148 THE CIRCULATION. 



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

 successive branches of every artery that, at length, the 

 intermittent 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. 



(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 calibre, whether this be caused by a temporary con- 

 traction of the muscular coat, or the application of a com- 

 pressing 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 cessa- 

 tion of post-mortem rigidity (p. 149). (5.) By means of 

 their elastic coat the arteries are enabled to adapt them- 

 selves to the different movements of the several parts of 

 the body. 



The evidence for the muscularity of arteries may be given 

 at some length. We have already referred to the mus- 

 cular structure of the inner layer of the middle coat of all 

 but the largest arteries, and to the fact, first observed by 

 Henle, that this layer is composed of fibres in all respects 

 similar to those of organic muscle, though mingled with 

 fine elastic filaments. The observation of the action of 

 arteries will show, ist, the operation of a contractile 

 power in arteries, essentially distinct from their elasticity ; 

 and, zndly, the identity of this power with muscular 

 contractility. 



