Of the Structure of the Heart and Lungs, and of the Circu⯑lation of the Blood.

IN order to have a right Underſtanding of the Circulation of the Blood, it is firſt neceſſary to be acquainted with the Structure of the Heart and Lungs; becauſe they are the Or⯑gans immediately concerned in carrying on that important Function.

Of the Heart.

The Heart is a ſelf-moving Muſcle of a pyramidal Figure, ſuſpended (in the Middle of the Breaſt between the two Lobes of the Lungs) by its principal Veſſels being connected to no other Part for the Conveniency of its Motions. Of the Heart [5]we may conſider the two Extremities; the one is uppermoſt and broad, call'd the Baſe; the other is lowermoſt, and con⯑tracting itſelf narrow becomes a little ſharp, and is called its Point: This more ſo in Brutes. The Situation of the Heart in Man is reſting on the Diaphragm, ſlanting much towards the left Side, ſo that the greateſt Part of it is placed under the left Breaſt.

It conſiſts chiefly of four principal Cavities, two of which are called Auricles, or Ears, from their Figure, placed on the right and left Sides of the Baſe of the Heart; the two other are the Ventricles, which are Cavities formed immediately in the Subſtance of the Heart: The one is the right Ventricle and the other the left: The right Ear opens into the right Ventricle; and the left Ear into the left Ventricle. The Heart, like all other Muſcles, is made up of a great Number of Fi⯑bres or fleſhy Threads, diſpoſed in a particular Manner, on the inſide of the Ears, where they form a kind of irregular Net-work, and in the Ventricles, beſides this Net-work, leve⯑ral conſiderable Eminences called Pillars. The right Ear opens into the right Ventricle, and the left into the left Ven⯑tricle; at the Entrance of the right Ear into the right Ventricle there are placed three Valves, which open from the right Ear into the right Ventricle, but cloſe the Return from that Ven⯑tricle into that Ear: From the left Ear into the left Ventricle there are placed two more, in the ſame manner as in the right Ventricle.

There are four principal Veſſels which come into and go out of the Heart; two Veins and two Arteries; the two Veins are the Vena Cava; or hollow Vein, which is made from the [6]Union of all the Veins of the Body, and the Pulmonary Vein, made from the Union of all the Veins of the Luags. The Vena Cava opens into the right Ear, and the Pulmonary Vein into the left. The two Arteries are, the Pulmonary Artery, which divides it⯑ſelf into every Part of the Lungs, and the Aorta magna, or great Artery, which divides itſelf into every Part of the Body; the Pulmonary Artery ariſes out of the right Ventricle, at the Beginning of which there are placed three Valves in the Figure of a Half-moon, which open from the Ventricle into this Ar⯑tery, but cloſe from the Artery into the Ventricle; the Aorta magna, or great Artery, ariſes out of the left Ventricle, at the Entrance of which there are placed three other Valves of the Half-moon Form, diſpoſed in the ſame Manner as thoſe in the Pulmonary Artery. Beſides theſe principal Veſſels, there are two Arteries and a Vein appropriated to the Uſe of the Heart; the Arteries are call'd Coronary; they ariſe from the great Ar⯑tery at its going out of the Heart, and are diſtributed through⯑out the whole Subſtance of the Heart for its Nouriſhment; and the Veins bear the ſame Name which return the Blood into the right Ear.

Of the Lungs.

The Lungs are two in Number, ſituate on each Side of the Heart, of an elaſtick ſpungy Texture: They are connected to the Veſſels of the Heart and Windpipe, of which they are chiefly compoſed. The Windpipe is the Paſſage for Air into the Lungs only; its upper Part is made up of fine ſmall Car⯑tilages, [7]or Griſtles, four of which form its Entrance, which is at the Root of the Tongue, before the Entrance of the Gul⯑let, and is the Paſſage for the Aliments. The Fifth is a car⯑tilaginous Valve placed at the Entrance of the Windpipe, which is preſſed down at the Time the Aliments are paſſing into the Gullet. The Windpipe is compoſed of many cartilaginous Rings, looſely connected to each other, in ſuch a Manner that they may draw one in another: It is placed in the Fore-part of the Neck, and, after its Entrance into the Breaſt, divides itſelf into two Branches, whereof the one enters the right Lobe of the Lungs, and the other the left: Theſe Branches divide them⯑ſelves throughout the whole Structure of the Lungs, and every where terminate in ſmall Veſicles or Bladders, which are all tied together by a Number of ſmall Productions, from the common Covering of the Lungs. The next Veſſels are the Pulmonary Arteries and Veins: The Pulmonary Artery divides itſelf above the Heart into two Branches, the one to the right and the other to the left Lobe of the Lungs, where they follow all the Diviſions of the Branches of the Windpipe, lay⯑ing on the Sides of them, and continuing their Courſe 'till they have attained to the ſmall Veſicles, where they ſpread them⯑ſelves over them in a Number of ſmall capillary Veſſels. From theſe ſmall capillary Arteries on the Veſicles ariſe the capillary-pulmonary Veins, which uniting themſelves from ſmall Branches into large, on the Sides of the Branches of the Windpipe, form one Trunk in the left Ear. As theſe conſi⯑derable Veſſels conduce nothing to the Nouriſhment of the Subſtance of the Lungs (as will more particularly be treated of in deſcribing the Circulation of the Blood in the Child while [8]in the Womb) there is a particular Artery for that Uſe, called the Bronchal Artery, and a Vein to return the Blood.

Of the Circulation of the Blood, and its Uſe.

From what hath been ſaid of the Structure of the Heart and the Diſpoſition of its Veſſels and Valves, placed at the coming in of the Ventricles from the Ears, as well as of thoſe placed at the going out of the Ventricles into the Arteries; if the Action of the Heart be rightly conſidered, its Uſe may be eaſily under⯑ſtood; for the Ears and Ventricles have both of them the Power of contracting and dilating themſelves. But neither of theſe two Actions can happen at the ſame time both in the Ears and Ventricles, but the Dilatation of the one muſt ſucceed the Con⯑traction of the other, for reaſons now to be mentioned: For the Blood is always flowing from the Vena Cava into the right Ear, and from the Pulmonary Vein into the left, the one from every Part of the Body, and the other from the Lungs. So that the Ears, if rightly conſidered, are two little Reſervoirs always ready to ſupply the Ventricles with a menſurated Quantity of Blood, leſt its Motion ſhould be ſuffocated by the Weight of too much Blood at one time. As to the Ventricles, they are to receive the Blood from the Ears, in given Quantities, in order to force it our of their Cavities into the Arteries; by the Force of the right Ventricle through the Pulmonary Artery into the Lungs, and by the left through the great Artery into every Part of the Body. When the Ears contract, the Ventricles di⯑late, becauſe that the Ventricles at that time are receiving the Blood from them; and when the Ventricles contract, the Ears [9]dilate, becauſe at that time the Blood endeavouring to return in⯑to the Auricles from the Ventricles preſſes againſt the Valves, and cloſes them; during which time the Auricles are filling and kept dilated: The Blood in the Ventricles, which cannot return into the Auricles, is forced over the Half-moon Valves into the Pulmonary Artery from the right Ventricle, as well as over thoſe from the left Ventricle into the great Artery, by which Means the Arteries are dilated and filled with Blood. So that the Dilatation of the Arteries ſucceeds the Contraction of the Ventricles; for no Blood can poſſibly return from the Ar⯑teries into the Ventricles, becauſe the Half-moon Valves ſhut up the Mouths of thoſe Arteries, whenever the Blood en⯑deavours to return; and this is the Reaſon of the Pulſation of the Arteries. Hence it plainly appears, that the Uſe of the Heart is to carry on two Circulations of the whole Maſs of Blood at one and the ſame time: The one from the right Ventricle through the Lungs to the left Ear, and the other from the left Ventricle through every Part of the Body to the the right Ear. The Manner in which theſe Circulations are carry'd on is the next thing to be conſidered; And firſt, of that through the Lungs:

The Lungs perform two Actions, that of Inſpiration, and that of Expiration; the firſt is when the Air is taken into the Lungs, and the other is when it is thruſt out.

The firſt Action is performed by the Means of many Muſcles which raiſe the Ribs, and widen the Breaſt, during which time the Midriff, from a Convex Surface towards the Breaſt, becomes plane; and at the ſame time the outward Air, being preſs'd, ruſhes in at the Noſe and Mouth, and paſſes through all the Branches of the Wind-pipe, which lead [10]to the ſmall Veſicles by which they are blown up. The Rings which compoſe the Branches of the Wind-pipe, which be⯑fore were contracted one within another, are now extended the one from the other; the Branches of the Pulmonary Ar⯑tery and Vein laying on the Sides of the Branches of the Wind-pipe, muſt of neceſſity be co-equally extended with them, which before lay in Folds, gives now to the Blood a free Opportunity to paſs from the right Ventricle of the Heart, through all the Ramifications of the Pulmonary Ar⯑tery, and fill their Capillaries, which are diſtributed on the Veſicles: By this time the Elevation of the Ribs ceaſes, and the antagoniſt Muſcles pull them down, aſſiſted by the weight of the external Air, which preſſes on their Surface, drives the Air violently out of the Veſicles, through all the Branches of the Wind-pipe; and their Sides being brought together, the Blood contained in the Capillary Ar⯑teries by this great Preſſure is broken, and its Globules grown ſo ſmall, as to be able to paſs in the Capillary Veins, which are [...]he Pulmonary Veins, from whence it is propelled by the conti⯑ [...]ual Contraction of the Lungs into the left Auricle of the Heart.

Hence we find the great Neceſſity of Reſpiration; for by [...] the Blood in its Paſſage through the Lungs undergoes a [...]iolent Trituration, breaking the Texture of its Globules to [...]eparate its Parts, by which Means they become diſſimilar to [...]ach other, and prepare a ſufficient Quantity of different Par⯑ [...]cles for the Nouriſhment of the different Parts of the Body, [...]hich is carried on by the ſecond Circulation from the left Ven⯑ [...]cle of the Heart through the Aorta magna, or great Artery.

The left Ventricle of the Heart has its Sides about four [...]mes as thick as the right; and neceſſary it was it ſhould [11]be ſo; a greater Force being required to drive the Blood through the great Artery into every Part of the Body, than through the Lungs only, which is the Uſe of the right Ventricle.

The Aorta magna, or great Artery, is an elaſtick cartila⯑ginous Tube ariſing out of the left Ventricle of the Heart, and divides itſelf to every Part of the Body, the Branches of which continue their Diviſion 'till they become ſo ſmall as to be as fine as a Hair, and are call'd Capillaries: From theſe ſmall Capillary Arteries ariſe another Kind of ſmall Veſſels, which are call'd Veins, which frequently unite into larger Branches from every Part of the Body, 'till they have formed one common great Trunk, call'd the Vena Cava, or great hol⯑low Vein, which opens itſelf into the right Ear of the Heart. The Blood, which has been ſufficiently ground and ſubtillized by the Action of the Lungs, as was now mentioned, is brought into the left Ear by the Pulmonary Veins. The Cavity of each Ear is capable of containing one Ounce of Blood, ſo that whenever the left Ear contracts, the great Artery muſt receive one Ounce of Blood, becauſe the left Ventricle can throw no more Blood into the great Artery, than what it re⯑ceives from its Ear. During the Time the great Artery is re⯑ceiving this Quantity of Blood from the contracting Ventricle, it is much dilated or ſtretched: But as its Subſtance is in⯑duced with a great Spring, as ſoon as the Reſiſtance is taken off, which is, when the left Ventricle dilates to receive a freſh Quantity of Blood from the left Ear, then the Artery con⯑tracts itſelf, by which Means the Blood is crowded into its Ca⯑vity, and cloſely embraced and preſſed againſt its Sides; the Blood at the ſame Time equally reſiſting every Way, preſſes [12]againſt the Half-moon Valves, and prevents its Return into the left Ventricle, and thus is obliged to paſs on thro' all the Ramifications of this Artery into every Part of the Body.

During the Time that the Artery is contracting to propel the the Blood into every Part of the Body, the left Ventricle is di⯑lating to receive another Quantity of Blood from the left Ear, in order to throw it into the great Artery again, by the Time that the Contraction of its Sides are over.

Thus by the alternate Dilatation and Contraction of the left Auricle and left Ventricle, the Blood, which returns from the Lungs, where it has been attenuated and rendered fit for the Nouriſhment of the different Parts of the Body, is ſent by the great Artery to be diſtributed to them, whilſt the excre⯑mentitious Parts are ſtrained off by particular Glands, and carried out of the Body by Sweat and other Evacuations: The remaining Part of the Blood, which has not been ſuffi⯑ciently ground by the Action of the Lungs, is taken up by the Veins, and brought back to the Cava, to be poured by the right Auricle into the right Ventricle, in order to be forced through the Lungs again, and there to undergo another Tri⯑turation, and afterwards to be diſtributed to every Part of the Body again, in the Manner it has already been deſcribed.

Of the Nouriſhment of the Child in the Womb.

During the Time that the Child is in the Womb, it is nouriſhed with Blood only, brought to it from the Placenta, or After-birth, through the Navel by the umbilical Rope or Navel-ſtring; but as there is always more Blood conveyed to it than is neceſſary for its Nouriſhment, there is a conſtant Return of this Blood through the Navel-ſtring to the After⯑birth again.

Therefore to have a right Underſtanding in what Manner this reciprocal Circulation is carried on, it is neceſſary to con⯑ſider the Child, during the Time it is in the Womb, to be precluded from all Communication with the outward Air, and cannot breathe, all the Veſicles or little Bladders of its Lungs being cloſely preſſed together, and the Extremities of the Pul⯑monary Arteries and Veins equally preſſed with them, ſo that the Courſe of the Blood through the Lungs is entirely ſtopp'd 'till Birth; and notwithſtanding that the Circulation of the Blood may be conveniently carried on (there are particu⯑lar Paſſages left open) without ever paſſing thro' the Lungs.

The firſt of theſe Paſſages is called Foramen Ovale, which is an oval Hole opening from the right Ear of the Heart in⯑to the left, where there is a Valve ſhutting from the left Ear into the right. The ſecond is a Canal of Communication, called Ductus arterioſus, which from the Trunk of the Pulmonary Artery opens into the Trunk of the Aorta magna, or great Artery.

The Placenta, or After-birth, is a large Maſs of Blood-Veſſels like a Cake, compoſed of Arteries and Veins, by the Extremities of which it is faſtened to the inward Surface of the Womb, ſo that the Extremities of the Arteries of the Womb are joined to thoſe of the Veins of the Placenta, and the Extre⯑mities of the Arteries of the Placenta to thoſe of the Veins the Womb, by which Means the maternal Blood, by the Ar⯑teries of the Womb, is tranſmitted into the Veins of the Pla⯑centa, which Veins unite into one conſiderable one in the mid⯑dle of the Placenta, about an Ell long, which enters the Navel of the Child, paſſing through the Fiſſure of the Liver, and [14]opens in the Vena Cava, to bring the Blood in the right Ear; one Part of this Blood paſſes through the Foramen Ovale, or oval Hole, into the left Ear; the two Ears contracting them⯑ſelves force the Blood in the right and left Ventricle of the Heart, ſo that the Blood contained in the left Ventricle is for⯑ced up the great Artery, and that in the right up the Pulmo⯑nary: This Blood finding no Paſſage through the Lungs paſſes the Canal of Communication, and mixes itſelf with the Blood of the left Ventricle in the great Artery, by which it is ſent to every Part of the Body of the Child for its Nouriſhment: The remaining Quantity of Blood, which is not employed in nouriſhing the Child, returns by two Arteries, which come out at the Navel of the Child, and are convoluted about the Vein, which three Veſſels compoſe the Navel-ſtring. The Ar⯑teries, having reached the After-birth, divide into an infinite Number of Ramifications, whoſe ſmalleſt Branches unload themſelves of their Blood into the Veins of the Womb of the Mother. Thus the Child is continually nouriſhed during the Time that it is contained in the Womb, and the Circulation carried on without ever paſſing through the Lungs.

But after the Child is born, the Air finding a Paſſage into its Lungs, blows up all the Veſicles; by which Means the Preſſure is taken off from the Blood-Veſſels; at which Time, the Blood in the right Ventricle finding a free Paſſage through the Pulmonary Artery into the Lungs, paſſes no more through the Duct of Communication, but returns, by the Pulmonary Vein, into the Left, and beating againſt the Valve of the Foramen Ovale for ever cloſes it up.