118 THE CIRCULATION. 



creases, but that it equalizes or diffuses the force derived from 

 the periodic contractions of the ventricles. The force with 

 which the arteries are dilated every time the ventricles con- 

 tract, might be said to be received by them in store, to be all 

 given out again in the next succeeding period of dilatation of 

 the ventricles. It is by this equalizing influence of the suc- 

 cessive branches of every artery that, at length, the intermit- 

 tent accelerations produced in the arterial current by the action 

 of the heart, cease to be observable, and the jetting stream is 

 converted into the continuous and equable movement of the 

 blood which we see in the capillaries and veins. 



In the production of a continuous stream of blood in the 

 smaller arteries and capillaries, the resistance which is offered 

 to the blood -stream in the capillaries (p. 136), is a necessary 

 agent. Were there no greater obstacle to the escape of blood 

 from the arteries than exists to its entrance into them from the 

 heart, the stream would be intermittent, notwithstanding the 

 elasticity of the walls of the arteries. 



It is the resistance which the left ventricle meets with in 

 forcing blood into the arteries that causes part of the force of 

 its contraction to be expended in dilating them, or, as before 

 remarked, in laying up in them a power which will act in the 

 intervals of the ventricle's contraction. 



(3.) By means of the elastic tissue in their walls (and of 

 the muscular tissue also), the arteries are enabled to dilate and 

 contract readily in correspondence with any temporary increase 

 or diminution of the total quantity of blood in the body ; and 

 within a certain range of diminution of the quantity, still to 

 exercise due pressure on their contents. 



The elastic coat, however, not only assists in restoring the 

 normal calibre of an artery after temporary dilatation, but 

 also (4), may assist in restoring it after diminution of the cali- 

 bre v whether this be caused by a temporary contraction of the 

 muscular coat, or the application of a compressing force from 

 without. This action of the elastic tissue in arteries, is well 

 shown in arteries which contract after death, but regain their 

 average patency on the cessation of post-mortem rigidity (p. 

 119). (5.) By means of their elastic coat the arteries are 

 enabled to adapt themselves to the different movements of the 

 several parts of the body. 



We have already referred to the fact that the middle coat 

 of the arteries is composed of unstriped muscular fibres, mingled 

 with fine elastic filaments. The evidence for the muscular 

 contractility of arteries may, however, be given briefly for the 

 sake of the physiological facts on which it hinges. 



^1.) When a small artery in the living subject is exposed 



