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the natural contraction is ; but that it is also of a different kind from that of the sphincters and muscles without antagonists; referring it, from its phænomena, to the contraction from a stimulus. And the 18th and last affirms, the mind may, by disuse, 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 instances of the former, and the muscles of the external ear of the last.
These neceffary affumptions and facts premised, the doctor proceeds to consider the most important and vital motion of the heart, by a previous examination of the opinions of a few of the most considerable authors concerning it. He begins with the theory of the celebrated Boerhaave, who deduced the alternate motions of the heart from the alternate pressure of the greater part of the nerves going to it between the auricles and large arteries, which compression would happen at the end of every systole, when their cavities were greatly diftended with blood; whence the motions of the fpirits being intercepted, the heart must become paralytic; but that this compression ceasing in the subsequent contraction of the auricles and arteries, the nervous fluid then pasfing freely on, the heart must contract anew. This hypothesis, however ingenious, Dr. Whytt rejects for many cogent reafons. First, because all the cardiac nerves are not in this compressible situation, as particularly two very considerable branches from the par vagum are distributed to the substance of the heart, and do not pass either between the auricles or large arteries. Secondly, from a consideration of the softness of the parts, and the fat upon the external coats of the arteries and auricles, which must greatly lefsen such compression; and from observing no paralytic affection in any other muscles, whose nerves were contiguous to a confiderable artery. Thirdly, from our certainty that a slight compression of a nerve is insufficient to render its muscle paralytic; as, the ulnar nerve must be pretty strongly compressed, even against a hard bone, before the fingers it serves lose their motion, which is attended besides with a disagreeable fenfation. Fourthly, because when this compression is renoved, the motion of these fingers is gradually, and not immediately, restored. Fifthly, that even granting a sufficient compression of the cardiac nerves, an effect, contrary to what the advocates for this theory suppose, must follow, as the spirits contained in the nervous tubuli, below the point of compreilion, must be squeezed faster towards the heart,
which quicker propulsion must occasion a stronger contraction at the very time its diastole is observed to begin. And experiment itself evinces, that a ligature on the par vagum, far from rendering the heart paralytic, produces strong, convulfive motions and palpitations of it. Sixthly, The fuppofed alternate compression of the cardiac rerves is inapplicable to the motion of the auricles, whose contraction happens when their nerves should be compreilied, and the nervous fluid consequently intercepted. Seventhly, belides that thë alternáte motions of the right ventricle and auricle are continued in dying animals after the left have ceased, when their nerves can suffer no compression, fince neither the great artery nor left auricle are distended with blood at the end of the systole of the right ventricle, and in the contraction of the right auricle the pulmonary artery is empty ; besides all this, the hearts of many animals, taken from their bodies, continue their alternate motions for some time with great regularity, when it is impossible to suppose any compresiion 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, whose nerves cannot justly be supposed liable to aliernate 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 such a structure of the involuntary muscles, that, upon a dilatation of their fibres from the immission of the spirits, their small nervous fibrills should be compressed ; -whence the spirits being intercepted, the muscle begins to be relaxed, which relaxation admitting a fresh ingress of them, the muscle is contracted anew, and fo relaxed alternately during life. But this hypothesis the doctor rejects, not only for such a struciure's being unsupported by experiment, or microscopical observation, but from the circumstance of all the vital organs not being contracted and rel.xed at the same inftant: from our being able to continue the diaphragm in the strongest contraction as long as we please : from observing, that even some muscles of voluntary motion may be and are occasionally employed, in the performance of the vital, as in the case of a difficult respiration from any infarction of the lungs; from which it may appear there is no such peculiarity of structure in the muscles of vital motion : from the pupil's (whose motions, from a stimulus, are as involuntary as those of the heart) not being iminediately relaxed after its contraction from the admission of light, but remaining in the
same degree of contraction, during the transmission of the fame quantity of light to the retina, which could not be, if there was such a structure of the muscles of the uvea, as De Gorter supposes in those of involuntary motion. The insufficiency then of these hypotheses being manifested, our author gives his own theory of the motion of the heart, beginning with its systole.
After observing, that some 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 otherwise as a stimulus, than in consequence of its weight and impulse, he very reasonably concludes both these causes to concur to the systole
. That the morbid quality of the blood has a tendency to increase the motion of the heart is evident from the small pox, and other contagious or acute diseases ; as well as from the manifest power which acrid and stimulating substances 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 cause 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 distension 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 stimulus, Dr. Whytt very clearly and diffusely evinces, from its constituent principles ; its heat; its intestine motion ; from the active vivifying spirit of the air, which it very probably contains; and from the internal structure 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 taste, (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 stimulus upon other nerves, differing from these in constitution and sensibility. This he abundantly illustrates, from the convulsive and even mortal operation of some mineral and vegetable substances, on the membranes of the stomach, which discover no acrimony to the taste, and fume of which are even sweetish and no ways difagrecable to it. The instances he adduces are strong and numerous, and clearly pregnant with this axiom, that various stimuli are very wisely adapted to various nerves and membranes, as some poisons act on the solids, and others on the fluids only. Having thus established this irritative power of the blood, the doctor observes, that a body, whether fluid or solid, and qualified to act as a slimulus, will excite
so much the stronger irritations, by how much the greater force it is applied with to the irritable part; since its active particles must then strike more strongly against the extremities of the tender nerves, This inference is at once so rational and obvious, that we shall omit the author's many just and ingenious arguments in support of it, and proceed to his next section concerning the relaxation and diastole of the heart.
Dr. Whytt had before observed, that of the three different states of the heart, its systole, relaxation, and diastole, the first and last might be termed violent, and only the second, which Bartholin termed its perisystole, natural. This relaxation then of the heart, he tells us, muft necessarily happen according to his tenth fact or principle, since the muscles of living animals, after being excited to contraction by a ftimulus, are quickly relaxed; the cause and confideration of which phenomenon he refers to a future section. It seems then to follow, a fortiori, that as the stimulating 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 consequence of their evacaation by the preceding systole, and the relaxation subsequent to it, give no resistance to any cause that begins to dilate them, but yet will not without violence allow their sides to be removed so much from each other, as happens during their diastole, which is produced
returning venous blood entering its cavities with a very considerable force; without which no relaxation of the heart could produce its utmost dilatation; as a hollow muscle, such as the heart or bladder of urine, can never be fully diftended by its own internal mechanism, or without a distending cause 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 insufficient to effect it without the previous relaxation of their fibres; notwithstanding the contraction of the auricles, and momentum of the venous blood, are, in some fenfe, antagonists to the ventricles; but being much weaker, there was a necessity for the operation of some relaxing cause, at the termination of every systole.'
Our author, after observing, that the contraction of the ventricles is in proportion to the cause dilating them, adds, that as the left, from its greater strength, must require a greater force to compleat its diastole 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
several experiments from Dr. Hales and others, and by several judicious arguments and inferences of his own.
Towards che end of this section the doctor reflects, that it is not frequently enquired, nor indeed easily determined, at what time the motion of the heart commences in nascent animals, nor what excites it; tho' he thinks, and makes it very probable, that heat is the cause, which, rarifying and agitating the particles of the fluids, enables them to stimulate its fibris into contraction.'
Having thus endeavoured to present our readers with a summary, 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 closer abridgement of his account of the other vital motions, as the great fimplicity and confistence of his whole theory, and the strong and unstrained analogy, that obtains thro' the different organs, continually illustrate his enquiry.
In his fifth section then, concerning the motion of the alimentary canal, and the bladder of urine, he observes, that tho' the act of deglutition is generally spontaneous, yet it is effected by the foods irritatiug the sensible membrane of the fauces, next that of the pharynx, and then the nerves of the oefophagus, till its final arrival into the stomach. He finds there, in the air contained in the aliment, and in its other contents, in the cool air swallowed with the saliva, and rarisied by the heat of the stomach, and in its contained humours, sufficient causes for such a gentle irritation of its nervous papilla, and such an extension of its fibres, as may excite them to contraction. This exactly agrees with Wepfer's observation from the dissection of living animals, which afferts, that the contraction of the stomach never happens, but in consequence of a preceding intumescence. These succeeding intumescences and contractions, though sensibly flower, preserve a remarkable analogy with the systole and diastole of the heart.'
But as Dr. IV bytt was sensible, the irritation of the blood in the ventricles of the heart muit determine, or pause, with the effects of each systole, till the diafioie consequent to its relaxation began to operate, he fupports some objector to ask, why the itomach does not suffer a new contracion from the stimulus of its contents, betore the intervention of a new intumescence? To which he answers, that to excite this new contraction of the stomach, or to prevent its yieldirg to the diluting force of the rarified air, the gentle stimulus of the aliment may require that additional irritation,