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' as the Fibres are ftretch'd beyond their Tone in every ' Cohftriclion ; for this Reafon, after that Nifits is over, ' the Heart relaxes again by a natural Motion of Reditu - ' tion, and is diftcnded by the Influx of new Blood from ' the Veins. The Diaftolc, then, is not cffeflcd by any ' farther Action of the Heart befide a Remiffion of its ' former Teniion, and the Influx of the Blood.'
New, if Contraction be the fole Action of thofe Fibres, as it certainly is, and as the Author here confeffes ; how can their Diftention, vulgarly tho' improperly, call'd their Relaxation, be a Motion of Reffitution ? For, from the Nature of thofc Fibres, and their Difpofition ; the Structure of the Heart manifeftly appears tonical, and its Dilatation a State ot Violence. So that the Conftriction is the true Motion of Rcftitution, and the only State to which it will fpontaneoully return when the Force is raken off; and thus we are left ftill to feck for the true Caufe of rhe Dia- ftole, which appears the moll difficult Phenomenon relating to the Heart.
Mr. CoivJ-er, in his Introduction to his Anatomy, im- proves on the Share Dr. Jjmer hints the Blood to have in that Action, and makes it the main Inftrument of the Dilatation of the Heart ; in which he is followed by Dr. T)rake ; who, however, differs from him as to the Manner, and Reafons of its being fo.
' The Heart of an Animal,' fays Mr. Copper, ' bears a great « Analogy to the Pendulums of thole artificial Automata, c Clocks and Watches, while its Motion is performed like that ' of other Mufcles ; the Blood doing the Office of a 'Pondm.'
— If he means that the Blood, in its Reflux, by gravi- tating on the Auricles and Ventricles, dilates and expands them ; acting therein as a Counterpoize to its Contraction as a Mufclc ; 'tis pity he had not given an Explication at large of fo abflrufe and important a Phenomenon : The fpecific Gravity of the Blood, not fceming a Caufe adequate to the Effect it is here fuppofed to produce.
— For if the Blood act only as a Weight, by meer Gra- vitation ; fo much of it only as defcends from above the Heart, can be employed in that Action ; which, at the largeft Computation, does not amount to above five Pound Weight, and mutt be able, according to the Com- putation of Sorellus, to overcome a Rcfiftance of 135,000 Pound.
Whatever, therefore, the Force that dilates the Heart and is the Caufe of the Diaftole, be, it mull be equal to that of the Heart, the intercoftal Mufcles and Diaphragm ; to all which it acts as an Antagonift.
Such a Power is hard, perhaps impoffible, to be found in the Machine of an animal Body ; and yet without fome fuch Antagonift, it is as impoffible the Circulation of the Blood fhould be maintained. — ■ All the Engines yet discovered within the Body, confpire towards the Conftriction of the Heart, which is the State of Quieicence, to which it na- turally tends ; yet we find it alternately in a State of Violence, or Dilatation : And on this Alternation does all animal Life depend.
Some external Caufe, therefore, mutt be found to produce this Phenomenon ; which Caufe mutt be either in fome Quality of the Air, or the Preffure of the Atmofphere, becaufe we have no conftanr and immediate Commerce, with any other Media.
Some Phyficians obferving this, and that deprived by whatfocver Means of Communication with the external Air, we became inftantly extinct ; have imagined, that in the Act of Infpiration, certain purer Parts of the Air were mixed with the Blood in the Lungs, and conveyed along with it to the Heart, where they nourifhed a Sort of •vital Flame, which was the Caufe of this reciprocal JEfins of the Heart. See Flame.
Others, rejecting an actual Flame, have fancied that thefe fine Parts of Air, mixing with the Blood in the Ventricles of the Heart, produced an Effervefcence which dilated it. See Effervescence.
But thefe Notions have been long exploded, upon ample Conviction ; and 'tis a Point yet undetermined, whether any Air- docs mix with the Blood at all in the Lungs or not. See Lungs, i5c.
But, fuppefing fome Air do infinuate it fclf into the pulmonary Vein, it can no other Way dilate the Heart, than by an Effervefcence in the left Ventricle, which would not dilate the right. — But even this Opinion is contradicted by Autopfy, and abundantly confuted by divers Authors. See Respiration.
Upon the whole, the grofs Body of the Atmofphere appears to be the true Antagonift to all the Mufcles fcrv- ing for ordinary Infpiration, and the Conftriction of the Heart ; which is confirmed not only from its fufficient Power, but the Neceflity of its Action upon animal Bodies. See Atmosphere.
The Heart, we have obferv'd, is a folitary Mufcle, of ■very great Strength 3 and the intercoftal Mufcles and
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Diaphragm, which likewife have no Antagonifts, are a vaft additional Force, which mutt be ballanced by rhe contrary Action of fome equivalent Power, or other. For tho' the Action of the intercottal Mufcles be voluntary, it does not exempt them from the Condition of all oihcr Mufcles ferving for voluntary Motion, which would be in a State of perpetual Contraction, notwithftanding any Infl'.i ;nce of the Will, were it not for the Libration of antagonift Mufcles. This Libration, between other Mufcles, is anfwered by the Weight of the incumbent Atmofphere, which prelTes 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 b fore equili- brated ; fo here it ferves ro enable thofe Mufcles to lift a Weight too ponderous for their Strength not fo affiiled : So that as foon as that Afliitance is withdrawn, the Ribs are again depreffed by the mete Gravitation of the Atmo- fphere, which would otherwife remain elevated thro' the natural Tendency of thofc Mufcles to Contraction.
This is evidently proved from the Toricellian Experi- ment, and thofe made upon Animals in Vacuo ; where, affoon as the Air is withdrawn, and the Preffure thereby taken off, the intercottal Mufcles and Diaphragm are con- tracted, and the Ribs elevated in an Inftant, and cannot by any Power of the Will be made to fubfide, till the Air is again let in to bear them forcibly down. See Vacuum.
As in the Elevation of the Ribs, the Blood, by the Paffage open'd for it, is in a Manner follicited into the Lungs ; fo in their Depreffion, by the Subfidencc of the Lungs, and Contraction of the Blood Veffels confequent thereof, the Blood is forcibly driven thro' the pulmonary Vein into the left Ventricle of the Heart. And this, toge- ther with the general Compreffion of the Body by the Weight of the Atmofphere which furrounds and preffes on its whole Surface, is rhat Power which caufes the Blood to mount in the Veins, after the Force imprefs'd upon it by the Heart is broken .and fpenr, and which fufficcs to force the Heart from its natural State to Dilatation.
Upon computing the Weight of a Column of Air equal to the Surface of the Body, it appears a Power fufficient for the Effects here afcribed to it : And confidering that the Bodies of Animals are compreffiblc Machines, it appears that it muft of Ncceffity affect them in the Manner here laid down. — ■ And yet, tho' our Bodies be enrirely compofed of Tstbuli, or Veffels filled with Fluids, this Preffure, how great foever, being equal every Way, could not affect them, were it not that the fuperficial Dimenfions are cafily variable - becaufc, being comprefs'd on all Parts with the fame Degree ot Force, the contained Fluids could not any where begin to recede, and make Way for the reft to follow, but would remain as fix'd and immoveable, as if they were actually folid. See Fluid. — But, by the Dilatation of the Thorax, Room is made for the Fluids to move ; and by the Coarctation of it, frefh Motion is impreft ; which is the great Spring whereby the Circulation is fet and kept going.
This reciprocal Dilatation and Contraftion of the fuper- ficial Dimenfions of the Body, feems fo neceffary to animal Life, that there is not any Animal fo imperfect as to want ir, at leaft none to which our anatomical Enquiries have yet rcach'd. — For tho' moll Kinds of Filh, and Infects, want both moveable Ribs and Lungs, and confequently have no dilatable Thorax ; yet that Delia is fupplied by an- ana- logous Mechanifm, anfwering fufficiently the Neceffities of their Life. — Thus, Fifnes, which have no Lungs, have Gills to do the Office of Lungs, receiving and expelling alternately the Water, whereby the Blood-Veffels fufter the fame Alteration of Dimenfions, as in the Lungs of more perfc-a Animals. See Gills.
Add, that the Lungs, or Air- Veffels of Infects, are much more different from thofe of perfect Animals, than thofe of Fifties arc j and yet in their Aaion and Ufe, agree per- feaiy with both ; that is, removing and expelling the Air, and varying the Dimenfions and Capacities of the Blood Veffels. Having no Thorax, or feparate Cavity for the Heart and Air- Veffels, the latter are diftributed thro' the whole Trunk ; by which they communicate with the ex- ternal Air through feveral Spiracles or vent Holes, to which are fitted fo many little Trache£ or Wind-Pipes, which thence fend their Branches to all the Mufcles ant Vifcertt, and feem to accompany the Blood- Ve'tels all over the Body, as they do in the Lungs of perfea Animals. By this Difpofition, in every Infpiration, the whole Body is inflated, and in every Expiration compreffed ; and con- fequently the Blood-Veffels muft fuffer a Viciffitude of Extention and Contraaion, and a greatet Motion he thereby impreffed on the contained Fluids, than the Heart, which in thefe Creatures does not appear to be mufcular, would be capable of giving. — The only Animal, exempted from this neceffary Condition of receiving and expelling alternately fome Fluid in and out of the Body, is a Fcetus :
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