22o PHYSIOLOGICAL PHYSICS. [Chap, xx, 



Now what effect on the velocity and rate of dis- 

 charge of the fluid does the elastic force produce 1 



Comparison between rigid and elastic 

 tubes. Suppose both tubes to be under precisely the 

 same conditions, except that the one tube is distensible 

 and the other not. Let both be filled with fluid, and 

 be under the influence of the same intermittent force, 

 projecting additional quantities of fluid into them. In 

 the case of the rigid tube there is no means of increasing 

 the accommodation of the tube for the new quantity 

 of fluid, because it is already full, is inextensible, and 

 the fluid is not compressible. It follows, then, that a 

 quantity of fluid must pass out of the tube precisely 

 equal to the quantity that enters, and at the same 

 moment. In short, the intermittent action of the 

 pressure is accompanied by an intermittent efflux, the 

 interval between the cessation of the pressure and its 

 recurrence being marked by no flow. The shock, that 

 is to say, which has been received is communicated at 

 the same instant to the fluid in every part of the tube; 

 it has its maximum in every part at the same time, 

 and it disappears at the same time. In an elastic 

 tube, the molecules in the immediate neighbourhood 

 of the point of afflux experience almost the same 

 effect of intermitten.ee. Their equilibrium is suddenly 

 disturbed by a shock, which passes off, leaving them, 

 after a few oscillations, to come to rest until they are 

 disturbed by another shock. But this effect is not 

 communicated to the parts of the tube at some dis- 

 tance from the point of afflux. The impulse is not 

 transmitted in full force throughout the whole tube. 

 Part only is so transmitted, and a large portion is 

 expended in distending the elastic walls of the tube 

 in the immediate neighbourhood of the point of entry 

 of the projected fluid. As soon as the pressure begins 

 to diminish, the elastic reaction of the walls of the 

 tube comes into play, the recoil of the walls of the 



