486 OF THE CIRCULATION OF THE BLOOD. 



vital and physical properties of the walls of the Arteries, upon the motion of 

 Blood through them. There is no sufficient proof that the vital Contractility of 

 these vessels enables them to exert a propulsive action in any degree supplement- 

 ary to that of the Heart ; and yet, looking to the general facts already stated, 

 as to the diffusion of the propulsive power through the arterial trunks in many 

 of the lower animals ( 493), and their experimentally-proved reaction upon a 

 distending force, it does not seem by any means improbable that some such 

 power should be preserved, even where there is the greatest concentration of the 

 propulsive force in the muscular walls of the heart. The contractility of the 

 arteries seems to be chiefly exercised, however, in regulating the diameter of the 

 tubes, in accordance with the quantity of blood to be conducted through them 

 to any part; which will depend upon its peculiar circumstances at the time. 

 Such local changes are continually to be observed, in the various phases of nor- 

 mal life, as well as in diseased states ; and they will be found to be constantly 

 in harmony with the particular condition of the processes of Nutrition, Secretion, 

 &c., to which the capillary circulation ministers. In such cases, it cannot be the 

 action of the Heart that increases the caliber of the vessels; since this is com- 

 monly unaltered, and is itself unable, as we have just seen, even to maintain 

 their permeability, when their contractility is excited. It must, therefore, be 

 by a power operating directly through themselves, that their dilatation is ef- 

 fected. The minute distribution of the Sympathetic nerve upon the walls of 

 the arteries, the known power which this has of producing contractions in their 

 fibrous coat ( 513), and the influence of mental states upon their dimensions 

 (as shown in the phenomena of blushing and erection), render it highly probable 

 that the caliber of the arteries is regulated in no inconsiderable degree through 

 its intervention. The permanent dilatation, however, which is seen in the 

 arteries supplying parts that are undergoing enlargement, must be due, not to 

 simple dilatation merely, but to increased nutrition; since we find that their 

 walls are thickened as well as extended. And, on the other side, when slow 

 contraction occurs in these tubes, as a consequence of disease, it must be in part 

 occasioned by atrophy; since their nutrition is so much diminished, that in 

 time they almost entirely disappear a portion of a large artery occasionally 

 shrivelling into a ligamentous band. 



516. The purpose served by the Elasticity of the Arteries is one of a purely 

 physical character; its effect being to convert the intermitting impulses which 

 the blood receives from the heart, into a continuous current. The former are 

 very evident in the larger trunks; but they diminish with the subdivision of 

 these, until they entirely disappear in the capillaries, in which the stream is 

 usually equable or nearly so. If a powerful force-pump were made to inject 

 water, by successive strokes, into a system of tubes with unyielding walls, the 

 flow of fluid at the farther extremities of these tubes would be as much inter- 

 rupted as its entrance into them. But if an air-vessel (like that of a fire-engine) 

 were placed at their commencement, the flow would be in a great degree equal- 

 ized ; since a part of the force of each stroke would be spent upon the compres- 

 sion of the air included in it ; and this force would be restored by the elasticity 

 of the air during the interval, which would propel the stream, until directly 

 renewed by the next impulse. A much closer imitation of the natural apparatus 

 would be afforded by a pipe which had elastic walls of its own ; thus if water 

 were forced by a syringe into a long tube of caoutchouc, for example, the stream 

 would be equalized before it had proceeded far. This effect is found to be accom- 

 plished, at any point of the arterial circulation, in a degree proportionate to its 

 distance from the Heart ; and in this mode it is that the intermitting force 

 of the ventricular contraction is almost equably distributed over the whole of 

 the interval between one systole and another, by the contraction of the elastic 

 tubes in the dilatation of which it was at first expended. Another effect of this 



