STRUCTURE OF ARTERIES. 135 



heart's action. In it, too, the little vasa vasorum find a 

 suitable tissue in which to subdivide for the supply of the 

 arterial coats. 



(2.) The purpose of the elastic tissue, which enters so 

 largely into the formation of all the coats of the arteries, is ; 

 1st, To guard the arteries from the suddenly exerted 

 pressure to which they are subjected at each contraction of 

 the ventricles. In every such contraction, the contents of 

 the ventricles are forced into the arteries more quickly 

 than they can be discharged into and through the capil- 

 laries. The blood therefore being, for an instant, resisted 

 in its onward course, a part of the force with which it was 

 impelled is directed against the sides of the arteries ; under 

 this force, which might burst a brittle tube, their elastic 

 walls dilate, stretching enough to receive the blood, and 

 as they stretch, becoming more tense and more resisting. 

 Thus, by yielding, they, as it were, break the shock of the 

 force impelling the blood, and exhaust it before they are 

 in danger of bursting, through being overstretched. Elas- 

 ticity is thus advantageous in all arteries, but chiefly so in 

 the aorta and its large branches, which are provided, as 

 already said, with a large proportional quantity of elastic 

 tissue, in adaptation to the great force of the left ventricle, 

 which falls first on them, and to the increased pressure of 

 the arterial blood in violent expiratory efforts. 



On the subsidence of the pressure, when the ventricles 

 cease contracting, the arteries are able, by the same elas- 

 ticity, to resume their former calibre ; and in thus doing, 

 they manifest the 2nd chief purpose of their elasticity, that, 

 namely, of equalizing the current of the blood by main- 

 taining pressure on the blood in the arteries during the 

 periods at which the ventricles are at rest or dilating. If 

 some such method as this had not been adopted if for 

 example the arteries had been rigid tubes, the blood, 

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

 have been propelled through the arterial system in a series 



