THE ARTERIAL FLOW 



197 



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- 



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



cles are at rest or are dilating. If the arteries were rigid tubes, the blood, 

 instead of flowing as it does in a constant stream, would be propelled through 

 the arterial system in a series of spurts corresponding in time to the ventric- 

 ular contractions and with intervals of almost complete rest during the in- 

 action of the ventricles. But in the actual condition of the vessels, the force 

 of the successive contractions of the ventricles is expended partly in the 

 direct propulsion of the blood, and partly in the dilatation of the elastic ar- 

 teries; and in the intervals between the contractions of the ventricles, the 

 force of the recoil is employed in continuing the flow onward. Of course 

 the pressure exercised is equally diffused in every direction, and the blood 



