquired beaks adapted to break the harder ſeeds, as ſparrows. Others for the ſofter ſeeds of flowers, or the buds of trees, as the finches. Other birds have acquired long beaks to penetrate the moiſter ſoils in ſearch of inſects or roots, as woodcocks; and others broad ones to filtrate the water of lakes, and to retain aquatic inſects. All which ſeem to have been gradually pro⯑duced during many generations by the perpetual endeavour of the crea⯑tures to ſupply the want of food, and to have been delivered to their poſterity with conſtant improvement of them for the purpoſes re⯑quired.

The third great want amongſt animals is that of ſecurity, which ſeems much to have diverſified the forms of their bodies and the colour of them; theſe conſiſt in the means of eſcaping other animals more powerful than themſelves. Hence ſome animals have acquired wings inſtead of legs, as the ſmaller birds, for the purpoſe of eſcape. Others great length of fin, or of membrane, as the flying fiſh, and the bat. Others great ſwiftneſs of foot, as the hare. Others have acquired hard or armed ſhells, as the tortoiſe and the echinus marinus.

The contrivances for the purpoſes of ſecurity extend even to ve⯑getables, as is ſeen in the wonderful and various means of their con⯑cealing or defending their honey from inſects, and their ſeeds from birds. On the other hand ſwiftneſs of wing has been acquired by hawks and ſwallows to purſue their prey; and a proboſcis of admirable ſtructure has been acquired by the bee, the moth, and the humming bird, for the purpoſe of plundering the nectaries of flowers. All [505] which ſeem to have been formed by the original living filament, ex⯑cited into action by the neceſſities of the creatures, which poſſeſs them, and on which their exiſtence depends.

From thus meditating on the great ſimilarity of the ſtructure of the warm-blooded animals, and at the ſame time of the great changes they undergo both before and after their nativity; and by conſidering in how minute a portion of time many of the changes of animals above deſcribed have been produced; would it be too bold to imagine, that in the great length of time, ſince the earth began to exiſt, perhaps millions of ages before the commencement of the hiſtory of mankind, would it be too bold to imagine, that all warm-blooded animals have ariſen from one living filament, which THE GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, at⯑tended with new propenſities, directed by irritations, ſenſations, vo⯑litions, and aſſociations; and thus poſſeſſing the faculty of continuing to improve by its own inherent activity, and of delivering down thoſe improvements by generation to its poſterity, world without end!

Sixthly, The cold-blooded animals, as the fiſh-tribes, which are furniſhed with but one ventricle of the heart, and with gills inſtead of lungs, and with fins inſtead of feet or wings, bear a great ſimila⯑rity to each other; but they differ, nevertheleſs, ſo much in their general ſtructure from the warm-blooded animals, that it may not ſeem probable at firſt view, that the ſame living filament could have given origin to this kingdom of animals, as to the former. Yet are there ſome creatures, which unite or partake of both theſe orders of animation, as the whales and ſeals; and more particularly the frog, who changes from an aquatic animal furniſhed with gills to an aerial one furniſhed with lungs.

The numerous tribes of inſects without wings, from the ſpider to the ſcorpion, from the flea to the lobſter; or with wings, from the gnat and the ant to the waſp and the dragon-fly, differ ſo totally from [506] each other, and from the red-blooded claſſes above deſcribed, both in the forms of their bodies, and their modes of life; beſides the organ of ſenſe, which they ſeem to poſſeſs in their antennae or horns, to which it has been thought by ſome naturaliſts, that other creatures have nothing ſimilar; that it can ſcarcely be ſuppoſed that this na⯑tion of animals could have been produced by the ſame kind of living filament, as the red-blooded claſſes above mentioned. And yet the changes which many of them undergo in their early ſtate to that of their maturity, are as different, as one animal can be from another. As thoſe of the gnat, which paſſes his early ſtate in water, and then ſtretching out his new wings, and expanding his new lungs, riſes in the air; as of the caterpillar, and bee-nymph, which feed on ve⯑getable leaves or farina, and at length burſting from their ſelf-formed graves, become beautiful winged inhabitants of the ſkies, journey⯑ing from flower to flower, and nouriſhed by the ambroſial food of honey.

There is ſtill another claſs of animals, which are termed vermes by Linnaeus, which are without feet, or brain, and are hermaphro⯑dites, as worms, leeches, ſnails, ſhell-fiſh, coralline inſects, and ſponges; which poſſeſs the ſimpleſt ſtructure of all animals, and ap⯑pear totally different from thoſe already deſcribed. The ſimplicity of their ſtructure, however, can afford no argument againſt their having been produced from a living filament as above contended.

Laſt of all the various tribes of vegetables are to be enumerated amongſt the inferior orders of animals. Of theſe the anthers and ſtigmas have already been ſhewn to poſſeſs ſome organs of ſenſe, to be nouriſhed by honey, and to have the power of generation like inſects, and have thence been announced amongſt the animal kingdom in Sect. XIII. and to theſe muſt be added the buds and bulbs which con⯑ſtitute the viviparous offspring of vegetation. The former I ſuppoſe to be beholden to a ſingle living filament for their ſeminal or amato⯑rial procreation; and the latter to the ſame cauſe for their lateral or [507] branching generation, which they poſſeſs in common with the poly⯑pus, taenia, and volvox; and the ſimplicity of which is an argument in favour of the ſimilarity of its cauſe.

Linnaeus ſuppoſes, in the Introduction to his Natural Orders, that very few vegetables were at firſt created, and that their numbers were increaſed by their intermarriages, and adds, ſuadent haec Crea⯑toris leges a ſimplicibus ad compoſita. Many other changes ſeem to have ariſen in them by their perpetual conteſt for light and air above ground, and for food or moiſture beneath the ſoil. As noted in Bo⯑tanic Garden, Part II. Note on Cuſcuta. Other changes of vege⯑tables from climate, or other cauſes, are remarked in the Note on Curcuma in the ſame work. From theſe one might be led to ima⯑gine, that each plant at firſt conſiſted of a ſingle bulb or flower to each root, as the gentianella and daiſy; and that in the conteſt for air and light new buds grew on the old decaying flower ſtem, ſhooting down their elongated roots to the ground, and that in proceſs of ages tall trees were thus formed, and an individual bulb became a ſwarm of vegetables. Other plants, which in this conteſt for light and air were too ſlender to riſe by their own ſtrength, learned by degrees to adhere to their neighbours, either by putting forth roots like the ivy, or by tendrils like the vine, or by ſpiral contortions like the honey⯑ſuckle; or by growing upon them like the miſleto, and taking nou⯑riſhment from their barks; or by only lodging or adhering on them, and deriving nouriſhment from the air, as tillandſia.

Shall we then ſay that the vegetable living filament was originally different from that of each tribe of animals above deſcribed? And that the productive living filament of each of thoſe tribes was different ori⯑ginally from the other? Or, as the earth and ocean were probably peopled with vegetable productions long before the exiſtence of ani⯑mals; and many families of theſe animals long before other families of them, ſhall we conjecture, that one and the ſame kind of living filaments is and has been the cauſe of all organic life?

[508] This idea of the gradual formation and improvement of the animal world accords with the obſervations of ſome modern philoſophers, who have ſuppoſed that the continent of America has been raiſed out of the ocean at a later period of time than the other three quarters of the globe, which they deduce from the greater comparative heights of its mountains, and the conſequent greater coldneſs of its reſpective climates, and from the leſs ſize and ſtrength of its animals, as the tygers and allegators compared with thoſe of Aſia or Africa. And laſtly, from the leſs progreſs in the improvements of the mind of its inhabitants in reſpect to voluntary exertions.

This idea of the gradual formation and improvement of the animal world ſeems not to have been unknown to the ancient philoſophers. Plato having probably obſerved the reciprocal generation of inferior animals, as ſnails and worms, was of opinion, that mankind with all other animals were originally hermaphrodites during the infancy of the world, and were in proceſs of time ſeparated into male and female. The breaſts and teats of all male quadrupeds, to which no uſe can be now aſſigned, adds perhaps ſome ſhadow of probability to this opi⯑nion. Linnaeus excepts the horſe from the male quadrupeds, who have teats; which might have ſhewn the earlier origin of his ex⯑iſtence; but Mr. T. Hunter aſſerts, that he has diſcovered the veſ⯑tiges of them on his ſheath, and has at the ſame time enriched natural hiſtory with a very curious fact concerning the male pidgeon; at the time of hatching the eggs both the male and female pidgeon undergo a great change in their crops; which thicken and become corrugated, and ſecrete a kind of milky fluid, which coagulates, and with which alone they for a few days feed their young, and afterwards feed them with this coagulated fluid mixed with other food. How this reſem⯑bles the breaſts of female quadrupeds after the production of their young! and how extraordinary, that the male ſhould at this time give milk as well as the female! See Botanic Garden, Part II. Note on Curcuma.

[509] The late Mr. David Hume, in his poſthumous works, places the powers of generation much above thoſe of our boaſted reaſon; and adds, that reaſon can only make a machine, as a clock or a ſhip, but the power of generation makes the maker of the machine; and pro⯑bably from having obſerved, that the greateſt part of the earth has been formed out of organic recrements; as the immenſe beds of lime⯑ſtone, chalk, marble, from the ſhells of fiſh; and the extenſive pro⯑vinces of clay, ſandſtone, ironſtone, coals, from decompoſed ve⯑getables; all which have been firſt produced by generation, or by the ſecretions of organic life; he concludes, that the world itſelf might have been generated, rather than created; that is, it might have been gradually produced from very ſmall beginnings, increaſing by the ac⯑tivity of its inherent principles, rather than by a ſudden evolution of the whole by the Almighty ſiat.—What a magnificent idea of the in⯑finite power of THE GREAT ARCHITECT! THE CAUSE OF CAUSES! PARENT OF PARENTS! ENS ENTIUM!

For if we may compare infinities, it would ſeem to require a greater infinity of power to cauſe the cauſes of effects, than to cauſe the effects themſelves. This idea is analogous to the improving ex⯑cellence obſervable in every part of the creation; ſuch as in the pro⯑greſſive increaſe of the ſolid or habitable parts of the earth from water; and in the progreſſive increaſe of the wiſdom and happineſs of its in⯑habitants; and is conſonant to the idea of our preſent ſituation being a ſtate of probation, which by our exertions we may improve, and are conſequently reſponſible for our actions.

V. 1. The efficient cauſe of the various colours of the eggs of birds, and of the hair and feathers of animals, is a ſubject ſo curious, that I ſhall beg to introduce it in this place. The colours of many animals ſeem adapted to their purpoſes of concealing themſelves either to avoid danger, or to ſpring upon their prey. Thus the ſnake and wild cat, and leopard, are ſo coloured as to reſemble dark leaves and their lighter interſtices; birds reſemble the colour of the brown [510] ground, or the green hedges, which they frequent; and moths and butterflies are coloured like the flowers which they rob of their honey. Many inſtances are mentioned of this kind in Botanic Garden, p. 2. Note on Rubia.

Theſe colours have, however, in ſome inſtances another uſe, as the black diverging area from the eyes of the ſwan; which, as his eyes are placed leſs prominent than thoſe of other birds, for the con⯑venience of putting down his head under water, prevents the rays of light from being reflected into his eye, and thus dazzling his ſight, both in air and beneath the water; which muſt have happened, if that ſurface had been white like the reſt of his feathers.

There is a ſtill more wonderful thing concerning theſe colours adapted to the purpoſe of concealment; which is, that the eggs of birds are ſo coloured as to reſemble the colour of the adjacent objects and their interſtices. The eggs of hedge-birds are greeniſh with dark ſpots; thoſe of crows and magpies, which are ſeen from beneath through wicker neſts, are white with dark ſpots; and thoſe of larks and partridges are ruſſet or brown, like their neſts or ſituations.

A thing ſtill more aſtoniſhing is, that many animals in countries covered with ſnow become white in winter, and are ſaid to change their colour again in the warmer months, as bears, hares, and par⯑tridges. Our domeſticated animals loſe their natural colours, and break into great variety, as horſes, dogs, pidgeons. The final cauſe of theſe colours is eaſily underſtood, as they ſerve ſome purpoſes of the animal, but the efficient cauſe would ſeem almoſt beyond con⯑jecture.

Firſt, the choroid coat of the eye, on which the ſemitranſparent retina is expanded, is of different colour in different animals; in thoſe which feed on graſs it is green; from hence there would appear ſome connexion between the colour of the choroid coat and of that con⯑ſtantly painted on the retina by the green graſs. Now, when the ground becomes covered with ſnow, it would ſeem, that that action [511] of the retina, which is called whiteneſs, being conſtantly excited in the eye, may be gradually imitated by the extremities of the nerves of touch, or rete mucoſum of the ſkin. And if it be ſuppoſed, that the action of the retina in producing the perception of any colour con⯑ſiſts in ſo diſpoſing its own fibres or ſurface, as to reflect thoſe colour⯑ed rays only, and tranſmit the others like ſoap-bubbles; then that part of the retina, which gives us the perception of ſnow, muſt at that time be white; and that which gives us the perception of graſs, muſt be green.

Then if by the laws of imitation, as explained in Section XII. 33. and XXXIX. 6. the extremities of the nerves of touch in the rete mucoſum be induced into ſimilar action, the ſkin or feathers, or hair, may in like manner ſo diſpoſe their extreme fibres, as to reflect white; for it is evident, that all theſe parts were originally obedient to irrita⯑tive motions during their growth, and probably continue to be ſo; that thoſe irritative motions are not liable in a healthy ſtate to be ſuc⯑ceeded by ſenſation; which however is no uncommon thing in their diſeaſed ſtate, or in their infant ſtate, as in plica polonica, and in very young pen-feathers, which are ſtill full of blood.

It was ſhewn in Section XV. on the Production of Ideas, that the moving organ of ſenſe in ſome circumſtances reſembled the object which produced that motion. Hence it may be conceived, that the rete mucoſum, which is the extremity of the nerves of touch, may by imitating the motions of the retina become coloured. And thus, like the fable of the camelion, all animals may poſſeſs a tendency to be coloured ſomewhat like the colours they moſt frequently inſpect, and finally, that colours may be thus given to the egg-ſhell by the imagination of the female parent; which ſhell is previouſly a mucous membrane, indued with irritability, without which it could not cir⯑culate its fluids, and increaſe in its bulk. Nor is this more wonder⯑ful than that a ſingle idea of imagination ſhould in an inſtant colour the whole ſurface of the body of a bright ſcarlet, as in the bluſh of [512] ſhame, though by a very different proceſs. In this intricate ſubject nothing but looſe analogical conjectures can be had, which may how⯑ever lead to future diſcoveries; but certain it is that both the change of the colour of animals to white in the winters of ſnowy coun⯑tries, and the ſpots on birds eggs, muſt have ſome efficient cauſe; ſince the uniformity of their production ſhews it cannot ariſe from a fortuitous concurrence of circumſtances; and how is this efficient cauſe to be detected, or explained, but from its analogy to other ani⯑mal facts?

2. The nutriment ſupplied by the female parent in viviparous ani⯑mals to their young progeny may be divided into three kinds, corre⯑ſponding with the age of the new creature. 1. The nutriment con⯑tained in the ovum as previouſly prepared for the embryon in the ovary. 2. The liquor amnii prepared for the fetus in the uterus, and in which it ſwims; and laſtly, the milk prepared in the pectoral glands for the new-born child. There is reaſon to conclude that variety of changes may be produced in the new animal from all theſe ſources of nutriment, but particularly from the firſt of them.

The organs of digeſtion and of ſanguification in adults, and after⯑wards thoſe of ſecretion, prepare or ſeparate the particles proper for nouriſhment from other combinations of matter, or recombine them into new kinds of matter, proper to excite into action the filaments, which abſorb or attract them by animal appetency. In this proceſs we muſt attend not only to the action of the living filament which receives a nutritive particle to its boſom, but alſo to the kind of par⯑ticle, in reſpect to form, or ſize, or colour, or hardneſs, which is thus previouſly prepared for it by digeſtion, ſanguification, and ſecre⯑tion. Now as the firſt filament of entity cannot be furniſhed with the preparative organs above mentioned, the nutritive particles, which are at firſt to be received by it, are prepared by the mother; and de⯑poſited in the ovum ready for its reception. Theſe nutritive particles muſt be ſuppoſed to differ in ſome reſpects, when thus prepared by different animals. They may differ in ſize, ſolidity, colour, and form; [513] and yet may be ſufficiently congenial to the living filament, to which they are applied, as to excite its activity by their ſtimulus, and its ani⯑mal appetency to receive them, and to combine them with itſelf into organization.

By this firſt nutriment thus prepared for the embryon is not meant the liquor amnii, which is produced afterwards, nor the larger exte⯑rior parts of the white of the egg; but the fluid prepared, I ſuppoſe, in the ovary of viviparous animals, and that which immediately ſur⯑rounds the cicatricula of an impregnated egg, and is viſible to the eye in a boiled one.

Now theſe ultimate particles of animal matter prepared by the glands of the mother may be ſuppoſed to reſemble the ſimilar ultimate particles, which were prepared for her own nouriſhment; that is, to the ultimate particles of which her own organization conſiſts. And that hence when theſe become combined with a new embryon, which in its early ſtate is not furniſhed with ſtomach, or glands, to alter them; that new embryon will bear ſome reſemblance to the mother.

This ſeems to be the origin of the compound forms of mules, which evidently partake of both parents, but principally of the male parent. In this production of chimeras the antients ſeem to have indulged their fancies, whence the ſphinxes, griffins, dragons, centaurs, and minotaurs, which are vaniſhed from modern cre⯑dulity.

It would ſeem, that in theſe unnatural conjunctions, when the nutriment depoſited by the female was ſo ill adapted to ſtimulate the living filament derived from the male into action, and to be received, or embraced by it, and combined with it into organization, as not to produce the organs neceſſary to life, as the brain, or heart, or ſto⯑mach, that no mule was produced. Where all the parts neceſſary to life in theſe compound animals were formed ſufficiently perfect, ex⯑cept [514] the parts of generation, thoſe animals were produced, which are now called mules.

The formation of the organs of ſexual generation, in contradiſtinc⯑tion to that by lateral buds, in vegetables, and in ſome animals, as the polypus, the taenia, and the volvox, ſeems the chef d'oeuvre, the maſter-piece of nature; as appears from many flying inſects, as in moths and butterflies, who ſeem to undergo a general change of their forms ſolely for the purpoſe of ſexual reproduction, and in all other animals this organ is not complete till the maturity of the creature. Whence it happens that, in the copulation of animals of different ſpecies, the parts neceſſary to life are frequently completely formed; but thoſe for the purpoſe of generation are defective, as requiring a nicer organization; or more exact coincidence of the particles of nu⯑triment to the irritabilities or appetencies of the original living fila⯑ment. Whereas thoſe mules, where all the parts could be perfectly formed, may have been produced in early periods of time, and may have added to the numbers of our various ſpecies of animals, as before obſerved.

As this production of mules is a conſtant effect from the conjunc⯑tion of different ſpecies of animals, thoſe between the horſe and the female aſs always reſembling the horſe more than the aſs; and thoſe, on the contrary, between the male aſs and the mare, always reſem⯑bling the aſs more than the mare; it cannot be aſcribed to the ima⯑gination of the male animal which cannot be ſuppoſed to operate ſo uniformly; but to the form of the firſt nutritive particles, and to their peculiar ſtimulus exciting the living filament to ſelect and com⯑bine them with itſelf. There is a ſimilar uniformity of effect in re⯑ſpect to the colour of the progeny produced between a white man, and a black woman, which, if I am well informed, is always of the mulatto kind, or a mixture of the two; which may perhaps be im⯑puted to the peculiar form of the particles of nutriment ſupplied to the embryon by the mother at the early period of its exiſtence, and their [515] peculiar ſtimulus; as this effect, like that of the mule progeny above treated of, is uniform and conſiſtent, and cannot therefore be aſcribed to the imagination of either of the parents.

When the embryon has produced a placenta, and furniſhed itſelf with veſſels for ſelection of nutritious particles, and for oxygenation of them, no great change in its form or colour is likely to be pro⯑duced by the particles of ſuſtenance it now takes from the fluid, in which it is immerſed; becauſe it has now acquired organs to alter or new combine them. Hence it continues to grow, whether this fluid, in which it ſwims, be formed by the uterus or by any other cavity of the body, as in extra-uterine geſtation; and which would ſeem to be produced by the ſtimulus of the fetus on the ſides of the cavity, where it is found, as mentioned before. And thirdly, there is ſtill leſs reaſon to expect any unnatural change to happen to the child after its birth from the difference of the milk it now takes; becauſe it has acquired a ſtomach, and lungs, and glands, of ſufficient power to decompoſe and recombine the milk; and thus to prepare from it the various kinds of nutritious particles, which the appetencies of the va⯑rious fibrils or nerves may require.

From all this reaſoning I would conclude, that though the imagi⯑nation of the female may be ſuppoſed to affect the embryon by pro⯑ducing a difference in its early nutriment; yet that no ſuch power can effect it after it has obtained a placenta, and other organs; which may ſelect or change the food, which is preſented to it either in the liquor amnii, or in the milk. Now as the eggs in pullets, like the ſeeds in vegetables, are produced gradually, long before they are im⯑pregnated, it does not appear how any ſudden effect of imagination of the mother at the time of impregnation can produce any conſiderable change in the nutriment already thus laid up for the expected or de⯑ſired embryon. And that hence any changes of the embryon, except thoſe uniform ones in the production of mules and mulattoes, more probably depend on the imagination of the male parent. At the ſame [516] time it ſeems manifeſt, that thoſe monſtrous births, which conſiſt in ſome deficiencies only, or ſome redundancies of parts, originate from the deficiency or redundance of the firſt nutriment prepared in the ovary, or in the part of the egg immediately ſurrounding the cicatri⯑cula, as deſcribed above; and which continues ſome time to excite the firſt living filament into action, after the ſimple animal is completed; or ceaſes to excite it, before the complete form is accompliſhed. The former of theſe circumſtances is evinced by the eggs with double yolks, which frequently happen to our domeſticated poultry, and which, I believe, are ſo formed before impregnation, but which would be well worth attending to, both before and after impregnation; as it is probable, ſomething valuable on this ſubject might be learnt from them. The latter circumſtance, or that of deficiency of original nu⯑triment, may be deduced from reverſe analogy.

There are, however, other kinds of monſtrous births, which neither depend on deficiency of parts, nor ſupernumerary ones; nor are owing to the conjunction of animals of different ſpecies; but which appear to be new conformations, or new diſpoſitions of parts in reſpect to each other, and which, like the variation of colours and forms of our domeſticated animals, and probably the ſexual parts of all animals, may depend on the imagination of the male parent, which we now come to conſider.

VI. 1. The nice actions of the extremities of our various glands are exhibited in their various productions, which are believed to be made by the gland, and not previouſly to exiſt as ſuch in the blood. Thus the glands, which conſtitute the liver, make bile; thoſe of the ſtomach make gaſtric acid; thoſe beneath the jaw, ſaliva; thoſe of the ears, ear-wax; and the like. Every kind of gland muſt poſſeſs a peculiar irritability, and probably a ſenſibility, at the early ſtate of its exiſtence; and muſt be furniſhed with a nerve of ſenſe, or of motion, to perceive, and to ſelect, and to combine the particles, which com⯑poſe the fluid it ſecretes. And this nerve of ſenſe which perceives [517] the different articles which compoſe the blood, muſt at leaſt be con⯑ceived to be as fine and ſubtile an organ, as the optic or auditory nerve, which perceive light or ſound. See Sect. XIV. 9.

But in nothing is this nice action of the extremities of the blood⯑veſſels ſo wonderful, as in the production of contagious matter. A ſmall drop of variolous contagion diffuſed in the blood, or perhaps only by being inſerted beneath the cuticle, after a time, (as about a quar⯑ter of a lunation,) excites the extreme veſſels of the ſkin into certain motions, which produce a ſimilar contagious material, filling with it a thouſand puſtules. So that by irritation, or by ſenſation in conſe⯑quence of irritation, or by aſſociation of motions, a material is formed by the extremities of certain cutaneous veſſels, exactly ſimilar to the ſtimulating material, which cauſed the irritation, or conſequent ſen⯑ſation, or aſſociation.

Many glands of the body have their motions, and in conſequence their ſecreted fluids, affected by pleaſurable or painful ideas, ſince they are in many inſtances influenced by ſenſitive aſſociations, as well as by the irritations of the particles of the paſſing blood. Thus the idea of meat, excited in the minds of hungry dogs, by their ſenſe of viſion, or of ſmell, increaſes the diſcharge of ſaliva, both in quantity and viſ⯑cidity; as is ſeen in its hanging down in threads from their mouths, as they ſtand round a dinner-table. The ſenſations of pleaſure, or of pain, of peculiar kinds, excite in the ſame manner a great diſcharge of tears; which appear alſo to be more ſaline at the time of their ſecre⯑tion, from their inflaming the eyes and eye-lids. The paleneſs from fear, and the bluſh of ſhame, and of joy, are other inſtances of the effects of painful, or pleaſurable ſenſations, on the extremities of the arterial ſyſtem.

It is probable, that the pleaſurable ſenſation excited in the ſtomach by food, as well as its irritation, contributes to excite into action the gaſtric glands, and to produce a greater ſecretion of their fluids. The ſame probably occurs in the ſecretion of bile; that is, that the pleaſur⯑able [518] ſenſation excited in the ſtomach, affects this ſecretion by ſenſitive aſſociation, as well as by irritative aſſociation.

And laſtly it would ſeem, that all the glands in the body have their ſecreted fluids affected, in quantity and quality, by the pleaſurable or painful ſenſations, which produce or accompany thoſe ſecretions. And that the pleaſurable ſentations ariſing from theſe ſecretions may conſtitute the unnamed pleaſure of exiſtence, which is contrary to what is meant by taedium vitae, or ennui; and by which we ſome⯑times feel ourſelves happy, without being able to aſcribe it to any mental cauſe, as after an agreeable meal, or in the beginning of in⯑toxication.

Now it would appear, that no ſecretion or excretion of fluid is at⯑tended with ſo much agreeable ſenſation, as that of the ſemen; and it would thence follow, that the glands, which perform this ſecretion, are more likely to be much affected by their catenations with pleaſur⯑able ſenſations. This circumſtance is certain, that much more of this fluid is produced in a given time, when the object of its excluſion is agreeable to the mind.

2. A forceable argument, which ſhews the neceſſity of pleaſurable ſenſation to copulation, is, that the act cannot be performed without it; it is eaſily interrupted by the pain of fear or baſhfulneſs; and no efforts of volition or of irritation can effect this proceſs, except ſuch as induce pleaſurable ideas or ſenſations. See Sect. XXXIII. 1. 1.

A curious analogical circumſtance attending hermaphrodite inſects, as ſnails and worms, ſtill further illuſtrates this theory; if the ſnail or worm could have impregnated itſelf, there might have been a ſaving of a large male apparatus; but as this is not ſo ordered by nature, but each ſnail and worm reciprocally receives and gives impregnation, it appears, that a pleaſurable excitation ſeems alſo to have been required.

This wonderful circumſtance of many inſects being hermaphro⯑dites, and at the ſame time not having power to impregnate them⯑ſelves, is attended to by Dr. Liſter, in his Exercitationes Anatom. de [519] Limacibus, p. 145; who, amongſt many other final cauſes, which he adduces to account for it, adds, ut tam triſtibus et frigidis animalibus majori cum voluptate perſiciatur venus.

There is, however, another final cauſe, to which this circumſtance may be imputed: it was obſerved above, that vegetable buds and bulbs, which are produced without a mother, are always exact re⯑ſemblances of their parent; as appears in grafting fruit-trees, and in the flower-buds of the dioiceous plants, which are always of the ſame ſex on the ſame tree; hence thoſe hermaphrodite inſects, if they could have produced young without a mother, would not have been capable of that change or improvement, which is ſeen in all other animals, and in thoſe vegetables, which are procreated by the male embryon received and nouriſhed by the female. And it is hence pro⯑bable, that if vegetables could only have been produced by buds and bulbs, and not by ſexual generation, that there would not at this time have exiſted one thouſandth part of their preſent number of ſpecies; which have probably been originally mule-productions; nor could any kind of improvement or change have happened to them, except by the difference of ſoil or climate.

3. I conclude, that the imagination of the male at the time of co⯑pulation, or at the time of the ſecretion of the ſemen, may ſo affect this ſecretion by irritative or ſenſitive aſſociation, as deſcribed in No. 5. 1. of this ſection, as to cauſe the production of ſimilarity of form and of features, with the diſtinction of ſex; as the motions of the chiſ⯑fel of the turner imitate or correſpond with thoſe of the ideas of the artiſt. It is not here to be underſtood, that the firſt living fibre, which is to form an animal, is produced with any ſimilarity of form to the future animal; but with propenſities, or appetencies, which ſhall produce by accretion of parts the ſimilarity of form, feature, or ſex, correſponding to the imagination of the father.

Our ideas are movements of the nerves of ſenſe, as of the optic nerve in recollecting viſible ideas, ſuppoſe of a triangular piece of [520] ivory. The fine moving fibres of the retina act in a manner to which I give the name of white; and this action is confined to a deſined part of it; to which figure I give the name of triangle. And it is a pre⯑ceding pleaſurable ſenſation exiſting in my mind, which occaſions me to produce this particular motion of the retina, when no triangle is preſent. Now it is probable, that the acting fibres of the ultimate terminations of the ſecreting apertures of the veſſels of the teſtes, are as fine as thoſe of the retina; and that they are liable to be thrown into that peculiar action, which marks the ſex of the ſecreted em⯑bryon, by ſympathy with the pleaſurable motions of the nerves of viſion, or of touch; that is, with certain ideas of imagination. From hence it would appear, that the world has long been miſtaken in aſcribing great power to the imagination of the female, whereas from this account of it, the real power of imagination, in the act of gene⯑ration, belongs ſolely to the male. See Sect. XII. 3. 3.

It may be objected to this theory, that a man may be ſuppoſed to have in his mind, the idea of the form and features of the female, rather than his own, and therefore there ſhould be a greater number of female births. On the contrary, the general idea of our own form occurs to every one almoſt perpetually, and is termed conſciouſneſs of our exiſtence, and thus may effect, that the number of males ſurpaſſes that of females. See Sect. XV. 3. 4. and XVIII. 13. And what further confirms this idea is, that the male children moſt frequently reſemble the father in form, or feature, as well as in ſex; and the fe⯑male moſt frequently reſemble the mother, in feature, and form, as well as in ſex.

It may again be objected, if a female child ſometimes reſembles the father, and a male child the mother, the ideas of the father, at the time of procreation, muſt ſuddenly change from himſelf to the mo⯑ther, at the very inſtant, when the embryon is ſecreted or formed. This difficulty ceaſes when we conſider, that it is as eaſy to form an idea of feminine features with male organs of reproduction, or of male [521] features with female ones, as the contrary; as we conceive the idea of a ſphinx or mermaid, as eaſily and as diſtinctly as of a woman. Add to this, that at the time of procreation the idea of the male or⯑gans, and of the female features, are often both excited at the ſame time, by contact, or by viſion.

I aſk, in my turn, is the ſex of the embryon produced by accident? Certainly whatever is produced has a cauſe; but when this cauſe is too minute for our comprehenſion, the effect is ſaid in common lan⯑guage to happen by chance, as in throwing a certain number on dice. Now that cauſe can occaſionally produce the male or female charac⯑ter of the embryon, but the peculiar actions of thoſe glands, which form the embryon? And what can influence or govern theſe actions of the gland, but its aſſociations or catenations with other ſenſitive motions? Nor is this more extraordinary, than that the catenations of irritative motions with the apparent vibrations of objects at ſea ſhould produce ſickneſs of the ſtomach; or that a nauſeous ſtory ſhould occaſion vomiting.

4. An argument, which evinces the effect of imagination on the firſt rudiment of the embryon, may be deduced from the production of ſome peculiar monſters. Such, for inſtance, as thoſe which have two heads joined to one body, and thoſe which have two bodies joined to one head; of which frequent examples occur amongſt our domeſti⯑cated quadrupeds, and poultry. It is abſurd to ſuppoſe, that ſuch forms could exiſt in primordial germs, as explained in No. IV. 4. of this ſection. Nor is it poſſible, that ſuch deformities could be pro⯑duced by the growth of two embryons, or living filaments; which ſhould afterwards adhere together; as the head and tail part of differ⯑ent polypi are ſaid to do (Blumenbach on generation, Cadel, Lon⯑don); ſince in that caſe one embryon, or living ſilament, muſt have begun to form one part firſt, and the other another part firſt. But ſuch monſtrous conformations become leſs difficult to comprehend, when they are conſidered as an effect of the imagination, as before [522] explained, on the living filament at the time of its ſecretion; and that ſuch duplicature of limbs were produced by accretion of new parts, in conſequence of propenſities, or animal appetencies thus acquired from the male parent.

For inſtance, I can conceive, if a turkey-cock ſhould behold a rab⯑bit, or a frog, at the time of procreation, that it might happen, that a forcible or even a pleaſurable idea of the form of a quadruped might ſo occupy his imagination, as to cauſe a tendency in the naſcent fila⯑ment to reſemble ſuch a form, by the appoſition of a duplicature of limbs. Experiments on the production of mules and monſters would be worthy the attention of a Spallanzani, and might throw much light upon this ſubject, which at preſent muſt be explained by con⯑jectural analogies.

The wonderful effect of imagination, both in the male and female parent, is ſhewn in the production of a kind of milk in the crops both of the male and female pigeons after the birth of their young, as obſerved by Mr. Hunter, and mentioned before. To this ſhould be added, that there are ſome inſtances of men having had milk ſe⯑creted in their breaſts, and who have given ſuck to children, as re⯑corded by Mr. Buffon. This effect of imagination, of both the male and female parent, ſeems to have been attended to in very early times; Jacob is ſaid not only to have placed rods of trees, in part ſtripped of their bark, ſo as to appear ſpotted, but alſo to have placed ſpotted lambs before the flocks, at the time of their copulation. Geneſis, chap. xxx. verſe 40.

5. In reſpect to the imagination of the mother, it is difficult to comprehend, how this can produce any alteration in the fetus, ex⯑cept by aſſecting the nutriment laid up for its firſt reception, as de⯑ſcribed in No. V. 2. of this ſection, or by affecting the nouriſhment or oxygenation with which ſhe ſupplies it afterwards. Perpetual anxiety may probably affect the ſecretion of the liquor amnii into the uterus, as it enfeebles the whole ſyſtem; and ſudden ſear is a frequent [523] cauſe of miſcarriage; for fear, contrary to joy, decreaſes for a time the action of the extremities of the arterial ſyſtem; hence ſudden paleneſs ſucceeds, and a ſhrinking or contraction of the veſſels of the ſkin, and other membranes. By this circumſtance, I imagine, the terminations of the placental veſſels are detached from their adheſions, or inſertions, into the membrane of the uterus; and the death of the child ſucceeds, and conſequent miſcarriage.

Of this I recollect a remarkable inſtance, which could be aſcribed to no other cauſe, and which I ſhall therefore relate in few words. A healthy young woman, about twenty years of age, had been about five months pregnant, and going down into her cellar to draw ſome beer, was frighted by a ſervant-boy ſtarting up from behind the bar⯑rel, where he had concealed himſelf with deſign to alarm the maid⯑ſervant, for whom he miſtook his miſtreſs. She came with difficulty up ſtairs, began to flood immediately, and miſcarried in a few hours. She has ſince borne ſeveral children, nor ever had any tendency to miſcarry of any of them.

In reſpect to the power of the imagination of the male over the form, colour, and ſex of the progeny, the following inſtances have fallen under my obſervation, and may perhaps be found not very un⯑frequent, if they were more attended to. I am acquainted with a gentleman, who has one child with dark hair and eyes, though his lady and himſelf have light hair and eyes; and their other four children are like their parents. On obſerving this diſſimilarity of one child to the others he aſſured me, that he believed it was his own imagination, that produced the difference; and related to me the following ſtory. He ſaid, that when his lady lay in of her third child, he became attached to a daughter of one of his inferior tenants, and offered her a bribe for her favours in vain; and afterwards a greater bribe, and was equally unſucceſsful; that the form of this girl dwelt much in his mind for ſome weeks, and that the next child, which was the dark-ey'd young [524] lady above mentioned, was exceedingly like, in both features and colour, to the young woman who refuſed his addreſſes.

To this inſtance I muſt add, that I have known two families, in which, on account of an intailed eſtate in expectation, a male heir was moſt eagerly deſired by the father; and on the contrary, girls were produced to the ſeventh in one, and to the ninth in another; and then they had each of them a ſon. I conclude, that the great deſire of a male heir by the father produced rather a diſagreeable than an agreeable ſenſation; and that his ideas dwelt more on the fear of ge⯑nerating a female, than on the pleaſurable ſenſations or ideas of his own male form or organs at the time of copulation, or of the ſecretion of the ſemen; and that hence the idea of the female character was more preſent to his mind than that of the male one; till at length in deſpair of generating a male theſe ideas ceaſed, and thoſe of the male character preſided at the genial hour.

7. Hence I conclude, that the act of generation cannot exiſt with⯑out being accompanied with ideas, and that a man muſt have at that time either a general idea of his own male form, or of the form of his male organs; or an idea of the female form, or of her organs; and that this marks the ſex, and the peculiar reſemblances of the child to either parent. From whence it would appear, that the phalli, which were hung round the necks of the Roman ladies, or worn in their hair, might have effect in producing a greater proportion of male children; and that the calipaedia, or art of begetting beautiful children, and of procreating either males or females, may be taught by affecting the imagination of the male-parent; that is, by the fine extremities of the ſeminal glands imitating the actions of the organs of ſenſe either of ſight or touch. But the manner of accompliſhing this cannot be unfolded with ſufficient delicacy for the public eye; but may be worth the attention of thoſe, who are ſeriouſly intereſted in the procreation of a male or female child.

Recapitulation.

VII. 1. A certain quantity of nutritive particles are produced by the female parent before impregnation, which require no further di⯑geſtion, ſecretion, or oxygenation. Such are ſeen in the unimpreg⯑nated eggs of birds, and in the unimpregnated ſeed-veſſels of vege⯑tables.

2. A living filament is produced by the male, which being inſerted amidſt theſe firſt nutritive particles, is ſtimulated into action by them; and in conſequence of this action, ſome of the nutritive particles are embraced, and added to the original living filament; in the ſame manner as common nutrition is performed in the adult animal.

3. Then this new organization, or additional part, becomes ſtimu⯑lated by the nutritive particles in its vicinity, and ſenſation is now ſuperadded to irritation; and other particles are in conſequence em⯑braced, and added to the living filament; as is ſeen in the new gra⯑nulations of fleſh in ulcers.

By the power of aſſociation, or by irritation, the parts already produced continue their motions, and new ones are added by ſenſa⯑tion, as above mentioned; and laſtly by volition, which laſt ſenſorial power is proved to exiſt in the fetus in its maturer age, becauſe it has evidently periods of activity and of ſleeping; which laſt is another word for a temporary ſuſpenſion of volition.

The original living filament may be conceived to poſſeſs a power of repulſing the particles applied to certain parts of it, as well as of embracing others, which ſtimulate other parts of it; as theſe powers exiſt in different parts of the mature animal; thus the mouth of every gland embraces the particles of fluid, which ſuits its appetency; and its excretory duct repulſes thoſe particles, which are diſagreeable to it.

[526] 4. Thus the outline or miniature of the new animal is produced gradually, but in no great length of time; becauſe the original nu⯑tritive particles require no previous preparation by digeſtion, ſecre⯑tion, and oxygenation: but require ſimply the ſelection and appoſi⯑tion, which is performed by the living filament. Mr. Blumenbach ſays, that he poſſeſſes a human fetus of only five weeks old, which is the ſize of a common bee, and has all the features of the face, every finger, and every toe, complete; and in which the organs of generation are diſtinctly ſeen. P. 76. In another fetus, whoſe head was not larger than a pea, the whole of the baſis of the ſkull with all its depreſſions, apertures, and proceſſes, were marked in the moſt ſharp and diſtinct manner, though without any oſſification. Ib.

5. In ſome caſes by the nutriment originally depoſited by the mo⯑ther the filament acquires parts not exactly ſimilar to thoſe of the fa⯑ther, as in the production of mules and mulattoes. In other caſes, the deficiency of this original nutriment cauſes deficiencies of the ex⯑treme parts of the fetus, which are laſt formed, as the fingers, toes, lips. In other caſes, a duplicature of limbs are cauſed by the ſuper⯑abundance of this original nutritive fluid, as in the double yolks of eggs, and the chickens from them with four legs and four wings. But the production of other monſters, as thoſe with two heads, or with parts placed in wrong ſituations, ſeems to ariſe from the ima⯑gination of the father being in ſome manner imitated by the extreme veſſels of the ſeminal glands; as the colours of the ſpots on eggs, and the change of the colour of the hair and feathers of animals by do⯑meſtication, may be cauſed in the ſame manner by the imagination of the mother.

6. The living filament is a part of the father, and has therefore certain propenſities, or appetencies, which belong to him; which may have been gradually acquired during a million of generations, even from the infancy of the habitable earth; and which now poſ⯑ſeſſes ſuch properties, as would render, by the appoſition of nutritious [527] particles, the new fetus exactly ſimilar to the father; as oc⯑curs in the buds and bulbs of vegetables, and in the polypus, and taenia or tape-worm. But as the firſt nutriment is ſupplied by the mother, and therefore reſembles ſuch nutritive particles, as have been uſed for her own nutriment or growth, the progeny takes in part the likeneſs of the mother.

Other ſimilarity of the excitability, or of the form of the male parent, ſuch as the broad or narrow ſhoulders, or ſuch as conſtitute certain hereditary diſeaſes, as ſcrophula, epilepſy, inſanity, have their origin produced in one or perhaps two generations; as in the progeny of thoſe who drink much vinous ſpirits; and thoſe heredi⯑tary propenſities ceaſe again, as I have obſerved, if one or two ſober generations ſucceed; otherwiſe the family becomes extinct.

This living filament from the father is alſo liable to have its pro⯑penſities, or appetencies, altered at the time of its production by the imagination of the male parent; the extremities of the ſeminal glands imitating the motions of the organs of ſenſe; and thus the ſex of the embryon is produced; which may be thus made a male or a female by affecting the imagination of the father at the time of impregnation. See Sect. XXXIX. 6. 3. and 7.

7. After the fetus is thus completely formed together with its um⯑bilical veſſels and placenta, it is now ſupplied with a different kind of food, as appears by the difference of conſiſtency of the different parts of the white of the egg, and of the liquor amnii, for it has now acquired organs for digeſtion or ſecretion, and for oxygenation, though they are as yet feeble; which can in ſome degree change, as well as ſelect, the nutritive particles, which are now preſented to it. But may yet be affected by the deficiency of the quantity of nutri⯑tion ſupplied by the mother, or by the degree of oxygenation ſupplied to its placenta by the maternal blood.

The augmentation of the complete fetus by additional particles of nutriment is not accompliſhed by diſtention only, but by appoſition [528] to every part both external and internal; each of which acquires by animal appetencies the new addition of the particles which it wants. And hence the enlarged parts are kept ſimilar to their prototypes, and may be ſaid to be extended; but their extenſion muſt be conceived only as a neceſſary conſequence of the enlargement of all their parts by appoſition of new particles.

Hence the new appoſition of parts is not produced by capillary at⯑traction, becauſe the whole is extended; whereas capillary attraction would rather tend to bring the ſides of flexible tubes together, and not to diſtend them. Nor is it produced by chemical affinities, for then a ſolution of continuity would ſucceed, as when ſugar is diſ⯑ſolved in water; but it is produced by an animal proceſs, which is the conſequence of irritation, or ſenſation; and which may be termed animal appetency.

This is further evinced from experiments, which have been inſti⯑tuted to ſhew, that a living muſcle of an animal body requires greater force to break it, than a ſimilar muſcle of a dead body. Which evinces, that beſides the attraction of coheſion, which all matter poſſeſſes, and beſides the chemical attractions of affinities, which hold many bodies together, there is an animal adheſion, which adds vigour to theſe common laws of the inanimate world.

8. At the nativity of the child it depoſits the placenta or gills, and by expanding its lungs acquires more plentiful oxygenation from the currents of air, which it muſt now continue perpetually to re⯑ſpire to the end of its life; as it now quits the liquid element, in which it was produced, and like the tadpole, when it changes into a frog, becomes an aerial animal.

9. As the habitable parts of the earth have been, and continue to be, perpetually increaſing by the production of ſea-ſhells and coral⯑lines, and by the recrements of other animals, and vegetables; ſo from the beginning of the exiſtence of this terraqueous globe, the [529] animals, which inhabit it, have conſtantly improved, and are ſtill in a ſtate of progreſſive improvement.

This idea of the gradual generation of all things ſeems to have been as familiar to the ancient philoſophers as to the modern ones; and to have given riſe to the beautiful hieroglyphic figure of the [...], or firſt great egg, produced by NIGHT, that is, whoſe origin is involved in obſcurity, and animated by [...], that is, by DI⯑VINE LOVE; from whence proceeded all things which exiſt.

Concluſion.

VIII. 1. CAUSE AND EFFECT may be conſidered as the pro⯑greſſion, or ſucceſſive motions, of the parts of the great ſyſtem of Nature. The ſtate of things at this moment is the effect of the ſtate of things, which exiſted in the preceding moment; and the cauſe of the ſtate of things, which ſhall exiſt in the next moment.

Theſe cauſes and effects may be more eaſily comprehended, if mo⯑tion be conſidered as a change of the figure of a group of bodies, as propoſed in Sect. XIV. 2. 2. inaſmuch as our ideas of viſible or tan⯑gible objects are more diſtinct, than our abſtracted ideas of their mo⯑tions. Now the change of the configuration of the ſyſtem of nature at this moment muſt be an effect of the preceding configuration, for a change of configuration cannot exiſt without a previous configuration; and the proximate cauſe of every effect muſt immediately precede that effect. For example, a moving ivory ball could not proceed onwards, unleſs it had previouſly began to proceed; or unleſs an impulſe had been previouſly given it; which previous motion or impulſe conſtitutes a part of the laſt ſituation of things.

As the effects produced in this moment of time become cauſes in the next, we may conſider the progreſſive motions of objects as a [530] chain of cauſes only; whoſe firſt link proceeded from the great Cre⯑ator, and which have exiſted from the beginning of the created uni⯑verſe, and are perpetually proceeding.

2. Theſe cauſes may be conveniently divided into two kinds, ef⯑ficient and inert cauſes, according with the two kinds of entity ſup⯑poſed to exiſt in the natural world, which may be termed matter and ſpirit, as propoſed in Sect. I. and further treated of in Sect. XIV. The efficient cauſes of motion, or new configuration, conſiſt either of the principle of general gravitation, which actuates the ſun and planets; or of the principle of particular gravitation, as in electricity, magnetiſm, heat; or of the principle of chemical affinity, as in com⯑buſtion, fermentation, combination; or of the principle of organic life, as in the contraction of vegetable and animal fibres. The inert cauſes of motion, or new configuration, conſiſt of the parts of mat⯑ter, which are introduced within the ſpheres of activity of the prin⯑ciples above deſcribed. Thus, when an apple falls on the ground, the principle of gravitation is the efficient cauſe, and the matter of the apple the inert cauſe. If a bar of iron be approximated to a mag⯑net, it may be termed the inert cauſe of the motion, which brings theſe two bodies into contact; while the magnetic principle may be termed the efficient cauſe. In the ſame manner the fibres, which conſtitute the retina may be called the inert cauſe of the motions of that organ in viſion, while the ſenſorial power may be termed the ef⯑ficient cauſe.

3. Another more common diſtribution of the perpetual chain of cauſes and effects, which conſtitute the motions, or changing con⯑figurations, of the natural world, is into active and paſſive. Thus, if a ball in motion impinges againſt another ball at reſt, and commu⯑nicates its motion to it, the former ball is ſaid to act, and the latter to be acted upon. In this ſenſe of the words a magnet is ſaid to at⯑tract iron; and the prick of a ſpur to ſtimulate a horſe into exertion; ſo that in this view of the works of nature all things may be ſaid ei⯑ther [531] ſimply to exiſt, or to exiſt as cauſes, or to exiſt as effects; that is, to exiſt either in an active or paſſive ſtate.

This diſtribution of objects, and their motions, or changes of po⯑ſition, has been found ſo convenient for the purpoſes of common life, that on this foundation reſts the whole conſtruction or theory of lan⯑guage. The names of the things themſelves are termed by gram⯑marians Nouns, and their modes of exiſtence are termed Verbs. The nouns are divided into ſubſtantives, which denote the principal things ſpoken of; and into adjectives, which denote ſome circumſtances, or leſs kinds of things, belonging to the former. The verbs are divided into three kinds, ſuch as denote the exiſtence of things ſimply, as, to be; or their exiſtence in an active ſtate, as, to eat; or their ex⯑iſtence in a paſſive ſtate, as, to be eaten. Whence it appears, that all languages conſiſt only of nouns and verbs, with their abbreviations for the greater expedition of communicating our thoughts; as explained in the ingenious work of Mr. Horne Tooke, who has unfolded by a ſingle flaſh of light the whole theory of lan⯑guage, which had ſo long lain buried beneath the learned lumber of the ſchools. Diverſions of Purley. Johnſon. London.

4. A third diviſion of cauſes has been into proximate and remote; theſe have been much ſpoken of by the writers on medical ſubjects, but without ſufficient preciſion. If to proximate and remote cauſes we add proximate and remote effects, we ſhall include four links of the perpetual chain of cauſation; which will be more convenient for the diſcuſſion of many philoſophical ſubjects.

Thus if a particle of chyle be applied to the mouth of a lacteal veſ⯑ſel, it may be termed the remote cauſe of the motions of the fibres, which compoſe the mouth of that lacteal veſſel; the ſenſorial power is the proximate cauſe; the contraction of the fibres of the mouth of the veſſel is the proximate effect; and their embracing the particle of chyle is the remote effect; and theſe four links of cauſation conſtitute abſorption.

Thus when we attend to the riſing ſun, firſt the yellow rays of [532] light ſtimulate the ſenſorial power reſiding in the extremities of the optic nerve, this is the remote cauſe. 2. The ſenſorial power is excited into a ſtate of activity, this is the proximate cauſe. 3. The fibrous extremities of the optic nerve are contracted, this is the prox⯑imate effect. 4. A pleaſureable or painful ſenſation is produced in conſequence of the contraction of theſe fibres of the optic nerve, this is the remote effect; and theſe four links of the chain of cauſation conſtitute the ſenſitive idea, or what is commonly termed the ſenſa⯑tion of the riſing ſun.

5. Other cauſes have been announced by medical writers under the names of cauſa procatarctica, and cauſa proegumina, and cauſa ſine quâ non. All which are links more or leſs diſtant of the chain of re⯑mote cauſes.

To theſe muſt be added the final cauſe, ſo called by many authors, which means the motive, for the accompliſhment of which the pre⯑ceding chain of cauſes was put into action. The idea of a final cauſe, therefore, includes that of a rational mind, which employs means to effect its purpoſes; thus the deſire of preſerving himſelf from the pain of cold, which he has frequently experienced, induces the ſavage to conſtruct his hut; the fixing ſtakes into the ground for walls, branches of trees for rafters, and turf for a cover, are a ſeries of ſucceſſive vo⯑luntary exertions; which are ſo many means to produce a certain ef⯑fect. This effect of preſerving himſelf from cold, is termed the final cauſe; the conſtruction of the hut is the remote effect; the action of the muſcular fibres of the man, is the proximate effect; the volition, or activity of deſire to preſerve himſelf from cold, is the proximate cauſe; and the pain of cold, which excited that deſire, is the remote cauſe.

6. This perpetual chain of cauſes and effects, whoſe firſt link is rivetted to the throne of GOD, divides itſelf into innumerable diverg⯑ing branches, which, like the nerves ariſing from the brain, permeate the moſt minute and moſt remote extremities of the ſyſtem, diffuſing [533] motion and ſenſation to the whole. As every cauſe is ſuperior in power to the effect, which it has produced, ſo our idea of the power of the Almighty Creator becomes more elevated and ſublime, as we trace the operations of nature from cauſe to cauſe, climbing up the links of theſe chains of being, till we aſcend to the Great Source of all things.

Hence the modern diſcoveries in chemiſtry and in geology, by having traced the cauſes of the combinations of bodies to remoter ori⯑gins, as well as thoſe in aſtronomy, which dignify the preſent age, contribute to enlarge and amplify our ideas of the power of the Great Firſt Cauſe. And had thoſe ancient philoſophers, who contended that the world was formed from atoms, aſcribed their combinations to cer⯑tain immutable properties received from the hand of the Creator, ſuch as general gravitation, chemical affinity, or animal appetency, inſtead of aſcribing them to a blind chance; the doctrine of atoms, as conſti⯑tuting or compoſing the material world by the variety of their com⯑binations, ſo far from leading the mind to atheiſm, would ſtrengthen the demonſtration of the exiſtence of a Deity, as the firſt cauſe of all things; becauſe the analogy reſulting from our perpetual experience of cauſe and effect would have thus been exemplified through univerſal nature.

I. Activity of the Retina in Viſion.

FROM the ſubſequent experiments it appears, that the retina is in an active not in a paſſive ſtate during the exiſtence of theſe ocular ſpectra; and it is thence to be concluded, that all viſion is owing to the activity of this organ.

1. Place a piece of red ſilk, about an inch in diameter, as in plate 1, at Sect. III. 1, on a ſheet of white paper, in a ſtrong light; look ſteadily upon it from about the diſtance of half a yard for a minute; then cloſing your eyelids cover them with your hands, and a green [536] ſpectrum will be ſeen in your eyes, reſembling in form the piece of red ſilk: after ſome time, this ſpectrum will diſappear and ſhortly re⯑appear; and this alternately three or four times, if the experiment is well made, till at length it vaniſhes entirely.

2. Place on a ſheet of white paper a circular piece of blue ſilk, about four inches in diameter, in the ſunſhine; cover the center of this with a circular piece of yellow ſilk, about three inches in dia⯑meter; and the center of the yellow ſilk with a circle of pink ſilk, about two inches in diameter; and the center of the pink ſilk with a circle of green ſilk, about one inch in diameter; and the centre of this with a circle of indigo, about half an inch in diameter; make a ſmall ſpeck with ink in the very center of the whole, as in plate 3, at Sect. III. 3. 6.; look ſteadily for a minute on this central ſpot, and then cloſing your eyes, and applying your hand at about an inch diſ⯑tance before them, ſo as to prevent too much or too little light from paſſing through the eyelids, you will ſee the moſt beautiful circles of colours that imagination can conceive, which are moſt reſembled by the colours occaſioned by pouring a drop or two of oil on a ſtill lake in a bright day; but theſe circular iriſes of colours are not only different from the colours of the ſilks above mentioned, but are at the ſame time perpetually changing as long as they exiſt.

3. When any one in the dark preſſes either corner of his eye with his finger, and turns his eye away from his finger, he will ſee a circle of colours like thoſe in a peacock's tail: and a ſudden flaſh of light is excited in the eye by a ſtroke on it. (Newton's Opt. Q. 16.)

4. When any one turns round rapidly on one foot, till he becomes dizzy, and falls upon the ground, the ſpectra of the ambient objects continue to preſent themſelves in rotation, or appear to librate, and he ſeems to behold them for ſome time ſtill in motion.

From all theſe experiments it appears, that the ſpectra in the eye are not owing to the mechanical impulſe of light impreſſed on the retina, nor to its chemical combination with that organ, nor to the [537] abſorption and emiſſion of light, as is obſerved in many bodies; for in all theſe caſes the ſpectra muſt either remain uniformly, or gra⯑dually diminiſh; and neither their alternate preſence and evaneſcence as in the firſt experiment, nor the perpetual changes of their colours as in the ſecond, nor the flaſh of light or colours in the preſſed eye as in the third, nor the rotation or libration of the ſpectra as in the fourth, could exiſt.

It is not abſurd to conceive, that the retina may be ſtimulated into motion, as well as the red and white muſcles which form our limbs and veſſels; ſince it conſiſts of fibres, like thoſe, intermixed with its medullary ſubſtance. To evince this ſtructure, the retina of an ox's eye was ſuſpended in a glaſs of warm water, and forcibly torn in a few places; the edges of theſe parts appeared jagged and hairy, and did [...] contract, and become ſmooth like ſimple mucus, when it is diſtended till it breaks; which ſhews that it conſiſts of fibres; and this its [...] conſtruction became ſtill more diſtinct to the ſight, by adding ſome cauſtic alkali to the water, as the adhering mucus was firſt eroded, and the hair-like fibres remained floating in the veſſel. Nor does the degree of tranſparency of the retina invalidate the evi⯑dence of its fibrous ſtructure, ſince Leeuwenhoek has ſhewn that the cryſtalline humour itſelf conſiſts of fibres. (Arcana Naturae, V. 1. p. 70.)

Hence it appears, that as the muſcles have larger fibres intermixed with a ſmaller quantity of nervous medulla, the organ of viſion has a greater quantity of nervous medulla intermixed with ſmaller fibres; and it is probable that the locomotive muſcles, as well as the vaſcular ones, of microſcopic animals have much greater tenuity than theſe of the retina.

And beſides the ſimilar laws, which will be ſhewn in this paper to govern alike the actions of the retina and of the muſcles, there are many other analogies which exiſt between them. They are both ori⯑ginally excited into action by irritations, both act nearly in the ſame [538] quantity of time, are alike ſtrengthened or fatigued by exertion, are alike painful if excited into action when they are in an inflamed ſtate, are alike liable to paralyſis, and to the torpor of old age.

II. OF SPECTRA FROM DEFECT OF SENSIBILITY.
The retina is not ſo eaſily excited into action by leſs irritation after having been lately ſubjected to greater.

1. WHEN any one paſſes from the bright daylight into a darkened room, the iriſes of his eyes expand themſelves to their utmoſt extent in a few ſeconds of time; but it is very long before the optic nerve, after having been ſtimulated by the greater light of the day, becomes ſenſible of the leſs degree of it in the room; and, if the room is not too obſcure, the iriſes will again contract themſelves in ſome de⯑gree, as the ſenſibility of the retina returns.

2. Place about half an inch ſquare of white paper on a black hat, and looking ſteadily on the center of it for a minute, remove your eyes to a ſheet of white paper; and after a ſecond or two a dark ſquare will be ſeen on the white paper, which will continue ſome time. A ſimilar dark ſquare will be ſeen in the cloſed eye, if light be admitted through the eyelids.

So after looking at any luminous object of a ſmall ſize, as at the ſun, for a ſhort time, ſo as not much to fatigue the eyes, this part of the retina becomes leſs ſenſible to ſmaller quantities of light; hence, when the eyes are turned on other leſs luminous parts of the ſky, a dark ſpot is ſeen reſembling the ſhape of the ſun, or other luminous object which we laſt beheld. This is the ſource of one kind of the dark-coloured muſcae volitantes. If this dark ſpot lies above the center of the eye, we turn our eyes that way, expecting to bring it into [539] the center of the eye, that we may view it more diſtinctly; and in this caſe the dark ſpectrum ſeems to move upwards. If the dark ſpectrum is found beneath the centre of the eye, we purſue it from the ſame motive, and it ſeems to move downwards. This has given riſe to various conjectures of ſomething floating in the aqueous hu⯑mours of the eyes; but whoever, in attending to theſe ſpots, keeps his eyes unmoved by looking ſteadily at the corner of a cloud, at the ſame time that he obſerves the dark ſpectra, will be thoroughly con⯑vinced, that they have no motion but what is given to them by the movement of our eyes in purſuit of them. Sometimes the form of the ſpectrum, when it has been received from a circular luminous body, will become oblong; and ſometimes it will be divided into two circular ſpectra, which is not owing to our changing the angle made by the two optic axiſes, according to the diſtance of the clouds or other bodies to which the ſpectrum is ſuppoſed to be contiguous, but to other cauſes mentioned in No. X. 3. of this ſection. The apparent ſize of it will alſo be variable according to its ſuppoſed diſtance.

As theſe ſpectra are more eaſily obſervable when our eyes are a little weakened by fatigue, it has frequently happened, that people of delicate conſtitutions have been much alarmed at them, fearing a be⯑ginning decay of their ſight, and have thence fallen into the hands of ignorant oculiſts; but I believe they never are a prelude to any other diſeaſe of the eye, and that it is from habit alone, and our want of at⯑tention to them, that we do not ſee them on all objects every hour of our lives. But as the nerves of very weak people loſe their ſenſibility, in the ſame manner as their muſcles loſe their activity, by a ſmall time of exertion, it frequently happens, that ſick people in the ex⯑treme debility of fevers are perpetually employed in picking ſomething from the bed-clothes, occaſioned by their miſtaking the appearance of theſe muſcae volitantes in their eyes. Benvenuto Celini, an Italian artiſt, a man of ſtrong abilities, relates, that having paſſed the whole night on a diſtant mountain with ſome companions and a conjurer, [540] and performed many ceremonies to raiſe the devil, on their return in the morning to Rome, and looking up when the ſun began to riſe, they ſaw numerous devils run on the tops of the houſes, as they paſſed along; ſo much were the ſpectra of their weakened eyes magnified by fear, and made ſubſervient to the purpoſes of fraud or ſuperſtition. (Life of Ben. Celini.)

3. Place a ſquare inch of white paper on a large piece of ſtraw⯑coloured ſilk; look ſteadily ſome time on the white paper, and then move the center of your eyes on the ſilk, and a ſpectrum of the form of the paper will appear on the ſilk, of a deeper yellow than the other part of it: for the central part of the retina, having been ſome time expoſed to the ſtimulus of a greater quantity of white light, is become leſs ſenſible to a ſmaller quantity of it, and therefore ſees only the yellow rays in that part of the ſtraw-coloured ſilk.

Facts ſimilar to theſe are obſervable in other parts of our ſyſtem: thus, if one hand be made warm, and the other expoſed to the cold, and then both of them immerſed in ſubtepid water, the water is per⯑ceived warm to one hand, and cold to the other; and we are not able to hear weak ſounds for ſome time after we have been expoſed to loud ones; and we feel a chillineſs on coming into an atmoſphere of tem⯑perate warmth, after having been ſome time confined in a very warm room: and hence the ſtomach, and other organs of digeſtion, of thoſe who have been habituated to the greater ſtimulus of ſpirituous liquor, are not excited into their due action by the leſs ſtimulus of common food alone; of which the immediate conſequence is indigeſtion and hypochondriaciſm.

III. OF SPECTRA FROM EXCESS OF SENSIBILITY.
The retina is more eaſily excited into action by greater irritation after having been lately ſubjected to leſs.

1. IF the eyes are cloſed, and covered perfectly with a hat, for a minute or two, in a bright day; on removing the hat a red or crimſon light is ſeen through the eyelids. In this experiment the retina, after being ſome time kept in the dark, becomes ſo ſenſible to a ſmall quan⯑tity of light, as to perceive diſtinctly the greater quantity of red rays than of others which paſs through the eyelids. A ſimilar coloured light is ſeen to paſs through the edges of the fingers, when the open hand is oppoſed to the flame of a candle.

2. If you look for ſome minutes ſteadily on a window in the be⯑ginning of the evening twilight, or in a dark day, and then move your eyes a little, ſo that thoſe parts of the retina, on which the dark frame-work of the window was delineated, may now fall on the glaſs part of it, many luminous lines, repreſenting the frame-work, will appear to lie acroſs the glaſs panes: for thoſe parts of the retina, which were before leaſt ſtimulated by the dark frame-work, are now more ſenſible to light than the other parts of the retina which were expoſed to the more luminous parts of the window.

3. Make with ink on white paper a very black ſpot, about half an inch in diameter, with a tail about an inch in length, ſo as to repre⯑ſent a tadpole, as in plate 2, at Sect. III. 8. 3.; look ſteadily for a minute on this ſpot, and, on moving the eye a little, the figure of the tadpole will be ſeen on the white part of the paper, which figure of the tadpole will appear whiter or more luminous than the other parts of the white paper; for the part of the retina on which the tad⯑pole was delineated, is now more ſenſible to light than the other parts [542] of it, which were expoſed to the white paper. This experiment is mentioned by Dr. Irwin, but is not by him aſcribed to the true cauſe, namely, the greater ſenſibility of that part of the retina which has been expoſed to the black ſpot, than of the other parts which had received the white field of paper, which is put beyond a doubt by the next experiment.

4. On cloſing the eyes after viewing the black ſpot on the white paper, as in the foregoing experiment, a red ſpot is ſeen of the form of the black ſpot: for that part of the retina, on which the black ſpot was delineated, being now more ſenſible to light than the other parts of it, which were expoſed to the white paper, is capable of per⯑ceiving the red rays which penetrate the eyelids. If this experiment be made by the light of a tallow candle, the ſpot will be yellow in⯑ſtead of red; for tallow candles abound much with yellow light, which paſſes in greater quantity and force through the eyelids than blue light; hence the difficulty of diſtinguiſhing blue and green by this kind of candle light. The colour of the ſpectrum may poſſibly vary in the daylight, according to the different colour of the meri⯑dian or the morning or evening light.

M. Beguelin, in the Berlin Memoires, V. II. 1771, obſerves, that, when he held a book ſo that the ſun ſhone upon his half-cloſed eye⯑lids, the black letters, which he had long inſpected, became red, which muſt have been thus occaſioned. Thoſe parts of the retina which had received for ſome time the black letters, were ſo much more ſenſible than thoſe parts which had been oppoſed to the white paper, that to the former the red light, which paſſed through the eyelids, was perceptible. There is a ſimilar ſtory told, I think, in M. de Voltaire's Hiſtorical Works, of a Duke of Tuſcany, who was playing at dice with the general of a foreign army, and, believing he ſaw bloody ſpots upon the dice, portended dreadful events, and retired in confuſion. The obſerver, after looking for a minute on the black [543] ſpots of a die, and careleſsly cloſing his eyes, on a bright day, would ſee the image of a die with red ſpots upon it, as above explained.

5. On emerging from a dark cavern, where we have long conti⯑nued, the light of a bright day becomes intolerable to the eye for a conſiderable time, owing to the exceſs of ſenſibility exiſting in the eye, after having been long expoſed to little or no ſtimulus. This occaſions us immediately to contract the iris to its ſmalleſt aperture, which becomes again gradually dilated, as the retina becomes accuſ⯑tomed to the greater ſtimulus of the daylight.

The twinkling of a bright ſtar, or of a diſtant candle in the night, is perhaps owing to the ſame cauſe. While we continue to look upon theſe luminous objects, their central parts gradually appear paler, owing to the decreaſing ſenſibility of the part of the retina expoſed to their light; whilſt, at the ſame time, by the unſteadineſs of the eye, the edges of them are perpetually falling on parts of the retina that were juſt before expoſed to the darkneſs of the night, and therefore tenfold more ſenſible to light than the part on which the ſtar or candle had been for ſome time delineated. This pains the eye in a ſimilar manner as when we come ſuddenly from a dark room into bright daylight, and gives the appearance of bright ſcintillations. Hence the ſtars twinkle moſt when the night is darkeſt, and do not twinkle through teleſcopes, as obſerved by Muſſchenbroeck; and it will after⯑wards be ſeen why this twinkling is ſometimes of different colours when the object is very bright, as Mr. Melvill obſerved in looking at Sirius. For the opinions of others on this ſubject, ſee Dr. Prieſtley's valuable Hiſtory of Light and Colours, p. 494.

Many facts obſervable in the animal ſyſtem are ſimilar to theſe; as the hot glow occaſioned by the uſual warmth of the air, or our clothes, on coming out of a cold bath; the pain of the fingers on ap⯑proaching the fire after having handled ſnow; and the inflamed heels from walking in ſnow. Hence thoſe who have been expoſed to much cold have died on being brought to a fire, or their limbs have become [544] ſo much inflamed as to mortify. Hence much food or wine given ſuddenly to thoſe who have almoſt periſhed by hunger has deſtroyed them; for all the organs of the famiſhed body are now become ſo much more irritable to the ſtimulus of food and wine, which they have long been deprived of, that inflammation is excited, which ter⯑minates in gangrene or fever.

IV. OF DIRECT OCULAR SPECTRA.
A quantity of ſtimulus ſomewhat greater than natural excites the retina into ſpaſmodic action, which ceaſes in a few ſeconds.

A CERTAIN duration and energy of the ſtimulus of light and colours excites the perfect action of the retina in viſion; for very quick mo⯑tions are imperceptible to us, as well as very ſlow ones, as the whirling of a top, or the ſhadow on a ſun-dial. So perfect darkneſs does not affect the eye at all; and exceſs of light produces pain, not viſion.

1. When a fire-coal is whirled round in the dark, a lucid circle remains a conſiderable time in the eye; and that with ſo much vivacity of light, that it is miſtaken for a continuance of the irritation of the object. In the ſame manner, when a ſiery meteor ſhoots acroſs the night, it appears to leave a long lucid train behind it, part of which, and perhaps ſometimes the whole, is owing to the continuance of the action of the retina after having been thus vividly excited. This is beautifully illuſtrated by the following experiment: fix a paper ſail, three or four inches in diameter, and made like that of a ſmoke jack, in a tube of paſteboard; on looking through the tube at a diſtant pro⯑ſpect, ſome disjointed parts of it will be ſeen through the narrow inter⯑vals between the ſails; but as the fly begins to revolve, theſe intervals [545] appear larger; and when it revolves quicker, the whole proſpect is ſeen quite as diſtinct as if nothing intervened, though leſs luminous.

2. Look through a dark tube, about half a yard long, at the area of a yellow circle of half an inch diameter, lying upon a blue area of double that diameter, for half a minute; and on cloſing your eyes the colours of the ſpectrum will appear ſimilar to the two areas, as in fig. 3.; but if the eye is kept too long upon them, the colours of the ſpectrum will be the reverſe of thoſe upon the paper, that is, the in⯑ternal circle will become blue, and the external area yellow; hence ſome attention is required in making this experiment.

3. Place the bright flame of a ſpermaceti candle before a black ob⯑ject in the night; look ſteadily at it for a ſhort time, till it is obſerved to become ſomewhat paler; and on cloſing the eyes, and covering them carefully, but not ſo as to compreſs them, the image of the blazing candle will continue diſtinctly to be viſible.

4. Look ſteadily, for a ſhort time, at a window in a dark day, as in Exp. 2. Sect. III. and then cloſing your eyes, and covering them with your hands, an exact delineation of the window remains for ſome time viſible in the eye. This experiment requires a little practice to make it ſucceed well; ſince, if the eyes are fatigued by looking too long on the window, or the day be too bright, the luminous parts of the win⯑dow will appear dark in the ſpectrum, and the dark parts of the frame-work will appear luminous, as in Exp. 2. Sect. III. And it is even difficult for many, who firſt try this experiment, to perceive the ſpectrum at all; for any hurry of mind, or even too great attention to the ſpectrum itſelf, will diſappoint them, till they have had a little experience in attending to ſuch ſmall ſenſations.

The ſpectra deſcribed in this ſection, termed direct ocular ſpectra, are produced without much fatigue of the eye; the irritation of the luminous object being ſoon withdrawn, or its quantity of light being not ſo great as to produce any degree of uneaſineſs in the organ of viſion; which diſtinguiſhes them from the next claſs of ocular ſpectra, [546] which are the conſequence of fatigue. Theſe direct ſpectra are beſt obſerved in ſuch circumſtances that no light, but what comes from the object, can fall upon the eye; as in looking through a tube, of half a yard long, and an inch wide, at a yellow paper on the ſide of a room, the direct ſpectrum was eaſily produced on cloſing the eye with⯑out taking it from the tube: but if the lateral light is admitted through the eyelids, or by throwing the ſpectrum on white paper, it becomes a reverſe ſpectrum, as will be explained below.

The other ſenſes alſo retain for a time the impreſſions that have been made upon them, or the actions they have been excited into. So if a hard body is preſſed upon the palm of the hand, as is practiſed in tricks of legerdemain, it is not eaſy to diſtinguiſh for a few ſeconds whether it remains or is removed; and taſtes continue long to exiſt vividly in the mouth, as the ſmoke of tobacco, or the taſte of gentian, after the ſapid material is withdrawn.

V. A quantity of ſtimulus ſomewhat greater than the laſt mentioned excites the retina into ſpaſmodic action, which ceaſes and recurs alternately.

1. ON looking for a time on the ſetting ſun, ſo as not greatly to fatigue the ſight, a yellow ſpectrum is ſeen when the eyes are cloſed and covered, which continues for a time, and then diſappears and re⯑curs repeatedly before it entirely vaniſhes. This yellow ſpectrum of the ſun when the eyelids are opened becomes blue; and if it is made to fall on the green graſs, or on other coloured objects, it varies its own colour by an intermixture of theirs, as will be explained in ano⯑ther place.

2. Place a lighted ſpermaceti candle in the night about one foot from your eye, and look ſteadily on the centre of the flame, till your [547] eye becomes much more fatigued than in Sect. IV. Exp. 3.; and on cloſing your eyes a reddiſh ſpectrum will be perceived, which will ceaſe and return alternately.

The action of vomiting in like manner ceaſes, and is renewed by intervals, although the emetic drug is thrown up with the firſt effort: ſo after-pains continue ſome time after parturition; and the alternate pulſations of the heart of a viper are renewed for ſome time after it is cleared from its blood.

VI. OF REVERSE OCULAR SPECTRA.
The retina after having been excited into action by a ſtimulus ſomewhat greater than the laſt mentioned falls into oppoſite ſpaſmodic action.

THE actions of every part of animal bodies may be advantageouſly compared with each other. This ſtrict analogy contributes much to the inveſtigation of truth; while thoſe looſer analogies, which com⯑pare the phenomena of animal life with thoſe of chemiſtry or me⯑chanics, only ſerve to miſlead our inquiries.

When any of our larger muſcles have been in long or in violent action, and their antagoniſts have been at the ſame time extended, as ſoon as the action of the former ceaſes, the limb is ſtretched the con⯑trary way for our eaſe, and a pandiculation or yawning takes place.

By the following obſervations it appears, that a ſimilar circumſtance obtains in the organ of viſion; after it has been fatigued by one kind of action, it ſpontaneouſly falls into the oppoſite kind.

1. Place a piece of coloured ſilk, about an inch in diameter, on a ſheet of white paper, about half a yard from your eyes; look ſteadily upon it for a minute; then remove your eyes upon another part of the white paper, and a ſpectrum will be ſeen of the form of the ſilk thus inſpected, but of a colour oppoſite to it. A ſpectrum nearly [548] ſimilar will appear if the eyes are cloſed, and the eyelids ſhaded by ap⯑proaching the hand near them, ſo as to permit ſome, but to prevent too much light falling on them.

• Red ſilk produced a green ſpectrum.
• Green produced a red one.
• Orange produced blue.
• Blue produced orange.
• Yellow produced violet.
• Violet produced yellow.

That in theſe experiments the colours of the ſpectra are the reverſe of the colours which occaſioned them, may be ſeen by examining the third figure in Sir Iſaac Newton's Optics, L. II. p. 1, where thoſe thin laminae of air, which reflected yellow, tranſmitted violet; thoſe which reflected red, tranſmitted a blue-green; and ſo of the reſt, agreeing with the experiments above related.

2. Theſe reverſe ſpectra are ſimilar to a colour, formed by a com⯑bination of all the primary colours except that with which the eye has been fatigued in making the experiment: thus the reverſe ſpectrum of red muſt be ſuch a green as would be produced by a com⯑bination of all the other priſmatic colours. To evince this fact the following ſatisfactory experiment was made. The priſmatic colours were laid on a circular paſteboard wheel, about four inches in dia⯑meter, in the proportions deſcribed in Dr. Prieſtley's Hiſtory of Light and Colours, pl. 12. fig. 83. except that the red compartment was entirely left out, and the others proportionably extended ſo as to com⯑plete the circle. Then, as the orange is a mixture of red and yellow, and as the violet is a mixture of red and indigo, it became neceſſary to put yellow on the wheel inſtead of orange, and indigo inſtead of violet, that the experiment might more exactly quadrate with the theory it was deſigned to eſtabliſh or confute; becauſe in gaining a green ſpectrum from a red object, the eye is ſuppoſed to have become inſenſible to red light. This wheel, by means of an axis, was made [549] to whirl like a top; and on its being put in motion, a green colour was produced, correſponding with great exactneſs to the reverſe ſpectrum of red.

3. In contemplating any one of theſe reverſe ſpectra in the cloſed and covered eye, it diſappears and re-appears ſeveral times ſucceſſively, till at length it entirely vaniſhes, like the direct ſpectra in Sect. V.; but with this additional circumſtance, that when the ſpectrum be⯑comes faint or evaneſcent, it is inſtantly revived by removing the hand from before the eyelids, ſo as to admit more light: becauſe then not only the fatigued part of the retina is inclined ſpontaneouſly to fall into motions of a contrary direction, but being ſtill ſenſible to all other rays of light, except that with which it was lately fatigued, is by theſe rays at the ſame time ſtimulated into thoſe motions which form the reverſe ſpectrum.

From theſe experiments there is reaſon to conclude, that the fa⯑tigued part of the retina throws itſelf into a contrary mode of action, like oſcitation or pandiculation, as ſoon as the ſtimulus which has fa⯑tigued it is withdrawn; and that it ſtill remains ſenſible, that is, liable to be excited into action by any other colours at the ſame time, ex⯑cept the colour with which it has been fatigued.

VII. The retina after having been excited into action by a ſtimulus ſome⯑what greater than the laſt mentioned falls into various ſucceſſive ſpaſmodic actions.

1. ON looking at the meridian ſun as long as the eyes can well bear its brightneſs, the diſk firſt becomes pale, with a luminous creſcent, which ſeems to librate from one edge of it to the other, owing to the unſteadineſs of the eye; then the whole phaſis of the ſun becomes blue, ſurrounded with a white halo; and on cloſing the eyes, and covering them with the hands, a yellow ſpectrum is ſeen, which in a little time changes into a blue one.

[550] M. de la Hire obſerved, after looking at the bright ſun, that the impreſſion in his eye firſt aſſumed a yellow appearance, and then green, and then blue; and wiſhes to aſcribe theſe appearances to ſome affection of the nerves. (Porterfield on the Eye, Vol. I. p. 343.)

2. After looking ſteadily on about an inch ſquare of pink ſilk, placed on white paper, in a bright ſunſhine, at the diſtance of a foot from my eyes, and cloſing and covering my eyelids, the ſpectrum of the ſilk was at firſt a dark green, and the ſpectrum of the white paper became of a pink. The ſpectra then both diſappeared; and then the internal ſpectrum was blue; and then, after a ſecond diſappearance, became yellow, and laſtly pink, whilſt the ſpectrum of the field va⯑ried into red and green.

Theſe ſucceſſions of different coloured ſpectra were not exactly the ſame in the different experiments, though obſerved, as near as could be, with the ſame quantity of light, and other ſimilar circumſtances; owing, I ſuppoſe, to trying too many experiments at a time; ſo that the eye was not quite free from the ſpectra of the colours which were previouſly attended to.

The alternate exertions of the retina in the preceding ſection re⯑ſembled the oſcitation or pandiculation of the muſcles, as they were performed in directions contrary to each other, and were the conſe⯑quence of fatigue rather than of pain. And in this they differ from the ſucceſſive diſſimilar exertions of the retina, mentioned in this ſec⯑tion, which reſemble in miniature the more violent agitations of the limbs in convulſive diſeaſes, as epilepſy, chorea S. Viti, and opiſtho⯑tonos; all which diſeaſes are perhaps, at firſt, the conſequence of pain, and have their periods afterwards eſtabliſhed by habit.

VIII. The retina, after having been excited into action by a ſtimulus ſome⯑what greater than the laſt mentioned, falls into a fixed ſpaſmodic action, which continues for ſome days.

1. AFTER having looked long at the meridian ſun, in making ſome of the preceding experiments, till the diſks faded into a pale blue, I frequently obſerved a bright blue ſpectrum of the ſun on other objects all the next and the ſucceeding day, which conſtantly occurred when I attended to it, and frequently when I did not previouſly attend to it. When I cloſed and covered my eyes, this appeared of a dull yellow; and at other times mixed with the colours of other objects on which it was thrown. It may be imagined, that this part of the retina was become inſenſible to white light, and thence a bluiſh ſpectrum became viſible on all luminous objects; but as a yellowiſh ſpectrum was alſo ſeen in the cloſed and covered eye, there can remain no doubt of this being the ſpectrum of the ſun. A ſimilar appearance was obſerved by M. Aepinus, which he acknowledges he could give no account of. (Nov. Com. Petrop. V. 10. p. 2. and 6.)

The locked jaw, and ſome cataleptic ſpaſms, are reſembled by this phenomenon; and from hence we may learn the danger to the eye by inſpecting very luminous objects too long a time.

IX. A quantity of ſtimulus greater than the preceding induces a temporary paralyſis of the organ of viſion.

1. PLACE a circular piece of bright red ſilk, about half an inch in diameter, on the middle of a ſheet of white paper; lay them on the floor in a bright ſunſhine, and fixing your eyes ſteadily on the center of the red circle, for three or four minutes, at the diſtance of four or [552] ſix feet from the object, the red ſilk will gradually become paler, and finally ceaſe to appear red at all.

2. Similar to theſe are many other animal facts; as purges, opiates, and even poiſons, and contagious matter, ceaſe to ſtimulate our ſyſtem, after we have been habituated to their uſe. So ſome people ſleep un⯑diſturbed by a clock, or even by a forge hammer in their neighbour⯑hood: and not only continued irritations, but violent exertions of any kind, are ſucceeded by temporary paralyſis. The arm drops down after violent action, and continues for a time uſeleſs; and it is probable, that thoſe who have periſhed ſuddenly in ſwimming, or in ſcating on the ice, have owed their deaths to the paralyſis, or extreme fatigue, which ſucceeds every violent and continued exertion.

X. MISCELLANEOUS REMARKS.

THERE were ſome circumſtances occurred in making theſe experi⯑ments, which were liable to alter the reſults of them, and which I ſhall here mention for the aſſiſtance of others, who may wiſh to re⯑peat them.

Concluſion.

IT was obſerved by the learned M. Sauvages (Noſol. Method. Cl. VIII. Ord. 1.) that the pulſations of the optic artery might be per⯑ceived by looking attentively on a white wall well illuminated. A kind of net-work, darker than the other parts of the wall, appears and vaniſhes alternately with every pulſation. This change of the colour of the wall he well aſcribes to the compreſſion of the retina by the di⯑aſtole of the artery. The various colours produced in the eye by the preſſure of the finger, or by a ſtroke on it, as mentioned by Sir Iſaac Newton, ſeem likewiſe to originate from the unequal preſſure on va⯑rious parts of the retina. Now as Sir Iſaac Newton has ſhewn, that all the different colours are reflected or tranſmitted by the laminae of ſoap bubbles, or of air, according to their different thickneſs or thin⯑neſs, is it not probable, that the effect of the activity of the retina may be to alter its thickneſs or thinneſs, ſo as better to adapt it to reflect or tranſmit the colours which ſtimulate it into action? May not muſ⯑cular fibres exiſt in the retina for this purpoſe, which may be leſs mi⯑nute than the locomotive muſcles of microſcopic animals? May not theſe muſcular actions of the retina conſtitute the ſenſation of light and colours; and the voluntary repetitions of them, when the object [566] is withdrawn, conſtitute our memory of them? And laſtly, may not the laws of the ſenſations of light, here inveſtigated, be applicable to all our other ſenſes, and much contribute to elucidate many phenomena of animal bodies both in their healthy and diſeaſed ſtate; and thus ren⯑der this inveſtigation well worthy the attention of the phyſician, the metaphyſician, and the natural philoſopher?