WORKING OF THE HEAET AND BLOOD-VESSELS. 237 



the capacity of the blood-vessels increases from the heart to 

 the capillaries, an acceleration of the flow during the ven- 

 tricular contraction which might be very manifest in the 

 vessels near the heart would become less and less obvious in 

 the more distant vessels. But if this were so, then when the 

 blood was collected again from the wide capillary sponge into 

 the great veins near the heart, which together are but little 

 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 sys- 

 tole. The rhythm of the 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. Suppose we 

 have two vessels, A and B (Fig. 

 97), containing water and con- 

 nected 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 

 #, 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 vessel. If now we work the pump, at 

 each stroke a certain 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 7). A, in these circum- 

 stances, would represent the venous system, from which the 

 heart constantly takes blood to pump it into B, representing 

 the arterial system ; and I would represent the capillary ves- 

 sels through which the return flow takes place; but, so far, 

 we should have as intermittent a flow through the capillaries, 

 b, as through the heart-pump, a. Now imagine 1) to be nar- 

 rowed at one point so as to oppose resistance to the back-flow, 

 while the pump goes on working steadily. The result will be 

 nn accumulation of water in B, and a fall of its level in A. 



at. 



FIG. 97. 



