THE CIRCULATION OF THE BLOOD 



321 



aL 



-d' 



FIG. 110. 



ought to find a pulse, but we do not : the venous pulse which some- 

 times 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 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 con- 

 tinuous flow. Suppose we have two 

 vessels, A and B (Fig. 110) contain- 

 ing 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, 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 result of the lower- 

 ing 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, repre- 

 senting 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, 6, 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 steadily. The result will be an accumula- 

 tion 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 6 to A, and more will 

 flow back, under the greater difference of pressure, in a given time, 

 until at last, when the water in B has reached a certain level, d', 

 and that in A has correspondingly fallen to d", the current through 

 b will carry back in one minute just so much -water as the pump 

 sends the other way, and this back-flow will be nearly constant; 



