I. Machinery of the sanguineous system. II. Moving powers of neous sys making the amount from 30lb. to 40lb. appear most reasonable; and perhaps fall not far short of the sum intended by Professor Blumenbach. The subject requires further examination, and a nicer estimate. II. There is another question which has also, in all the sangui- ages, greatly occupied the attention of physiologists, but upon which we still remain in a very considerable degree of indecision; and that is, the MOVING POWERS employed in the circulation; or, in other words, the projectile force by which the blood is sent forward. tem. What excites the heart to contract. Hunter's stimulus of necessity, what. The heart forms the salient point of motion, and with its systole or contraction the circulation commences. But what is it that excites the heart to contract? One of the most common answers to this question in the writings of physiologists is the flow of the blood into the ventricles. But this is merely to argue in a circle; for the question still returns, what is it that makes the blood flow into the ventricles? Others have referred the cause to an immediate impulse from the brain. Now in contractions of the voluntary muscles, there is no doubt of the existence of such an impulse, for we are conscious of it, and assent to it; but we are neither conscious of nor assent to any thing of the kind in respect to the contraction of the heart; and are perfectly sure that no such power of the will takes place during sleep. It is a mere assumption; and an assumption which can only apply to a part of the great animal kingdom even during wakefulness; for, as it is only in mammals and birds that the nerves can be thus influenced in their passage to the heart, the postulate does not account for the contraction or dilatation of the heart in other classes of animals*. Mr. John Hunter ascribes this action of the heart, or rather the whole career of the circulation, of which he regards the action of the heart as a single and ordinary link in the general chain, to what he calls a stimulus of necessity; by which he seems to mean an instinctive power dependent on the general sympathy of the system which in every part is craving or demanding such an alternation; or, in other terms, is uneasy without it. His words are as follow: "The alternate contraction and relaxation of the heart constitutes a part of the circulation; and the whole takes place in consequence of a necessity, the constitution demanding it, and becoming the stimulus. It is rather, therefore, the want of repletion, which makes a negative impression on the constitution, which becomes the stimulus, than the immediate impression of something applied to the heart. This we see to be the case, wherever a constant supply or some kind of aid is wanted in consequence of some action. We have as regularly the stimulus for respiration, the moment one is finished an immediate demand taking place; and if prevented, as this action is under the influence of the will, the stimulus of want is increased. We have the stimulus of want of food which takes place regularly in health, and so it is with the circulation. The heart, we find, can rest one stroke, but the constitution feels it; even the mind and the heart is thereby stimulated to action. The constant want in the constitution of this action in the heart, is as much as the constant action of the spring of a clock is to its pendulum, all hanging or depending on each other."* II. Moving powers of the sanguineous sys tem. Mr. Hunter's "Treatise on the Blood", is a work of such sterling merit, so rich in its facts, and so valuable in its remarks, that notwithstanding a few nice-spun and chimerical speculations that occasionally bewilder it, there is no book on physiology which a student ought to study more assiduously. Yet I am much afraid that the Little meaning language now read has no great deal of meaning in it; furnished and that it does little more than tell us that the heart by such an explanation. contracts because it contracts, or, rather, that the circulation takes place because it takes place. Few physiologists indeed seem to have adopted this opinion: and hence a far more plausible and intelligible hypothesis has been since offered. This consists in sup- Oxygene posing the heart to be stimulated by the oxygene of the * On Blood, p. 149. received from the lungs re powers of the sangui neous system. II. Moving blood introduced into it at the lungs by the process of respiration. Such was the favourite opinion of Dr. Darwin: and such appears to have been the opinion of Professor Blumenbach, who was so fully persuaded of the the primum oxygenized state of the blood when first received by the heart and poured into the arteries, that he expresses a desire of changing the terms arterial and venous blood for oxygenized and carbonized. garded as mobile. That oxygene, if introduced into the blood, would stimulate the heart, there can be no doubt, from numerous experiments which prove that a very small quantity of any foreign body whatever, even an ounce or two of solution of gum Arabic, infused into the blood by opening a vein, will not only stimulate the heart, but the stomach, intestinal canal, and other organs with which the heart readily sympathizes *. But, unfortunately for Dr. Opposed by Darwin's hypothesis, Mr. Ellis, as we observed at some length in the proem to the preceding class, has advanced a variety of arguments so stubborn and cogent, though not conclusive, to prove that no oxygene whatever is introduced into the blood in its transit through the lungs, that, till these arguments are disposed of, the present hypothesis, beautifully simple as it is, is entitled to the claim of ingenuity, and nothing more. Ellis's ex periments on respiration. By what power is the circulation maintained after it has once commenced? Harvey's opinion. At first re But passing by, till this question is settled, the doctrine of the primum mobile, or first moving power of the blood from the heart--by what means is the motion, thus mysteriously commenced, maintained afterwards through the whole circulatory course? Harvey replied to this question by asserting that it is maintained by the action of the heart alone, which propels the blood equally through the entire length of the arteries and veins, both which he regarded as tubes alike inert, and in no respect contributing to the propulsive energy. This dictum was at first received with universal assent; ceived with and the mechanical physiologists immediately set to work universal assent. * De Chirurgiâ Infusoriâ renovendâ. Aut. J. M. Regnaudot. 8vo. Lugd. Bat. 1779. But no com in order to calculate the force with which the heart acts II. Moving at every contraction, in the same manner as they had powers of the sanguiendeavoured to calculate the force of the stomach in the neous process of digestion. It is not necessary to enter into system. the details of these estimates. It is sufficient to observe, that from Michelot to Sauvages or Cheselden, they all differed from each other as widely as in calculating arrived at. the quantity of blood in the system; and that while Keil estimated the projectile power of the heart at eight ounces, Borelli fixed it at no less than one hundred and eighty thousand pounds. mon result itself not Proofs of There are various facts, however, which sufficiently The heart prove that the heart cannot be the sole propulsive power the sole through the entire range of the circulation; the chief of propulsive which are: Firstly, that the pulse, if the systole of the power. heart were the only projectile force, must take place, not this asserSYNCHRONOUSLY all over the system, as it is well known tion. to do, except in a few morbid cases in which local causes interfere, but SUBSEQUENTLY to the contraction of the heart, and SUCCESSIVELY through the whole line of the arterial tubes, in proportion as they lie more remote from the salient point. And, secondly, that whatever may be the projectile power of the heart, it must altogether cease with the arteries, and cannot reach the veins. of a vis à And hence arose another hypothesis, which ascribed Hypothesis. the propulsive power to a progressive vis à tergo, or a tergo. force communicated from the ventricles of the heart to the commencement of the arteries, producing a vibration or alternate dilatation and contraction of their tunics, through their whole length to the veins; and thus acting in conjunction with the projectile force of the heart itself. In proof of this auxiliary power afforded by the coats Supposed proof deof the arteries, the phænomenon of pulsation was trium- rived from phantly appealed to; which, it was maintained, gave a pulsation. direct and incontrovertible evidence that an alternate dilatation and contraction, or enlargement and diminution in the diameter of the arteries, is constantly taking place. This, by Bichat, is attributed solely to the loco- Bichat's motion of the arterial tubes, propagated to their terminal explanation. II. Moving powers of the sangui neous system. Arteries sustain no change of bulk from pulsation. Pulsation alone produced by pressure from without. The pulse of an in ramifications, and thence continued to the veins; but by most modern physiologists to a joint power compounded of the action of the heart and the arteries. M. Bichat's doctrine has of late been incontrovertibly refuted by one or two very simple experiments of M. Magendie*. Besides which, however, it is now a wellascertained fact, and one that has been thoroughly elucidated by Dr. Parry of Bath, that no increase of size, or indeed change of bulk of any kind takes place in arteries during either the systole or diastole of the heart's ventricles in a state of health+. The arteries of animals, to ascertain this point, have been exposed in different parts, and to considerable lengths, without evincing the least apparent increase of size. And hence it is the pressure of the finger, or of some other substance, against the side of an artery that alone occasions pulsation, in consequence of the resistance hereby made to the regular flow of the blood; the alternating beat being produced by the greater momentum with which the current strikes against the finger or other cause of obstruction, during the systole than during the diastole of the heart. Professor Dollinger has confirmed the experiments of Dr. Parry by laying bare the carotid of a dog before his pupils which gave to the eye no proof of altered form or motion, though a pulse was distinctly felt by the finger. And in like manner a pulsatory motion is always felt by the fingers when applied to a leaden water-pipe while a pump is at work upon it at one end, and alternately giving a fresh pressure to the column of water it contains by forcing in a fresh supply: yet the pipe is all this time incompressible. It may be still further observed that, in a state of flamed part inflammation, the pulse of the inflamed part, in conserarely syn- quence of local excitement, is much more frequent than with that of that of the heart or of any other organ. Thus in a the heart or whitlow, the radial artery may give to the finger a hun chronizes of the neighbour ing parts. * Précis Elementaire de Physiologie. Tom. 11. p. 320. + Experimental Inquiry into the Nature, Cause, and Varieties of the Arterial Pulse, &c. By C. H. Parry, M.D. F.R.S., &c. Bath, 1816. |