230 



THE HUMAN BODY. 



-d" 



are very littie bigger than the aorta, we ought to find a 

 pulse, but we do not: the venous pulse which sometimes 

 occurs having quite a different cause, being due to a back- 

 flow from the auricles, or a checking of the on-flow into 

 them, during the cardiac systole. The rhythmic flow 

 caused by the heart is therefore not merely cloaked in the- 

 small arteries and capillaries but abolished in them. 



We can, however, readily contrive conditions outside the; 

 Body under which an intermittent supply is transformed 



into a continuous flow. Sup- 

 pose we have two vessels, A 

 and B (Fig. 86), containing; 

 water and connected below in 

 two ways; through the tube 

 a on which there is a pump- 

 provided with valves so that 

 it can only drive liquid from 

 ^ A to B, and through b,. 

 )j which may be left wide open 

 or narrowed by the clamp c, 

 at will. If the apparatus be 

 left at rest the water will lie at the same level, d, in each 

 vesssel. If now we work the pump, at each stroke a cer- 

 tain amount of water will be conveyed from A to B, and 

 as a result of the lowering of the level of liquid in A and 

 its rise in B, there will be immediately a return flow from 

 B to A through the tube b. A, in these circumstances, 

 would represent the venous system, from which the heart 

 constantly takes blood to pump it into B, representing the 

 arterial system; and b would represent the capillary vessels 

 through which the return flow takes place: but, so far, we 

 should have as intermittent a flow through the capillaries, 

 b y as through the heart-pump, a. Now imagine b to be 

 narrowed at one point so as to oppose resistance to the 

 back-flow, while the pump goes on working steadity. The re- 

 sult will be an accumulation of water in B, and a fall of its 

 level in A. But the more the difference of level in the two 

 vessels increases, the greater is the force tending to drive 

 water back through b to A, and more will flow back, under 



Fia. 86. 



