THE ARTERIES. 



271 



carried into the reservoir, and they fill it half full of water, 

 C ; the mouth of the pipe, D, which is to convey away 

 the water, reaches into the water in the reservoir. As 

 the water rises, the air is compressed : so that, although 

 the pumps act alternately, the elasticity of the contained 

 air acts uninterruptedly in pressing on the surface of the 

 water, and raising it by the tube, D, in an equable stream. 

 The elasticity of the contained air, fills up the interval 

 between the actions of the pumps, and admits of no in- 

 terruption to the force with which the water is propelled 

 upwards. 



Surely these are sufficient indications of the necessity 

 of three powers acting in propelling the blood from the 

 heart. The first, is a sudden and powerful action of 

 the ventricle : the second, is a contraction of the artery, 

 somewhat similar, excited by its distention : the third, 

 though a property independent of fife, is a power permit- 

 ting no interval or alternation ; it is the elasticity of the 

 coats of the artery : and these three powers, duly adjust- 

 ed, keep up a continued stream in the blood-vessels. It 

 is true, that when an artery is wounded, the blood flows 



