The results of actual observation of the flow of the blood and of the measurement of the relative capacities of different arteries, afford as yet very unsatisfactory data upon which to found an estimate of the relative velocity of the blood in the trunks and branches of the arte ries. In the absence of more direct means of calculation, an approximative estimate may be made in another way, viz. by comparing the quantity of blood which occupies a known space of the larger vessels with the whole quan tity of blood contained in the body.
We have already seen that the whole blood in the body may be estimated at nearly thirty pounds : now, let us suppose the aorta and pulmonary arteries, together with their return ing veins, to form a continuous tube of the length of the two courses of the blood, in the systemic and pulmonic circulations, and of the same diameter as these vessels at their point of junction with the heart; a very simple calcula tion shews us that such a tube is capable of holding only about six pounds and a quarter, or less than a fourth part of' the whole blood of the body ; or in other words, were the aggre gate capacities of the small vessels no more than equal to that of the larger, they would be capable of holding only a fifth of the blood contained in the body.
- The velocity of the blood in the commence ment of the aorta may be considered as two and a half inches in a second, for this is the space occupied by all the blood which is pro pelled into the aorta from the left ventricle in that time, and according to the arbitrary modes of estimating the relative capacity of the aorta and its branches here employed, the velocity of the blood in the aortic capillaries generally, might be considered as one-fourth of that in the commencement of the aorta, or nearly half ail inch in a second, a result widely different from that obtained by Hales.
Attempts have also been made to estimate the velocity of the flow of blood, by ob serving the time which certain substances, when introduced into one part of the vascular system, take to pass to another. The most remarkable series of experiments of this na ture with which we are acquainted were per formed by Hering. This author states that he has been able to detect prussiate of potassa, which he had introduced into one of the jugu lar veins of a horse, in the blood drawn from the opposite jugular vein in the space of from twenty to thirty seconds ; and he has formed the conclusion from this experiment that the prussiate of potass, in order to gain the jugu lar vein on the opposite side of the body, had passed in this remarkably short space of time through the whole course of the double circu lation : that it was first carried to the heart, then passed through the pulmonary arteries and veins, and returned to the heart, from which it must have been transmitted through the ultimate ramifications of the systemic ar teries before being brought back by the veins, in which it was found on tbe opposite side of the body. Hering states, as the result of other
experiments of a similar nature made upon different bloodvessels, that the prussiate of potassa passed from the jugular vein to the saphena vein in twenty seconds ; to the mas seteric artery, in fifteen to twenty seconds; to the external maxillary artery, in ten to tvventy five seconds ; to the metatarsal artery, in twenty to forty seconds.
We consider these curious experiments as important in many points of view, but do not feel inclined to concur in the conclusion de duced from them by their author, that the circulation of the blood, rapid as it may be, takes place in this remarkably short space of time, and we are disposed to suspect that the experiments themselves are liable to several sources of fallacy. The tendency of the prus siate of potass to permeate the textures of the body, more freely than any other substance known, has been proved by many expe riments, and it is therefore necessary that Hering's experiments should be performed with some other substances, before they can be re garded as a correct means of estimating the rapidity of the circulation.
The velocity of the blood is generally be lieved to be greater in the pulmonic than in the systemic circulation,—an opinion founded chiefly on the supposed less capacity of the vessels belonging to the pulmonary trunks. Actual measurements of the velocity of the blood in the capillaries of the lungs of cold blooded animals by Hales, Spallanzani, and others would seem to give support to this view, 'but it must at the same time be re collected that the course through which the blood passes in the pulmonary or lesser circu lation, is considerably shorter upon the whole than that of the systemic or greater,—a circum stance which must diminish to a certain extent the disproportion in the velocity.* b. Force of the blood in the arteries and force of the heart.—Another interesting inquiry con nected with this subject relates to the force with which the blood is impelled in the arte ries and the calculations that have been made of ;he power of the heart itself, from the ob servation of the force of the blood in the arte ries. The experiments made with a view to discover these forces appear sufficiently simple in their nature ; but the calculations founded upon the experiments have differed so widely, as to have- furnished a plausible pretext for throwing ridicule on the application of physical la.ws to the living animal functions.