136 THE CIRCULATION. 



of jerks corresponding to the ventricular contractions, with 

 intervals of almost complete rest during the inaction of the 

 ventricles. But in the actual condition of the arteries, 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 arteries ; and in the 

 intervals between the contractions of the ventricles, the 

 force of the recoiling and contracting arteries is employed 

 in continuing the same direct propulsion. Of course, the 

 pressure exercised by the recoiling arteries is equally 

 diffused in every direction through the blood, and th 

 blood would tend to move backwards as well as onwards 

 but that all movement backwards is prevented by the 

 closure of the semi-lunar arterial valves, which takes place 

 at the very commencement of the recoil of the arterial 

 wills. 



By this exercise of the elasticity of the arteries, all the 

 force of the ventricles is made advantageous to the circula- 

 tion ; for that part of their force which is expended in 

 dilating the arteries, is restored in full, according to that 

 law of action of elastic bodies, by which they return to the 

 state of rest with a force equal to that by which they were 

 disturbed therefrom. There is thus no loss of force ; but 

 neither is there any gain, for the elastic walls of the artery 

 cannot originate any force for the propulsion of the blood 

 they only restore that which they received from the ventri- 

 cles; they would not contract had they not first been 

 dilated, any more than a spiral spring would shorten itself 

 unless it were first elongated. The advantage of elasticity 

 in this respect is, therefore, not that it increases, but that 

 it equalizes or diffuses the force derived from the periodic 

 contractions of the ventricles. The force with which the 

 arteries are dilated every time the ventricles contract, 

 might be said to be received by them in store, to be all 

 given out again in the next succeeding period of dilatation 

 of the ventricles. It is by this equalizing influence of the 



