THE CIRCULATION 



125 



illustration (Fig. 19) which accompanies this may be help- 

 ful. The pump delivers water intermittently to the leaky 

 trough, keeping it filled to a level which is nearly constant, 

 though fluctuating a little in the rhythm of the strokes. 

 Meanwhile the escape of water through the cracks is 

 all but uniform in its rate. One important difference be- 

 tween the pump and trough, on the one hand, and the cir- 

 culatory system, on the other, lies in the fact that in the 



Fig. 19. At 7 the discharge is in gushes with pauses between 

 the type of the expulsion of blood from the heart At C the escape 

 through the cracks is at a practically constant rate; this is true of the 

 blood flow through the capillaries. 



first case the driving force is gravity; in the second, it is 

 the reaction of the enclosing elastic walls. 



A set of facts which it is well to separate in one's thought 

 as completely as possible from considerations of pressure 

 is the body of data respecting the linear velocity of the 

 blood. By this is meant the rate of advance of the aver- 

 age corpuscle. In any vessel the stream runs more swiftly 

 in the central axis and lags along the walls. The velocity 

 in the arteries rises and falls as does the pressure, but, on 



