In the present instance, as far as respects the heart, the only answer that can be given to the proposed question, is one that is itself, in some measure, an acknowledgment of our ignorance. We know that distention excites a mus cle to contract, and that when a muscle has contracted, relax ation ensues. We must conclude, then, that in the original formation of the body, the degree of contractility bestowed upon the heart, the quantity of distention which it receives from the blood, the size and strength of the arteries which are to transmit the blood, and the quantity of resistance which it has to overcome, are so nicely balanced, that each individual action is kept in due subjection to the rest, and contributes to the perfect performance of the func tion.
I have already stated that the heart is the great source of motion to the circulating system, and the main cause of the transmission of the blood to all parts of the body, but other powers have been pointed out, which are con ceived to assist in the operation, some of which at least have probably a real .efficacy. The principal of these is the contractility of the arteries, a point which, although it has been much controverted, and has even heen denied by some very eminent physiologists, is now generally admit ted to exist. Between the coats of membranous matter which compose the principal substance of the arteries, there is a layer of transverse fibres, which bear a consider able resemblance to the fibres of muscles, and have been generally conceived to possess a contractile power. The action of these fibres, if muscular, must be to diminish the cavity of the vessels, and, by the alternation of contraction and dilatation, to propel their contents.
Haller performed many experiments on the muscularity of the arteries, which consisted in applying to them the stimulants that produce the contraction of the proper muscular fibres ; but his results were, for the most part, unsuccessful. Other experimentalists, however, since his time, have been more fortunate ; in the first instance, Verschuir, and more lately Drs. Philip, J. Thomson, and Hastings, have, in the most unequivocal manner, proved that the arteries possess a power, in all respects analogous to that of muscular contraction ; and it may be fairly pre sumed that the transverse fibres, or, as they have been usually termed, the muscular coat, is the seat of this power. Probably one cause of Haller's failure was, that he per formed his experiments principally upon the larger ves sels, which appear to possess this property in a small de gree only, while it is the minute capillary extremities of the arterial system that are the most contractile, and where the operations are principally carried on that depend upon this power.
Before the contractility of the arteries had been prov ed by direct experiment, or rather, while experiments seemed averse to the doctrine, a very ingenious physiolo gical argument in support of it was advanced by Cullen. This acute physiologist remarks, that if the motion of the blood depended entirely upon the impetus impressed on it by the heart, its force and velocity, in the different parts of the body, must, at all times, bear the same ratio to each other. If the quantity and velocity of the blood in any two parts, as for example in the two arms, be the same, provided the arteries were membranous tubes, the mo mentum of the blood in the arms must always be precisely similar, and this will continue to be the case, whatever be the strength or velocity of the heart's motion, and how ever these may vary at different times.
But we do not observe this constant ratio to exist ; on the contrary, the relative momentum of the blood in the different parts of the body is always varying, so that the qtlantity of blood and the velocity with which it moves, are perpetually altered, both from the effect of external stimuli, and from a number of internal causes. The vari able state of the contraction of the capillary branches of the arteries, constitutes what is termed the action of the vsssels, a circumstance which forms a very prominent feature in the modern doctrines of medicine, and for which we are chiefly indebted to Cullen, who substituted this ac tion in place of the humoral or mechanical pathology of his predecessors. It is to an altered state of the capil laries that we refer many of the phenomena of fever and local inflammation, the morbid changes in the secretions, and the derangement of most of the functions which serve for the growth and nutrition of the body.
Hunter perlbrnied an elaborate set of experiments upon the vessels, by which he endeavoured to discriminate be tween their elastic and their muscular power. They con sisted in examining portions of the artery of an animal that had been bled to death, by which it was supposed that the vessels would be reduced to a state of extreme contrac tion, for the purpose of ascertaining how far they were able to recover themselves from a certain state of distention. Dead membranous matter retains the whole of its elasti city, and after being stretched, will restore itself to its former size, but the contractile power of muscles ceases with life ; and therefore, when a muscular fibre is stretch ed, it remains permanently distended. The results of Hun ter's experiments sanctioned Cullen's hypothesis, that the elasticity of the arteries diminishes, and their elasticity increases, as we recede from the heart.