146 THE CIRCULATION. 



muscles. But elasticity is a property not exclusively, 

 though especially, seated in the elastic portion of the 

 middle coat ; indeed, all the coats are in some measure 

 elastic, and will recoil after being distended; and the 

 effect their elasticity produces is yet further assisted by 

 -the elasticity of the tissues around them. 



The purposes of the elasticity of arteries are chiefly these ; 

 ist, To guard them from the suddenly exerted pressure to 

 which they are subjected at each contraction of the ven- 

 tricles. In every such contraction, the contents of the ven- 

 tricles are forced into the arteries more quickly than they 

 can be discharged into and through the capillaries. 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 yield- 

 ing, 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. Elasticity 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 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, 



