A Discourse concerning Some Influence of Respiration on the Motion of the Heart, Hitherto Unobserved. By J. Drake, M. D. F. R. S.

II. A Discourse concerning some Influence of Respiration on the Motion of the Heart, hitherto unobserved. By J. Drake, M.D. F.R.S. Tho divers accurate Treatises of the Heart, and its action, have been written by Learned Men of several Nations, especially by two of our own Country; the Great Dr Harvey, to whose happy Sagacity this Nation owes the Glory of the Invention of the Circulation of the Blood; and the incomparable Dr Lower, to whom we are beholding for a compleat display of the Mechanical Structure of the Heart, and a most ingenious Rationale of its Action. Yet there remain several doubts and difficulties about it (in my opinion) not sufficiently accounted for, towards the resolving some of which I shall offer what my own thoughts have suggested to me, and leave it to the consideration of the Reader. The Learned Dr Lower (whose accurate piece on this Argument will insure his Reputation so long as Physical knowledge shall last in esteem) has so well accounted for the Systole, or Contraction of the Heart from the Mechanical Structure of it, that he seems almost to have exhausted the subject, and had he been as happy in discovering the true cause of the Diastole, he had left little room for the Industry and Sagacity of others about this Viscus. But having judiciously and solidly explain'd the Systole, he contents himself to ascribe the Diastole to a motion of Restitution, which account gives me no satisfaction: Because the Systole being the proper, and (as himself confesses) the only motion of the Heart, a state of Contraction seems to be the natural state, and consequently without external violence, it shou'd have no Diastole at all. This will appear more plain, if we consider the circumstances of it, and its motion, as a Muscle, with respect to other Muscles. That Contraction is the proper Action, and state of all Muscles, is evident from Experience of Fact, as well as Reason. For, if any Muscle be freed from the power of its Antagonist, it is immediately contracted, and is not by any action of the Will, or Spirits to be reduc'd to a state of Dilatation. Thus, if the Musculi Flexores of any Joint be divided, the Extensores of that Joint being by that means freed from the contrary Action of their Antagonists, that Joint is immediately extended without any consent of the Will, and in that state it remains; and so Vice versa if the Extensores be divided. From whence it is plain, that the Muscles have no restitutive motion, but what they derive from the Action of their Antagonists by which they are ballanc'd. Thus likewise the Sphincters of the Gula, Anus and Vesica, having no proper Antagonists, are always in a state of Contraction, and suffer nothing to pass 'em, but what is forced thro' em by the contrary Action of some stronger Muscles, which, tho not properly to be call'd Antagonists, yet on all necessary occasions perform the office of such. That the Heart is a Muscle, furnish'd and instructed for motion like other Muscles, is (in my opinion at least) demonstrated beyond contradiction by Dr Lower and others. And, as it is a Solitary Muscle without any proper Antagonist, and not directly under the power of the Will, nor exercising Voluntary motion, it approaches nearest to the Sphincter kind, which only has these conditions in common with it. But in constant and regular alternations of Contraction and Dilatation, it differs exceedingly from all the Muscles of the Body. This reciprocal Effect of the Heart has given the Learned abundance of trouble; who, finding nothing peculiar in the structure, which shou'd necessarily occasion it, nor any Antagonist, whose reaction shou'd produce it, have been extremely perplexed to find out the cause of it. But passing over the various opinions of Authors, to avoid being tedious, I shall take notice here only of the very Learned Dr Lower's, in whose account of the Sytote, however solid and ingenious, I observe something deficient, and whose Hypothesis of the Diafote I think to be precarious and false. This excellent Author, having by sound Arguments, drawn from the Structure and Mechanism of the Heart, established the certainty of its Muscular Motion, rests satisfied, without taking notice of any Assistance, that the Heart receives from any other Part, except from the Brain, by the means of the eighth Pair of Nerves. The Accurate Borellus in his Oeconomia Animalis computes the Motive Power of the Machine of the Heart to be equal to, or to surmount that of a Weight of 3000 l. The Obstacles to the Motion of the Blood thro the Arteries he esteems equivalent to 180,000 l. which is 60 times as much as he rates the Force of the Heart at. Then deducting 45,000 l. for the Adventitious Help of the Muscular Elastick Coat of the Arteries, he leaves the Heart with a Force of 3,000 l. to overcome a resistance of 135,000 l. that is, with 1 to remove 45. This stupendous effect he contents himself to ascribe to the Energy of Percussion. But, had he proceeded in his Calculation to the Veins, which he allows to contain constantly a quantity of Blood, quadruple to the Contents of the Arteries, and to which this Energy of Percussion does either not reach at all, or but very languidly, he might probably have seen a necessity for some other Expedient to remove so insuperable a difficulty. But not to insist rigorously on the exactness of this Calculation (tho the great Abilities of the Author in this way, and his Ingenuity and Modesty, are a sufficient Warrant for the accuracy of his Computations, and the fidelity of his Accounts) we may allow a much greater Deduction, than would would be justifiable, without lessening the difficulty. But this account I have taken notice of purely for the sake of the Calculation, which may be of use in the Sequel, the account itself being in other respects more defective than Dr Lower's, to which we will return. The Dr, notwithstanding his great Sagacity, appears (to me) to have overlooked something of very great moment; and importance in the explication of the Action of the Heart. For, tho it shou'd be granted, that the Muscular Fibres of the Heart acted by the Nerves, are the immediate Instruments of its Constriction or Systole, yet it must not be denied, that the Intercostal Muscles and Diaphragm are of great service to aid and facilitate this Contraction, by opening a Passage for the Blood thro the Lungs, which denied would be an invincible obstacle. Neither do they promote it that way only. The manner how they farther assist the Heart in its Contraction, will appear manifestly, if we consider the different Posture, Situation and Capacity of the Blood Vessels of the Lungs in the several times of Elevation and Depression of the Costæ. The Pulmonary Artery rises from the right Ventricle of the Heart, and runs in one Trunk, till it comes to the Aspera Arteria, where it is divided, and sends a Branch along with each division of the Aspera Arteria, according to all the minutest subdivisions, of which it is likewise subdivided, accompanying all the Bronchi, in their whole progress thro the Lungs. The Pulmonary Vein, which empties itself into the Left Ventricle of the Heart, spreads itself on the Aspera Arteria and Bronchi, in the same manner that the Artery does. The necessary consequence of this disposition is, that this Artery and Vein being coextended with, and fasten'd to the Bronchi, must needs suffer such alteration of Superficial Dimensions, as the Bronchi do in the Elevation or Depression of the Costæ. While the Ribs are in a state of Depression (whether before Commerce with the external Air or after) the Annular Cartilages of the Bronchi shrink one into another, and by that that means their Dimensions are exceedingly contracted. In conformity to this condition of the Bronchi the Pulmonary Artery and Vein must likewise, either by means of their Muscular Coats contract themselves to the same Dimensions, or lyce in Folds or Corrugations, which is less probable. On the other hand, when the Ribs are elevated, and the Diaphragm bears downward, the Air rushing into the Lungs, shoots out the Cartilaginous Rings, and Divaricates the Branches of the Trachea, and by them extends and divaricates the several divisions of the Pulmonary Artery and Veins, and thereby lengthens and enlarges their Cavities. This enlargement of their Cavities is very considerable, not only upon the score of the addition, which they receive in length thereby, but also upon the account of their Divarication. For whereas, when the Ribs are depress'd, and the Lungs subside, the Blood Vessels are not only contracted, (as I have already observ'd) but their Branches, which are exceeding numerous, approach one another, and lie in juxta-position, by which their Cavities are very much compress'd and straighten'd: when the Ribs are elevated, and the Lungs turgid with Air, not only the Fibres, by which their Coats in the opposite State were contracted, are extended, but those innumerable Vessels, which lying before in lines almost parallel upon one another, compress'd one another, making an acute Angle at their Junctures, are divaricated and separated from each other, and make an obtuse, whereby their Channels are widen'd. Thus a passage is open'd to the Blood, from the Right Ventricle of the Heart to the Left, thro the Lungs, to which it cou'd not otherwise pass; and the opposition, which the Blood, contain'd in that Ventricle, must otherwise necessarily have made to its constriction, is taken off, and the Systole thereby facilitated. Nor is that all. For the Diastole being caus'd (as I shall in the sequel shew) by the Force of the Blood rushing into the Ventricles, this Ampliation and Extension of the Pulmonary Pulmonary Artery is a sort of Check or Counterpoise to it, and prevents an endeavour towards two contrary Actions at once, which must necessarily frustrate both. For the Heart being a Springy, Compressible Body, whose proper Action, which is Contraction, depends on the influx of certain Fluids into its Fibres, or Substance; and containing besides a Fluid in its Ventricles, or great Cavities, in one of which is the Mouth of this Artery, the action of this Vessel must in great measure resemble that of a Syringe, whose extremity is immersed in Water, the Enlargement or Expansion of the Channels of the Artery answering the drawing of the Embolus, as the Constrictive Motion of the Muscle of the Heart does the Pressure of the Atmosphere upon the Surface of the Water, the one making way for the fluid, and the other forcing it to follow, where the resistance is least. In this sense we may allow a sort of Attraction to the Pulmonary Artery, depending wholly upon the Action of the Intercostal Muscles and Diaphragm, which we must therefore confess to be very serviceable and instrumental in promoting the Systole of the Heart. But if the Learned Author be deficient in his account of the Systole; that is, if he has not observ'd all the Mechanism and Contrivance of Nature for the Contraction of the Heart; much less sufficiently has he accounted for the Diastole, or Dilatation of it, which he ascribes to a Motion of restitution of the over-strain'd Fibres, which yet he confesses are made for Constriction only. 'Tis true, he immediately after joins the influx of the Blood as a concurrent cause; but from the slight notice that he takes of it, 'tis plain, that he did not so much as dream of any great share it had in that action. His words are these. Quin & (ut obiter hoc moneam) cum omnis motus contractione persiciatur, Cordis Fibrae ad constrictionem solum factae sint, apparet quoque Cordis motum totum in Systole positum esse; cumq; Fibrae ultra tonum suum in omni constrictione ejus tendantur, idcirco ubi nixus iste absolviatur, motu quasi restitutionis Cor iterum iterum relaxatur, & sanguine a Venis influente rursus distenditur; a nullo enim cordis motu, nisi tensionem suam remittente, & ab irruente sanguine Diastole ejus libris ad eo viribus succedit. I have transcribed the entire Paragraph, because it contains his whole Hypothesis of the Diastole, and all the notice that he takes of it thro his whole Work. But how slender soever this may prove, it is the most substantial that I have anywhere met with, except a late one of Mr Cowper, which is properly an improvement of this, and shall be considered in the sequel. But, if Contraction be the sole Action of these Fibres (as this Great Man confesses it to be) and as indeed it is of all Muscular Fibres, I wonder how so judicious a Writer came to slip into such an absurdity, as to call their Distension (vulgarly but improperly called Relaxation) a Motion of Restitution. For from the Nature of those Fibres, and their disposition in the Structure of the Heart, the Natural State of the Heart appears manifestly to be Tonical, and its Dilatation a State of Violence; and consequently the Constriction is the true motion of Restitution, and the State to which it will spontaneously return, when the Force is taken off, which is the work of the Intercostal Muscles and Diaphragm. Thus we are left still to seek for the true cause of the Diastole, which seems to me to be the main and most difficult Phenomenon, relating to the Heart and the Circulation of the Blood. But in Mr Cowper's ingenious Introduction to his Anatomy of Human Bodies, I find the Share which Dr Lower hints the Blood to have in that Action, further prosecuted, and improv'd into the main Instrument of the Dilatation of the Heart, wherein I agree entirely with him. But as to the manner, and reasons of its being so very Instrumental, I can't be so perfectly of his mind. The Heart (says this accurate Anatomist) of an Animal bears a great Analogy to the Pendulums of those Artificial... Automata, Clocks and Watches, whilst its Motion is performed like that of other Muscles, the Blood doing the Office of a Pondus. This Explication, being but a Simile without a distinct application to particulars, is beside so very short, that I can at best but give a conjecture at the meaning, which if I mistake, I shall deserve to be excused, and expect to be better inform'd. By the Blood's doing the Office of a Pondus, I suppose he means, that the Blood contributes in the same manner to the Motion of the Heart, as the Weights do to that of the Pendulum of a Clock. If so, the Blood, according to him, must be the Instrument of Constriction; and Dilatation must be the Natural State, or Spontaneous Motion, to which it would, when under no Violence, return; the contrary of which, I presume, will appear e're I have done. But if he means, that the Blood in its reflux, by gravitating on the Auricles and Ventricles, dilates and expands 'em, acting therein as a Counterpoise to its contraction as a Muscle, I cou'd wish his design had not bound him up to so narrow a compass, and that he had given us an explication at large of so abstruse and so important a Phenomenon. Because the Specifick Gravity of the Blood seems to me a cause by no means alone adequate to the effect, which it is here suppos'd to produce. For, if the Blood acts only as a weight by meer gravitation, then that part of it only which descends from the Parts above the Heart can be employ'd in that action. This at the largest computation can't amount to five pound weight, and must, according to the computation of Borellus, force a Machine, that is able to overcome a resistance of 135,000l. I leave every man to deduct what he shall upon examination find reasonably to be deducted, and yet shall rest secure, that it is not to be effected in the least with so small a Weight. But neither does the Refluent Blood gravitate in any such proportion, as I have here assign'd. For to make a true estimate of its Gravitation, we must consider the circumstances of the Liquor suppos'd to gravitate; in which it very much resembles Water inclos'd in a recurve Tube, of which, if the length of the two Legs be equal, it may be suspended in the Air full of Water, with the Extremities downwards, without losing a drop, altho the Diameter of those Legs shou'd be very unequal. The Case of the Arteries and Veins is pretty near a parallel to a Tube, so fill'd and inverted. For, if the Arteries and Veins be continued Tubes, (as by the Microscope they are made to appear) then supposing their contents to have no other determination of Motion, than their own weight wou'd give 'em, the contain'd Fluids must be counterpoises to each other. For the Veins and Arteries being join'd at the smaller extremities, and the larger of both terminating in the same Parallel Line, it is impossible, according to the Laws of Hydrostatics, that the contents of either shou'd overballance t'other. How far then must it fall short of forcing the natural Power and Resistance of so strong a Muscle as the Heart, by meer Gravitation. The Blood indeed has a Progressive Motion thro its Vessels, wherein it differs from Water, in a recurve Tube, in the experiment above stated. But, if the natural Gravitation of the Blood contributes nothing to the Dilatation of the Heart, this Progressive Motion will not be found much more sufficient. For, as this Motion is deriv'd entirely from the Heart's Constriction (as all accounts hitherto derive it) cou'd the Blood be suppos'd to re-act upon the Heart, with all the force first impress'd upon it by the Heart, it would be insufficient, unlets we will suppose the Force communicated to be superior to the Power Communicant, which is absurd. But when the just and necessary Deductions for the Impediments, which the Blood meets with in its Progres thro the Vessels, shall be made, the remaining Force will be found so exceeding weak, that to prop the Blood thro the Veins may be a task alone too great for so small a Power, without charging it with the additional difficulty of forcing the Muscle of the Heart. Alphonsus Borellus, after a great deal of solemn pains taken to shew his care and exactness, and to possess his Reader of the truth of his Calculations, casts up the Force of the Heart, and the Muscular Coat of the Arteries, to be together equal to a weight of 3,750 l. and allots 'em a Resistance equal to 180,000 l. to overcome which is 45 to 1. To make up for a disproportion, by his own confession, incredible to those who have not considered the matter as he had done, he flings into the Scale the additional Force of Percussion, which he leaves indefinite, and thinks sufficient to force any quiescent finite Resistance whatsoever. But as this Account and Hypothesis are part of a Posthumous Work (if a liberty of Conjecture may be allow'd in so uncertain a matter,) I shou'd suspect, that these Papers were left unfinished by Borellus; or at least, that in many places the last hand was never put to 'em. For, neither in this Place, nor any other of this Work, does he account for any more than the Systole of the Heart, and the resistance which is made to the Progressive Motion of the Blood in the Arteries only. This alone he found to exceed the Power of the Heart so prodigiously, that he seems to shuffle it off his hands with a general and precarious Solution, as a difficulty that he was desirous to be rid of. For, having ascrib'd this stupendous (as he himself calls it) effect to the Energy of Percussion, he takes no care to satisfy his Reader any farther about it, or to refer him, or give him the expectation of satisfaction anywhere else; altho he has an express Treatise on the Force of Percussion, which was written preparatory to this, and to which he frequently refers in other Places of this Work. But what confirms my suspicion, that this part was intended for a farther Revise by the the Author, is, that he has left the Progress of the Blood thro the Veins, and the Diastole of the Heart, absolutely untouched, tho they are difficulties of a much greater Magnitude, than this which he has attempted to account so lightly for. For, in these he is excluded the benefit of Percussion, and has yet a greater resistance to overcome without it. Omissions of this kind are so unusual with this Author, where-ever he knows himself to go upon sure grounds, that it is to me an Argument, that he doubted the sufficiency of his Percussion, and reserv'd these important Phenomena for farther consideration, without plunging himself into such an Absurdity as to ascribe to Percussion any such Energy as to be able (so broken as it returns to the Heart) by its re-action to force that Power, from whence only it was at first deriv'd. Dr Lower and Mr Comper deliver their Opinions of the Cause of the Dilatation of the Heart so very short, and without any Arguments to support 'em, that by exposing 'em naked, they seem rather to discourse of it transiently, as men obliged by the nature of their Subjects to say something of it, than solicitous to give any full or satisfactory Account, and therefore I shall proceed no farther upon 'em here. But tho the Hypothesis of Borellus may in this case be found precarious or insufficient (a misfortune that has befallen him in divers other particulars) his Theory holds still good. At least it ought to be allowed in justice to his great Abilities and Exactness, till some body convicts him of some material Error in his Calculations, which has not as yet been done by any body, that I know of. Supposing then the Force of the Heart, and of the Muscular Coat of the Arteries, as likewise of the resistance, which they must overcome, to be computed with any degree of accuracy, there remains yet such a prodigious disproportion to be accounted for, as requires some more powerful Agent, than any yet assign'd, to make up the deficiency. What assistance the Heart receives from the action of the Thorax towards facilitating its contraction, without which assistance there could have been no Systole, has been already shewn. But neither the Intercostal Muscles, or Diaphragm, which are so instrumental in that part of its action, can contribute anything to the Diastole; because they serve only to enlarge the cavity of the Thorax, and thereby to open a Passage to the Blood from the Heart, and promote its Constriction. Whatever therefore the Force is, that dilates the Heart, and is the Cause of the Diastole, it must be equal to that of the Heart, the Intercostal Muscles and Diaphragm; to all which it acts as an Antagonist. I take no notice of the Serratus Major Anticus, and other Muscles, which have an obscure share in the Elevation of the Costæ, because as much may reasonably be deducted upon the account of the Obliquus externus Abdominis, and other Muscles; which, having their insertions on some of the lower Ribs, are as instrumental towards the Depression of them, and so balance the account. But the chief use of these is in violent Respiration. In ordinary Respiration their share is small. Such a real Power (which may in the least be suspected of any share in this Action) is hard, perhaps impossible to be found in the Machine of any Animal Body; and yet without some such Antagonist, it is as impossible the Circulation of the Blood should be maintain'd. All the Engines yet discover'd within the Body conspire towards the Constriction of the Heart, which is the State of Quiescence, to which it naturally tends. Yet we find it alternately in a State of Violence, that is, of Dilatation; and this upon necessity, because upon this Alternation depends all Animal Life. Some sufficient Cause external must therefore be found, to produce this great Phænomenon, which Cause must be either in the Air, or Atmosphere, because we have no constant and immediate Commerce with any other Medians. Some great Physicians observing this, and that deprived by whatsoever Means of Communication with the external Air, we became instantly extinct, have imagin'd, that in the Act of Inspiration certain purer parts of the Air mixed with the Blood in the Lungs, and was convey'd with it to the Heart, where it nourish'd a sort of Vital Flame, which was the cause of this reciprocal Aëtus of the Heart. Others not quite so gross, rejecting an Actual Flame, have fancied that these fine Parts of Air mixing with the Blood in the Ventricle of the Heart, produc'd an Effervescence which dilated it. But these Fancies have been long since exploded and condemn'd upon ample Conviction, and 'tis a Point yet undetermined, whether any Air does mix with the Blood at all in the Lungs, or not. But supposing that some Air may insinuate itself into the Pulmonary Vein, it can no other way dilate the Heart than by an Effervescence in the Left Ventricle, which would not dilate the Right. But this opinion is contradicted by Autopsie, and too laboriously confuted by others, to be brought upon the Stage again here. There remains therefore only the gross Body of the Atmosphere to be consider'd, which is undoubtedly the true Antagonist to all those Muscles, which serve for ordinary Inspiration and the Constriction of the Heart. This will appear more evidently, if we consider not only the Power, but the necessity of its Action upon Animal Bodies, as well as the want of other sufficient Agents. The Heart is a Solitary Muscle of very great strength, and the Intercostal Muscles and Diaphragm, which likewise have no Antagonists, are a vast additional Force, which must be ballanc'd by the contrary action of some equivalent Power or other. For, tho' the Action of the Intercostal Muscles be voluntary, that does not exempt 'em from the condition of all other Muscles serving for voluntary Motion, which would be in a State of perpetual contraction, notwithstanding any Influence of the Will, were it not for the libration of Antagonist Muscles. This libration between other Muscles is answer'd by the Weight of the incumbent Atmosphere, which presses upon the Thorax and other Parts of the Body. And, as in all other voluntary Motions the influence of the Will only gives a Prevalence to one of two Powers before equilibrated, so here it serves to enable those Muscles to lift up a Weight too ponderous for their strength not so assisted; and therefore as soon as that assistance is withdrawn, the Costæ are again depress'd by the meer Gravitation of the Atmosphere, which wou'd otherwise remain elevated thro' the natural tendency of those Muscles to contraction. This is evidently prov'd from the Torricellian experiments, and those made upon Animals in Mr Boyle's Engine; where, as soon as the Air is withdrawn, and the pressure thereby taken off, the Intercostal Muscles and Diaphragm are contracted, and the Ribs elevated in an instant, and can't by any power of the Will be made to subside, till the Air is again let in to bear 'em forcibly down. It were scarce worth while to take notice here of a Mistake of the Learned Dr Willis, were it not for the Great Authority of the Man, which is almost sufficient to keep Error in countenance. The Dr having observ'd that the Fibres of the External and Internal Intercostal Muscles ran in a contrary order, as it were, decussating each other, takes occasion from thence to fancy, that there was an opposition in their Office, and that as the External serv'd to raise up the Ribs, the Internal drew 'em down again, forgetting at that time, That, when a contractile Body is fasten'd at the several ends to Points unequally movable, let the Contraction happen in what part or manner foever, the more movable Point must be drawn towards the less movable: By which rule, whether External or Internal Intercostals be contracted, the lower Ribs will be forc'd to approach the upper, that is, be rais'd up. As in the Elevation of the Costæ, the Blood, by the passage that is open'd for it, is in a manner solicited into the Lungs, so in the Depression of 'em, by the subsidence of the Lungs and the Contraction of the Blood Vessels, both which are consequent thereof, the Blood is forcibly driven, as it were with an Embolus, thro' the Pulmonary Vein into the Left Ventricle of the Heart. And this, together with the general Compression of the Body by the weight of the Atmosphere, which surrounds and presses upon the whole Surface of it, is that Power which causes the Blood to mount in the Veins, after the force impress'd upon it by the Heart is broken and spent, and which is sufficient to force the Heart from its natural State to dilatation. He that is able to compute the weight of a Column of Air, equal to the Surface of the whole Body, will readily grant it a Power sufficient for the Effects, which are here ascrib'd to it. And, when he considers, that the Bodies of Animals are compressible Machines, he will find that it must of necessity affect them in the manner here laid down. But tho' our Bodies be entirely compos'd of Tubuli, or Vessels fill'd with Fluids, yet this Pressure, how great soever, being equal, cou'd have no effect upon 'em, if the superficial Dimensions were not easily variable; because being compris'd on all parts with the same degree of Force, the contain'd Fluids cou'd not any where begin to recede, and make way for the rest to follow, but wou'd remain as fixt and immovable as if they were actually solid. But by the Dilatation of the Thorax, room is made for the Fluids to move, and by the Coarctation of it fresh motion is imprest, which is the main Spring whereby the Circulation is set and kept going. This reciprocal Dilatation and Contraction of the Superficial Dimensions of the Body seems so necessary to Animal Life, that there is not any Animal so imperfect as to want it, at least none to the inward Structure, of which our Anatomical Discoveries have yet reach'd. For, the most kinds of Fish, and Insects, wants both moveable Ribs, and Lungs, and consequently have no dilatable Thorax, yet that want is made up to 'em by an Analogous Mechanism, answering sufficiently the Necessities of their Life. Those Fishes, which have no Lungs, have Gills, which do the Office of Lungs, receiving and expelling alternately the Water, whereby the Blood Vessels suffer the same Alteration of Dimensions, that they do in the Lungs of more perfect Animals. The Lungs or Air Vessels of Insects are yet exceedingly more different in Structure, Distribution and Situation from those of perfect Animals, than those of Fishes are, and yet in their Use and Action agree perfectly with both, that is, receiving and expelling the Air, and varying the Dimensions and Capacities of the Blood Vessels. These having no Thorax, or separate Cavity for the Heart and Air Vessels, have the latter distributed thro the whole Trunk of their Bodies, by which they communicate with the External Air thro several Spiracula or Vent Holes, to which are fasten'd so many little Tracheæ, or Wind-pipes, which thence send their Branches to all the Muscles, and Viscera, and seem to accompany the Blood Vessels all over the Body, as they do in the Lungs only of Perfect Animals. By this disposition in every Inspiration, the whole Body of these little Animals is inflated, and in every expiration compress'd, and consequently the Blood Vessels must suffer a vicissitude of Extension and Contraction, and a greater motion must thereby be impress'd upon the Fluids contain'd in 'em, than the Heart, which does not in these Creatures appear to be Muscular, seems capable of giving. The only Animal that is exempted from this necessary condition of Breathing, or receiving and expelling alternately some fluid into and out of the Body, is a Fetus. But this, while included in the Womb, has little more than a vegetative Life, and ought scarce to be reckon'd among the number of Animals. For, were it not for that small share of of Muscular Motion, which it exercises in the Womb, it might without absurdity be accounted for as a Graft upon, or Branch of the Mother. Concerning the immediate matter, and means of Life, and Nutrition, Authors are not agreed, nor is it the business of this place to reconcile, or decide their differences, but to account for the motion of the Blood thro the Vessels only. In order to this, it will be necessary to observe, that the Pulsation of the Heart in a Fœtus is so very weak and obscure, and the motion of the Blood so extreme slow and languid, as to be scarce, if at all, perceivable, as has been experienc'd in the Dissection of Puppies before Respiration had. To produce such a feeble Palpitation, and creeping Motion, no greater force seems to be requir'd than may be deriv'd from the Communication between the Vessels of the Mother and Fœtus in the Placenta. I am not ignorant, that divers very Learned Anatomists (whom the Crowd have implicitly follow'd) have absolutely rejected all Communication between these Vessels. But, with submission to Great Authorities, I think they have acted arbitrarily, and without sufficient Warrant from Reason or Experiment. For neither are the Arguments which they bring against it conclusive, nor the Office which they assign to the Umbilical Vessels in lieu of it, proper, or natural to those Vessels, or the reality of the Fact made out by any substantial Reasons. Those that reject this Communication usually do it in favour of one or both of these Opinions, that the Arteries of the Uterus do deposite a Nutritive Juice, or a Juice impregnate with Air in the Placenta, which is suck'd in by the Umbilical Vein, and convey'd to the Fœtus, for the necessary Uses of Nutrition and Life. Now those that patronize either of these Opinions lead Nature an unnecessary dance. For if the Maternal Blood does really contain any such Nutritious, or any such necessary Aerial Particles, why should they be separated and extravasated, to be with difficulty receiv'd into the Umbilical Vein, and again mixt with the Blood, when they might more easily have been imparted by the plain simple way of Transfusion from the Arteries of the Mother to the Veins of the Fetus. And, that this is the course which Nature takes in this case, I am perswaded from the easiness and simplicity of the Method, which readily performs what might be perhaps in vain expected from another, and would over and above find 'em, what they seem to grope so blindly about for, a first Mover of the Blood in a Fetus. Those that contend for the conveyance of a Nutritious Juice, thro the Umbilical Vein from the Placenta, are forc'd upon two difficulties next to Absurdities. For first, they are oblig'd to make this Vein, which, as all other Veins, seems dedicated to the re-conveyance of Blood only, the proper and immediate Channel, thro which a very different Liquor is to be carried; and next to give a Power of Attraction or Suction to it; because the Nutritious Juice, which it is thus destin'd to carry is both Viscous and Stagnant, and has neither force to drive, nor subtilty to penetrate, or insinuate itself into the Capillary Veins; and therefore must be drawn or suckt as Milk is from the Breast, to which the Placenta and its Nutritious Juice are by the favourers of 'em expressly compared. But if this were the sole use of the Placenta, and Umbilical Vessels, why were the Umbilical Arteries sent along with the Vein? Their business is not to bring any thing back to the Fetus, nor can they contribute any thing to the benefit of the Mother; for the Uterine Arteries bring all to the Placenta, the Umbilical Vein carries it to the Fetus, and the Uterine Veins convey back again the Surcharge of the Mothers Blood; the Umbilical Arteries only, have nothing to do, and are superfluous and impertinent, which is contrary to the constant practice of Nature. Yet if Autopsie did in the least countenance this Hypothesis, some defence might still be made; but we find in the Umbilical Vein of a Fetus nothing but Florid Blood, such as in all probability it received immediately mediately from the Arteries of the Mother without any mixture. And therefore I can't help concluding, that this opinion engages its favourers in some Absurdity, without Necessity and without Proof. They that from the Placenta supply the Body of the Fetus with Air, are as much distress'd as t'other, for they are forc'd to beg the question twice, which, even when granted, will not answer their ends. First, they suppose, that an intimate mixture or confusion of Air with the Blood, is necessary for the support of Animal Life, a Postulate, which perhaps the former part of this Discourse may have render'd unnecessary; and next that the Fetus is supplied with Air from, and its Blood mixt with it in the Placenta. But here again they fetch a Compass without necessity or proof. For if a mixture of Air were necessary to a Fetus, why should it be separated from the Mothers Blood, and not rather both communicated together, since it is so much more easy and commodious. But neither does the Placenta seem to be instructed and provided for the separation of Air, but of a much groser Fluid, destin'd to some other use, which Autopsie confirms. Yet were both these opinions true, they are however defective, and the Circular Motion of the Blood unprovided for. By the way of Transfusion this great Phænomenon is naturally accounted for, and the ends, for which the other two Hypotheses were devis'd, might both be answered with more ease. For the Hysterick Arteries transmitting their Blood immediately to the Umbilical Vein, may very easily transmit such Nutritious Juices or Aerial Particles as are contain'd in the Blood, along with it, without depositing 'em by the way. By this means so much of the Impulse of the Mothers Blood is preserv'd, as suffices to maintain that languid circulation, which a Fetus enjoys. For the Blood being driven thro' the Arteries of the Uterus into the Umbilical Vein, is convey'd directly to the Sinus of the Porta, and thence by a short and direct passage thro' the Cava to the Heart; where passing thro' the Foramen Ovale to the Left Ventricle, and thro' the Canalis Arteriosus from the Right and Pulmonary Artery, it is all deliver'd without coming at the Lungs, to the Aorta, and from thence again by the Umbilical Arteries to the Veins of the Uterus, making a sort of Epicycle to the main Circulation in the Mother. As this Opinion is favour'd by the structure and disposition of the Blood Vessels on both parts, so there is nothing in it difficult to be conceived, or repugnant to experience. Late discoveries have made it appear, that the Arteries and Veins are continued Tubes, and that the latter contain nothing but what they receive from the former, and no reason appears why we shou'd think this Method to be varied in the Placenta. On the other hand, if the Arteries of the Uterus were continued to the Veins of the same part, and those of the Fœtus in like manner, without communicating with each other, their confluence in the Placenta seems to be altogether impertinent and of no use, and the Umbilical Arteries and Vein fram'd for no other service or purpose, than to give the Blood room for an idle Sally. Thus the reasonableness of this old Opinion may be vindicated, but the certainty of it rests upon stronger proof. Mr Comper, to whose happy Industry we owe the Confirmation of many Antient Discoveries, and the benefit of some new ones, has the honour to re-establish this old, but long exploded Truth. For by pouring Mercury into a Branch of the Uterine Artery of a Cow, that went into one of the Cotyledones of the Uterus, he fill'd those Branches of the Umbilical Veins, which went from that Cotyledon to the Navel of the Fœtus; which with a part of the Uterus he keeps prepar'd by him. It wou'd be a weak objection, to alledge that the Observation and Experiment being made on the Uterus of a Cow, the inference wou'd not hold from thence to a Woman, the one being Glanduliferous, and the other Placentiferous; since since ev'ry one of these Cotyledones, or Uterine Glandules, is in all respects a little Placenta, and all the difference between 'em is in number, name, and magnitude. Why Ruminants differ in this particular from other Viviparous Animals; is beside the subject of our present Enquiry. But the great Flux of Blood which constantly follows upon drawing the Placenta from Women (which is frequently so great as to cost 'em their Lives) is as plain a demonstration to Reason of the Continuity of the Vessels, as Mr Comper's Experiment is to the Eye. I have heard it objected by very learned men, that if there were such a Continuity of Vessels, and such Transfusion of Blood, the Fetus must necessarily perish thro loss of Blood, upon the separation of the Placenta from the Uterus; but that on the contrary no visible flux of Blood does follow while the Fetus continues wrapt in the Membrane, in which condition it may be kept alive some hours. To this it may be answer'd, that the Circulation in the Fetus being deriv'd from the Mother, may be suppos'd wholly to cease upon the cutting off the communication between 'em, till it is again renew'd more forcibly by respiration. But if we allow the Motion already impress'd upon the Blood to be sufficient to keep it going a little while, yet it must needs be so exceeding languid, that the meer resiftance of the external Air must be more than enough to hinder any Efflux of Blood from a Fetus before Respiration. How long Life may be preserv'd without an actual Circulation of the Blood, is a question not of this place. But we have been convinc'd by many and notorious Observations and Experiments, that Life has been recover'd a long time after all tokens of Respiration, Circulation, or even Life itself have disappear'd, so that we can't think the first solution either impossible or improbable. I expect to be told, that in the early days of Gestation in Viviparous Animals there is no Placenta, or any Adhesion of the Umbilical Vessels to any part of the Mother, and consequently no such Transfusion; and that in Oviparous there is no continuity, or communication of Vessels of any kind, during the whole time of Incubation. But these Objections carry neither the Weight nor Difficulty along with 'em, that they may be suppos'd to do; for in those days there is neither Blood nor Blood Vessels, and consequently there can be no Circulation of the Blood; and the Embryo, of what Species soever, is no more than a Vegetable at that time; nor does the Fetus of any Viviparous Creature enjoy any Circulation, or shew any signs of Animal Life, till after those Vessels, as well as others requisite to the Circulation, are compleated. It must be confess'd, that Oviparous Animals are denied the Benefit of this Communication: But that want is sufficiently compensated by a peculiar Mechanism, which directly answers the ends of Respiration, and the Pressure of the Atmosphere upon the Fetus. There is at the Obtuse end of an Egg a small Cavity fill'd with Air, which is the Succedaneous Instrument to the Respiratory Organs. For as soon as the Contents begin to be warm'd by the Incubation of the Hen, or any analogous Heat of Furnace or Dunghill, the several Humours of the Egg require a Fermentative Motion, and the Air contain'd in the Cavity or Vesicle at the obtuse end of the Egg is rarefied, and the Vesicle extended and enlarg'd, and consequently the other contents are compris't; to which the Fermentative Motion naturally resists. But both Bodies being as well compressible as dilatable, and both having an Expansive Motion impress upon 'em by Incubation, the compression and renitency will be mutual; but varied in degree, according as either, thro' the variation of Circumstances, shall prevail. By this means, an Alternation of Compression and Dilatation will be produc'd in both answering the Respiratory Motion, by which a Motion will be communicated, which, as soon as the Organs by which it shou'd be regulated are compleated, will in the Body of the Pullus be regular and circulatory. Fabritius ab Aquapendente, and after him our Great Dr Harvey, have assign'd divers uses to this Cavity or Air Vesicle, the Extravagance of which have perhaps deter'd others others from enquiring so much into the Use, as the importance of it requir'd. But tho I can't agree to that Perspiration, Refrigeration, and Respiration, which they make it the Instrument of, yet perhaps the Air, that was inclos'd in that Cavity, may thro the augmentation of the Body of the Pullus, and its own Rarefaction (which is at last so great as to occupy half the Shell) break the Membrane, which separated it from the Pullus, and thereby give so much Respiration as to form the chirping Voice, which is often heard before the breaking of the Shell, and with it give an Addition of Strength to enable it to break the Shell. But how it shou'd respire sooner is to me inconceivable. There are many Problems of great seeming difficulty, the solutions of which flow naturally from what has been laid down here: But intending to prosecute this Subject farther, and to treat of the Impediments of Respiration, and the consequences of Respiration obstructed or intermitted, I shall reserve 'em for that opportunity, and content myself here to attempt the Harveyan Problem only, which has given abundance of Authors so much perplexity. That incomparable Philosopher enquires, Why a Foetus, taken out of the Uterus with the Membranes entire, shall live in Water some hours without communication with the external Air; whereas if it be taken out and suffer'd once to breath, it can't afterwards survive a moment without the benefit of Respiration. Granting the Fact to be as he has deliver'd it, which yet is not so in all cases, the main difficulty is grounded on a Mistake, which from the stating of the Question I find this Great Man to have slipt into. For he thinks, that a Foetus is sooner suffocated after having once breath'd, than if it had not breath'd at all, and that by breathing it had contracted something which render'd it more perishable. Idem tamen Harv. de Gen. secundis exutus, (says he) si semel acrem intra Pulmones attraxerit, postea ne momentum quidem temporis absq; eo durare de parte possit, sed confestim moriatur? And presently after, Siquidem The Dr, observing a Fœtus to live longer without Respiration, and to dispence better with the want of Air while included in the Membranes entire, than it cou’d afterwards; infers thence, that the Air does in the first Act of Inspiration impress upon the Lungs some quality, which renders it ever after more indispensably necessary. But allowing his Observation, I must yet deny his Inference to be good: For, deprive a Fœtus of means of respiring, and then take it out of the Membranes, and it shall be as soon suffocated, as if it had respired before. This proves, that this Necessity of intercourse with the Air by way of the Lungs is not the Offspring, but the Parent of Respiration, and that, that Learned Man was drawn into a Fallacy of Non causa pro causa. The reason of this Necessity is the pressure of the External Air upon the Surface of the Body, from which it was defended by the interposition of the Membranes and the Humours contain’d, which are not so compressible, as the Body of the Fœtus itself. So soon therefore as the Fœtus is excluded, and expos’d to the immediate contact of the ambient Atmosphere, the Vessels and all the Cavities of the Body must necessarily be so compress’d, that the Fluids can’t have room for Motion, and consequently the Fœtus cou’d have no Life, if Nature had not contriv’d by the Motion of the Thorax to remove and admit that pressure alternately, and thereby to impress a Motion on the Fluids, which is the Spring of Life. But this Motion of the Thorax being any way suppress’d, the equal pressure of the Atmosphere on all parts occasions a total Cessation of Motion, which is Death. I shall prosecute this Subject no farther now, nor trouble the Reader with any Apology, for dissenting from those great Men herein named: Because, I hope, I have done it with Modesty, and all the Respect due to so great Authorities, and have assign’d nothing which is not matter of Fact uncontroverted, or deduc’d from it by plain Mechanical Necessity.