With respect to the use of the circulation, it is sufficient for the present to say, that it is necessary for all parts of the body to be provided with a regular supply of arterial blood, as it is termed, or blood that has undergone its ap propriate change in its passage through the lungs. The object of the lesser, or the pulmonic circulation, is to effect this change ; and the object of the larger, or the systemic circulation, is to convey the blood, when changed, to all parts of the body.
Many causes have been assigned to account for the power by which this great machine is primarily moved. Some of the ancients conceived that there was an innate fire in the heart which produced its motion : Sylvius, who is ce lebrated as being the founder of a noted chemical sect in medicine, attributed it to the effervescence excited by the mixture of the different kinds of blood, and many other equally absurd ideas prevailed, until the publication of Se nac's treatise on the heart. He correctly ascribed the power by which the circulation is carried on to muscular contractility, principally residing in the heart ; and he show ed that, by the blood being poured into its cavities, and causing a certain degree of distention of its fibres, the heart is stimulated to contraction, and by this act expels its con tents. The stimulating cause being thus removed, the heart relaxes ; but, in the mean time, the blood having tra versed the vessels, is poured into the auricles, and from them into the ventricles, which being again distended, are again made to contract and expel their contents into the artery. We frequently observe, that even in human con trivances, the most important effects are produced by the most simple means, and the same remark will apply to the mechanism of the heart.
An important point connected with the motion of the blood, and one which has given rise to much controversy, is the inquiry into the cause of its regularity and constancy. How, it has been asked, can the heart proceed for years to gether, pulsating at equal intervals, and propelling the same quantity of blood ? Willis, who was one of the first physiologists that was fully sensible of the importance of the nervous system in the animal economy, advanced the opinion to which we have already alluded, that the nerves of the heart, as well as of all the muscles which are in con stant motion, are derived from a different part of the brain from the nerves of those parts which are called into occa sional action only, and which are under the control of the will. There are no doubt some facts which countenance the opinion, that the nerves which supply the voluntary and the involuntary muscles, are derived from different sources, but there is some doubt how far these facts will apply to the heart, and even were they admitted, they would scarcely solve the difficulty. Other hypotheses have been
proposed, to which it will not be necessary for us to advert; but we must not pass by without noticing the remarkable doctrine which was maintained on this subject by Stahl. He ascribed the regularity of the heart's motion to the agency of the anima, or principle of intelligence which re sides in man, and superintends all his functions, though without exciting any sensation, or producing any conscious ness of its existence. This principle, aware of the fatal consequences that would ensue from the intermission of so important an operation as the circulation of the blood, is careful always to preserve it in due action, and at all times to regulate its operations according to the demands of the system.
The doctrine of a superintending intelligent principle residing in the body, and directing its operations, seems to have originated with Vanhelmont, who gave it the name of archeus; Stahl refined upon Vanhelmont's notion, and applied it to many parts of the animal economy, un der the title anima, and it has borne a distinguished share in the hypotheses of many learned physiologists down to the present day. An agent of this kind, although not so distinctly brought into view as by Stahl and some of his im mediate followers, forms a leading feature in the writings of John Hunter ; and many of Darwin's speculations, when divested of their poetical garb, must be referred to the same principle. Yet nothing surely can be more con trary to the spirit of true philosophy, than to assume the existence of an intelligent agent in the body, of which we are entirely unconscious, or to adduce, as the cause of cer tain effects, a power of which we have no knowledge, or intimation, and which is only had recourse to as a commo dious method of removing the present difficulty. The Stahlian doctrine in all its ramifications, and under every form in which it has been exhibited, seems to have origi nated partly from the want of an accurate discrimination between the physical and the final causes of the operations of the body, and partly from an indistinct conception of the difference between the agency of the first great cause of all things, and the secondary or physical cause, into which alone it is the province of natural philosophy to in,• quire. It affords no explanation of the physical cause, or the nature of any phenomenon, to say, that the Supreme Being has thought fit to order it so ; the object of our re searches is, to examine to which of the general laws that he has impressed upon matter the action in question is to be referred.