Besides the muscularity of the small branches of the arte ries,many of the old writers conceived that the elasticity of the vessels contributed to the motion of the blood ; but this idea depended upon an erroneous conception of the nature of elasticity, which can never actually generate power, but only restore what has been impressed upon the blood in a contrary direction. Even if the arteries were perfectly elastic, they could only re-act upon the blood with exact ly the same degree of force which the heart had expended in their dilitation; and so far as the mere motion of the blood is concerned, it would have been more promoted if the arteries had possessed a rigid texture similar to that of metallic tubes. But the loss of power which is occa sioned by the flexible and imperfectly elastic nature of the arteries is no doubt amply compensated by other advan tages.
The passage of the blood along the veins, is accelerated by the contraction of the muscles. The veins, as has been remarked above, are furnished with numerous valves in different parts of their course, which are so constructed that they permit the blood to pass in one direction only towards the heart ; and as many of the large veins are so situated that when the muscles contract they are pressed upon, and their contents partially expelled, we perceive how the circulation of the blood is actually promoted by the contraction of the muscles. The contraction of the heart, that of the capillaries of the arteries, and the pi es sure of the muscles upon the veins, are all of them causes obviously contributing to the motion of the blood ; but it may be questioned whether there are any other circum stances which are to be considered as real sources of power to the circulation ; for although the elasticity of the parts concerned in this function may tend to distribute the force in a more commodious manner, yet it cannot actual ly generate power.
The causes which retard the blood's motion along the vessels are numerous, and some of them very efficient, so that the force employed greatly exceeds the effect produc ed. The following may be considered as some of the most important of the retarding causes : the resistance which the quantity of blood previously in the vessels op poses to the entrance of a fresh quantity, the imperfect elasticity of the vessels, their flexibility, their winding course, the adhesive nature of the blood, the friction which it must occasion in passing along the vessels, the ramifi cation of the arteries into smaller branches which go off at considerable angles from the trunk, the union of two streams of blood at a large angle with each other, and the increased diameter of all the branches compared to that of the trunk. We know from the laws of hydraulics that these
are real causes of retardation, which must act powerfully on the motion of the blood ; but they are all independent of the vital actions of the organs, and apply to them as they would do to a system of tubes, possessed of the same physical properties, but without any of those powers which are peculiar to the living animal.
The degree in which the sanguiferous system is subject ed to the physical laws of matter, induced the mathemati cal physiologists to bestow an unusual share of attention upon every circumstance connected with the action of the heart and the motion of the blood. There is little doubt that, by the cautious application of 'mathematical reasoning, we have arrived at some physiological truths which we could not have attained by other means, and which are beyond the reach of actual observation. But when we call in the aid of mathematics to assist us in our researches, it is essen tial to our success that the data which we employ be well founded, and that we are not misled by false analogies, or by the misapplication of principles which may be in them selves correct. Unfortunately, however, the mechanical physiologists fell into all these errors ; and the conse quence was, that, compared to the labour employed, the advantage resulting from it was very inconsiderable ; while the mistakes which they committed, and the erroneous statements which they advanced, which were for a long time received with little hesitation, were both very im portant and very numerous.
A point which was very minutely attended to by the ma thematicians, was the estimate of the force of the heart, or the power which it exerts upon the propulsion of the blood along the great arteries which immediately commu nicate with the ventricles. Some of the most learned men of the seventeeth and eighteenth centuries engaged in this nv iestigation, and employed upon it all the force of their genius ; yet it may be confidently asserted that they en tirely failed in their object, and have in fact given us little or no information upon the point in question.