the natural contraction is; but that it is alfo of a different kind from that of the sphincters and mufcles without antagonifts; referring it, from its phenomena, to the contraction from a ftimulus. And the 18th and laft affirms, the mind may, by difufe, not only lose the power of moving the voluntary muscles, except in a particular way, but even that of contracting them at all. The uniform motions of the eyes are mentioned as inftances of the former, and the muscles of the external ear of the last.

Thefe neceffary affumptions and facts premifed, the doctor proceeds to confider the most important and vital motion of the heart, by a previous examination of the opinions of a few of the moft confiderable authors concerning it. He begins with the theory of the celebrated Boerhaave, who deduced the alternate motions of the heart from the alternate preffure of the greater part of the nerves going to it between the auricles and large arteries, which compreffion would happen at the end of every fyftole, when their cavities were greatly diftended with blood; whence the motions of the fpirits being intercepted, the heart muft become paralytic ; but that this compreffion ceafing in the fubfequent contraction of the auricles and arteries, the nervous fluid then paffing freely on, the heart muft contract anew. This hypothefis, however ingenious, Dr. Whytt rejects for many cogent reafons. First, because all the cardiac nerves are not in this compreffible fituation, as particularly two very confiderable branches from the par vagum are diftributed to the substance of the heart, and do not pafs either between the auricles or large arteries. Secondly, from a confideration of the foftness of the parts, and the fat upon the external coats of the arteries and auricles, which muft greatly leffen fuch compreffion; and from obferving no paralytic affection in any other mufcles, whofe nerves were contiguous to a confiderable artery. Thirdly, from our certainty that a flight compreffion of a nerve is infufficient to render its muscle paralytic; as, the ulnar nerve must be pretty ftrongly compreffed, even against a hard bone, before the fingers it ferves lose their motion, which is attended befides with a difagreeable fenfation. Fourthly, becaufe when this compreffion is removed, the motion of thefe fingers is gradually, and not immediately, reftored. Fifthily, that even granting a fufficient compreffion of the cardiac nerves, an effect, contrary to what the advocates for this theory fuppofe, muft follow, as the spirits contained in the nervous tubuli, below the point of compreffion, must be fqueezed fafter towards the heart,

which quicker propulfion muft occafion a ftronger contraction at the very time its diaffole is obferved to begin. And experiment itself evinces, that a ligature on the par vagum, far from rendering the heart paralytic, produces firong, convulfive motions and palpitations of it. Sixthly, The fuppofed alternate compreffion of the cardiac nerves is inapplicable to the motion of the auricles, whofe contraction happens when their nerves fhould be compressed, and the nervous fluid confequently intercepted. Seventhly, befides that the alternate motions of the right ventricle and auricle are continued in dying animals after the left have ceafed, when their nerves can fuffer no compreffion, fince neither the great artery nor left auricle are diftended with blood at the end of the fiftole of the right ventricle, and in the contraction of the right auricle the pulmonary artery is empty; befides all this, the hearts of many animals, taken from their bodies, continue their alternate motions for fome time with great regularity, when it is impoffible to fuppofe any comprefiion of its nerves. And lastly, the doctor thinks it a confiderable defect of this theory, that it reflects no light on the manner of fpontaneous motion in other organs, whofe nerves cannot juftly be fuppofed liable to alternate compreffion.'

He proceeds next to examine the theory of the learned De Gorter, who imagined vital motion in the heart and the other organs to depend on fuch a ftructure of the involuntary muscles, that, upon a dilatation of their fibres from the immiffion of the fpirits, their fmall nervous fibrills fhould be compreffed; whence the fpirits being intercepted, the muscle begins to be relaxed, which relaxation admitting a fresh ingrefs of them, the muscle is contracted anew, and fo relaxed alternately during life. But this hypothefis the doctor rejects, not only for such a structure's being unfupported by experiment, or microscopical obfervation, but from the circumftance of all the vital organs not being contracted and relaxed at the fame inftant: from our being able to continue the diaphragm in the ftrongest contraction as long as we please: from obferving, that even fome mufcles of voluntary motion may be and are occafionally employed, in the performance of the vital, as in the cafe of a difficult refpiration from any infarction of the lungs; from which it may appear there is no fuch peculiarity of ftructure in the mufcles of vital motion: from the pupil's (whofe motions, from a ftimulus, are as involuntary as those of the heart) not being immediately relaxed after its contraction from the admiffion of light, but remaining in the

fame

fame degree of contraction, during the tranfmiffion of the fame quantity of light to the retina, which could not be, if there was fuch a structure of the muscles of the uvea, as De Gorter fuppofes in thofe of involuntary motion. The infufficiency then of thefe hypothefes being manifefted, our author gives his own theory of the motion of the heart, beginning with its fyftole.

After obferving, that fome have imagined the blood contracted the heart, only by an irritation of the internal surface of its ventricles; and that others supposed it to act no otherwife as a ftimulus, than in confequence of its weight and impulfe, he very reasonably concludes both thefe caufes to concur to the fyftole. That the morbid quality of the blood has a tendency to increafe the motion of the heart is evident from the small pox, and other contagious or acute diseases; as well as from the manifeft power which acrid and stimulating fubftances have of renewing the motion of the heart, when separated from the body. And, on the other hand, the increase of the motion of the blood from exercise, or from any other caufe that returns it to the heart in greater quantity, and with more force; as well as the diminution of its motion from bleeding, prove, that even the diftenfion of hollow muscles has a remarkable influence towards exciting them into action. But that the blood, even in its healthy. ftate, is well qualified for a ftimulus, Dr. Whytt very clearly and diffufely evinces, from its conftituent principles; its heat; its inteftine motion; from the active vivifying fpirit of the air, which it very probably contains; and from the internal ftructure of the heart, and its tendineo-carnous chords: And to those who may object against its stimulative power, from its want of acrimony to the tafte, (tho' it is fenfibly falt) and from its scarcely irritating the membrane of the eye, he very juftly replies, that, notwithstanding this, it may be fitted to act as a powerful ftimulus upon other nerves, differing from thefe in conftitution and fenfibility. This he abundantly illuftrates, from the convulfive and even mortal operation of fome mineral and vegetable fubftances, on the membranes of the ftomach, which difcover no acrimony to the tafte, and fome of which are even sweetish and no ways difagreeable to it. The inftances he adduces are ftrong and numerous, and clearly pregnant with this axiom, that various ftimuli are very wifely adapted to various nerves and membranes, as fome poifons act on the folids, and others on the fluids only. Having thus established this irritative power of the blood, the doctor obferves, that a body, whether fluid or folid, and qualified to act as a flimulus, will excite

fo

fo much the stronger irritations, by how much the greater force it is applied with to the irritable part; fince its active particles must then strike more strongly against the extremities of the tender nerves, This inference is at once fo rational and obvious, that we shall omit the author's many just and ingenious arguments in fupport of it, and proceed. to his next fection concerning the relaxation and diastole of the heart.

Dr. Whytt had before obferved, that of the three different states of the heart, its fyftole, relaxation, and diastole, the first and last might be termed violent, and only the fecond, which Bartholin termed its perifyftole, natural. This relaxation then of the heart, he tells us, muft neceffarily happen according to his tenth fact or principle, fince the muscles of living animals, after being excited to contraction by a ftimulus, are quickly relaxed; the cause and confideration of which phænomenon he refers to a future fection. It seems then to follow, a fortiori, that as the ftimulating blood is expelled during the systole out of the ventricles, their fibres will endeavour to return, out of a violent, into their most natural condition. The ventricles then, in confequence of their evacuation by the preceding fiftole, and the relaxation fubfequent to it, give no refiftance to any cause that begins to dilate them; but yet will not without violence allow their fides to be removed fo much from each other, as happens during their diaftole, which is produced. by the returning venous blood entering its cavities with a very confiderable force; without which no relaxation of the heart could produce its utmoft dilatation ; as a hollow mufcle, fuch as the heart or bladder of urine, can never be fully diftended by its own internal mechanism, or without a diftending caufe introduced into its cavities. And tho' the full dilatation of the ventricles is owing to the force of the refluent blood, that alone would have been infufficient to effect it without the previous relaxation of their fibres; notwithstanding the contraction of the auricles, and momentum of the venous blood, are, in fome fenfe, antagonists to the ventricles; but being much weaker, there was a neceffity for the operation of fome relaxing caufe, at the termination of every Systole.'

Our author, after obferving, that the contraction of the ventricles is in proportion to the caufe dilating them, adds, that as the left, from its greater ftrength, muft require a greater force to compleat its diaftole than the right, the blood ought to return to it with a greater momentum, which he affirms it does, and indeed renders it highly probable by

feveral

feveral experiments from Dr. Hales and others, and by feveral judicious arguments and inferences of his own.

Towards the end of this fection the doctor reflects, that it is not frequently enquired, nor indeed easily determined, at what time the motion of the heart commences in nafcent animals, nor what excites it; tho' he thinks, and makes it very probable, that heat is the caufe, which, rarifying and agitating the particles of the fluids, enables them to ftimulate its fibres into contraction.'

Having thus endeavoured to prefent our readers with a fummary, and yet clear, view of this author's very ingenious and rational account of the motions of the heart, we shall be able to give a still clofer abridgement of his account of the other vital motions, as the great fimplicity and confiftence of his whole theory, and the ftrong and unftrained analogy, that obtains thro' the different organs, continually illuftrate his enquiry.

In his fifth fection then, concerning the motion of the alimentary canal, and the bladder of urine, he obferves, that tho' the act of deglutition is generally fpontaneous, yet it is effected by the foods irritatiug the fenfible membrane of the fauces, next that of the pharynx, and then the nerves of the oefophagus, till its final arrival into the ftomach. He finds there, in the air contained in the aliment, and in its other contents, in the cool air fwallowed with the faliva, and rarified by the heat of the ftomach, and in its contained humours, fufficient caufes for fuch a gentle irritation of its nervous papilla, and fuch an extenfion of its fibres, as may excite them to contraction. This exactly agrees with Wepfer's obfervation from the diffection of living animals, which afferts, that the contraction of the ftomach never happens, but in confequence of a preceding intumefcence.-Thefe fucceeding intumefcences and contractions, though fenfibly flower, preferve a remarkable analogy with the systole and diastole of the heart.'

But as Dr. Whytt was fenfible, the irritation of the blood in the ventricles of the heart muft determine, or paufe, with the effects of each fyftole, till the diafiole confequent to its relaxation began to operate, he fuppofes fome objector to afk, why the ftomach does not fuffer a new contraction from the ftimulus of its contents, before the intervention of a new intumefcence? To which he answers, that to excite this new contaction of the ftomach, or to prevent its yielding to the dilating force of the rarified air, the gentle stimulus of the aliment may require that additional irritation,

wh