ELASTICITY OF ARTERIES. 137 



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. 



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. 161) 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 contrac- 

 tion. 



(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. 140). (5.) By means of 

 their elastic coat the arteries are enabled to adapt them- 



