352 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



onwards or from the heart, and backwards or to the heart. Its 

 course, in the latter direction, is speedily checked by the sudden 

 and forcible closure of the aortic valves under the pressure of the 

 regurgitating current. Therefore, the great mass of the blood rushes 

 onwards towards the capillary system, propelled first by the 

 heart's impulse, and, secondly, by the elastic reaction of the 

 arterial walls. 



This elastic reaction of the parietes of the arteries does not come 

 into play until the heart has ceased to contract, and begun to 

 dilate. It is, therefore, synchronous with the diastole of the heart, 

 and corresponds with it in duration ; so that while the ventricle is 

 inactive, the blood in the arteries is still being pressed upon by the 

 reacting arterial walls. Thus the blood is ever moving onwards 

 throughout the arterial system, during the diastole, as well as 

 during the systole of the heart ; and the jerking impulses commu- 

 nicated to it by the successive contractions of the ventricle are 

 gradually converted into that continuous uniform forward move- 

 ment, which is observed under ordinary circumstances in the ulti- 

 mate arterial ramifications, the capillaries and the veins. 



An analogous application of the reacting force of an elastic 

 agent to convert a jerking movement into a continuous stream is 

 found in the mechanism of the fire-engine, and of the organ. In 

 the one water, in the other air, is forced into a chamber in which 

 air already exists. This air undergoes compression by the sudden 

 introduction of a new quantity of water or air. Its elasticity causes 

 it to react, and thus to supply an expulsive force during the subsi- 

 dence of the action of the piston in the one case, and of the bellows 

 in the other .* 



The heart, by its propulsion of blood into the arterial system, 

 not only dilates the arteries, but elongates them likewise. This is 

 generally better seen than their dilatation, but it is most apparent 

 in arteries which are curved. Under the influence of the heart's 



* This explanation of the influence of the elastic reaction of the arterial 

 wall in promoting a continuous stream, and converting the jerking current of 

 the blood in the large arteries into a uniform one in the small ones, is very com- 

 monly attributed by modern writers to Weber. English physiologists ought 

 not to have overlooked John Hunter's remarks (on the Blood, &c., 4to ed. 

 p. 129), nor Sir C. Bell's observations, in his "Animal Mechanics," p. 44. But 

 the following passage from Hales will show that that able observer held much 

 the same views long prior to either of those last named. * * * " The blood 

 in the arteries," he says, " being forcibly propelled forward, with an accelerated 

 impetus, thereby dilates the canal of the arteries, which begin again to contract 

 at the instant the systole ceases : by which curious artifice of nature, the blood 



