2l8 



THE CIRCULATION OF THE BLOOD 



The Arterial Flow. The character of the flow of blood through the 

 arterial system depends to a very considerable extent upon the structure 

 of the arterial walls, and particularly upon the elastic tissue which is so highly 

 developed in them. 



The elastic tissue of the arteries, first of all, guards them from the sud- 

 denly exerted pressure to which they are subjected at each contraction of the 

 ventricles. In every such contraction, as is above seen, the contents of the 



FIG. 192. Cross Section of the Aorta to Show Elastic Tissue; e, elastic elements. (Bailey.) 



ventricles are forced into the arteries more quickly than they are discharged 

 through the capillaries. The blood, therefore, being for an instant resisted 

 in its onward course, a part of the force with which it is impelled is directed 

 against the sides of the arteries; under this force their elastic walls dilate, 

 stretching enough to receive the blood, and becoming more tense and more 

 resisting as they stretch. Thus by yielding they break the shock of the 

 force impelling the blood. On the subsidence of the pressure, should the 

 ventricles cease contracting, the arteries are able by the same elasticity to 

 resume their former caliber. 



The elastic tissue in the same way equalizes the current of blood by main- 

 taining pressure on it in the arteries during the period at which the ventri- 



