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as Des Cartes and others would have it; but by the Coats of the Arteries themselves, which

Cartes Ep. V. Ep. 776 77 & Seq.

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the phyfics of a certain Lo

vain (the first whereof is Galen's) do, in my Opinion, make good against him. Firft, faith he, if you fit the Artery, and thruft into it a Pipe, fo big as to fill the Cavity of it, and cafta ftrait Ligature upon that Part of the Artery containing the Pipe, and fo bind it faft to the Pipe; notwithstanding, the Blood hath free Paffage through the Pipe, yet will not the Artery beat below the Ligature; but do but take off the Ligature, it will commence again to beat immediately. But because one might be ready to reply to this Experiment, that the Reafon why when bound it did not beat, was, becaufe the Current of the Blood being straitned by the Pipe, when beneath the Pipe it came to have more Liberty, was not fufficient to stretch the Coat of the Artery, and fo caufe a Pulfe, but when the Ligature was taken off, it might flow between the enclofed Tube and the Coat of the Artery; therefore he adds another, which clearly evinces that this could not be the Reafon, but that it is fomething flowing down the Coats of the Artery that caufes the Pulfe: That is, if you ftraiten the Artery never fo much, provided the Sides of it do not quite meet, and ftop all Paffage of the Blood, the Veffel will, notwithstanding, continue still to beat below, or beyond the Coractation. So we fee fome Phyficians, both Ancients, (as Ga

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len) and Modern, were of Opinion, that the Pulfe of the Arteries was owing to their Coats; though the first, that I know of, who obferved the third Coat of an Artery to be a muscular Body, compofed of annular Fibres, was Dr. Willis. This Mention of the peristaltick Motion, puts me in mind of an ocular Demonftration of it in the Gullet of Kine when they chew the Cud, which I have often beheld with Pleasure. For, after they have fwallowed one Morfel, if you look ftedfaftly upon their Throat, you will foon fee another afcend, and run pretty swiftly all along the Throat up to the Mouth, which it could not do, unless it were impelled by the fucceffive Contraction, or periftaltick Motion of the Gullet, continually following. And it is remarkable, that thefe ruminant Creatures have a Power by the Imperium of their Wills, of directing this peri ftaltick Motion upwards or downwards. I fhall add no more concerning the Heart, but that it and the Brain do mutuas operas tradere, enable one another to work: For, Firft, the -Brain cannot itfelf live, unless it receive continual Supplies of Blood from the Heart, much lefs can it perform its Functions of preparing and diftributing the Animal Spirits, nor the Heart pulfe, unless it receives Spirits, or fomething else that defcends from the Brain by the Nerves. For do but cut afunder the Nerves that go from the Brain to the Heart, the Motion thereof, in more perfect and hot CreaT 2 tures,

Part II. tures, ceaseth immediately. Which Part began this Round is the Queftion.

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I find, in the Philofophical Tranfactions, Numb. 280. fome notable Obfervations of the famous Anatomift Mr. William Cowper, concerning the Artifice of Nature in regulating the Motion of the Blood in the Veins and Arreries, to affift and promote it in the one, and moderate it in the other, which I fhall give you in his own Words.

As the Arteries (faith be) are known to export the Blood, fo the Veins to carry it backagain to the Heart; but having already defcribed their Extremities, we come now to the large Trunks of the Veins, and here, as in the Arteries, we find the common Practice of Nature, in difpofing the Branches of Veins to difcharge the refluent Blood into the next adjacent Trunk, and fo on to the Heart. As the Arteries afford abundance of Inftances of Checks given to the Velocity of the Current of Blood through feveral Parts, fo the Veins fupply us with as many Artifices, to affift its regular Return to the Heart, as well as favour thofe Contrivances in the Arteries.

The carotid, vertebral, and splenick Arteries, are not only variously contorted, but also here and there dilated to moderate the Motion of the Blood; fo the Veins that correspond to thofe Arteries are alfo variously dilated. The Beginnings of the internal Jugulars have a bulbous Cavity, which are diverticula to the refluent Blood in the Sinus's of the dura Mater,

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left it should defcend too faft into the Jugulars. The like has been taken Notice of by Dr. Lower in the vertebral Sinus's. The Splenick Vein has divers Cells opening into it, near its Extremities in Humane Bodies; but in Qua drupeds, the Cells open into the Trunks of the Splenick Veins.

The Spermatick Veins do more than equal the Length of the Arteries of the Teftes in Men; their various Divifions and feveral Inofculations, and their Valves are admirably contrived to fufpend the Weight of the Blood, in order to discharge it into the larger Trunks of the Veins; and were it not that, the refluent Blood from the Teftes is a Pondus to the influent Blood from the Arteries, and ftill leffens its Current in the Teftes, thefe Spermatick Veins, like those of other Parts, might have difcharged the Blood into the next adjacent Trunk.

Who can avoid Surprize at the Art of Nature, in contriving the Veins that bring Part of the refluent Blood from the lower Parts of the Body, when they confider the neceffity of placing the Humane Heart, as well as that of moft Quadrupeds, fo far from the Center of the Body towards its upper Part. It is for that End neceffary, that the large Trunks of the Veins and Arteries fhould not affociate each other. For if all the Blood fent to the lower Parts by the defcending Trunk of the Aorta, fhould return to the Heart again by one fingle Trunk, (as it is fent out from thence) the

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Part II. Weight of fo much Blood in the afcending Trunk of the vena Cava would oppofe all the Force the Heart could give it from the Arteries, and hinder its Afcent. For this Reafon the vena Azygos, or fine Pari, is contrived to convey the Blood fent to the Muscles of the Back and Thorax, into the defcending Trunk of the Vena Cava above the Heart. Hence, it's evident, that more Blood comes into the Heart by the defcending or upper Trunk of the Vena Cava, than paffes out by the afcending Trunk of the Aorta. Nor does the Quantity of Blood convey'd to the Heart by the fuperiour Trunk of the Cava, feem, without fome other Defign in Nature befides tranfporting it thither, to free the inferiour Trunk from its Weight. But perhaps it was neceffary fo much Blood fhould be ready there to join with the Chyle, for its better Mixture, before it reaches the right Auricle of the Heart. So far Mr. Cowper.

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Eighthly, The next Part I fhall treat of fhall be the Hand, this opravou oprávy, or fuperlative Inftrument, which ferves us for fuch a multitude of Ufes, as it is not eafy to enumerate, whereto, if we confider the Make and Structure of it, we fhall find it wonderfully adapted. First, It is divided into four Fingers bending forward, and one oppofite to them bending backwards, and of greater Strength than any of them fingly, which we call the Thumb, to join with them feverally, or united; whereby it is fitted to lay hold of Objects

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