672 THE POPULAR SCIENCE MONTHLY. 



much water will leave the tube as enters it under the two conditions of 

 no resistance and of the presence of such obstacle to the flow. If now 

 we substitute for our rigid pipe an elastic one, the resistance to the 

 water-flow is diminished, no doubt, but the fluid will, as before, issue 

 in jets ; that is, in an intermittent and not continuous fashion. There 

 is "easy come and easy go" in the elastic tube, as in the rigid one 

 where no resistance exists. The elasticity, in other words, is not called 

 upon to act in modifying the flow because the course of the fluid is 

 clear and open. Suppose, now, that some obstacle or resistance is in- 

 troduced into the elastic tube. The fluid can not escape so readily as 

 before, and it tends, as a matter of course, to accumulate on the near 

 or pump side of the obstacle. The tube gives, so to speak, and accom- 

 modates the water which is forced to wait its turn for exit. Each 

 stroke of the pump, it is true, sends its quantity into the tube, but 

 between the strokes the swollen and expanded tubes, in virtue of their 

 elasticity, act as an aid to the pump, and by exercising their power 

 force the accumulated fluid past the point of resistance. There is rest 

 in the rigid tube between the pump-strokes. There is, contrariwise, 

 activity in the elastic tube, due to the overcoming by its elasticity of 

 the obstacle to the flow, and to its work of keeping the fluid moving 

 and of avoiding distention and blockage. It is possible, moreover, to 

 conceive of the elastic reaction of the tube being so great that the 

 accumulated fluid will be made to pass the knotty point before the 

 next stroke of the pump occurs. Let us imagine, lastly, that the strokes 

 succeed one another in rapid succession, and that the elasticity of the 

 tube is powerful enough to overcome the resistance opposing the flow 

 of fluid, and we shall arrive at a state of matters wherein not only 

 will the obstacle become practically non-existent while as much fluid 

 leaves the tube as enters it, but the flow from the far end of the tube 

 will also be converted into a continuous and stable stream. 



This latter condition of matters is exactly reproduced in the circula- 

 tion of the blood. There is great resistance found on the arterial side 

 of the heart. Each impulse has to send blood into a vessel which is 

 clastic in itself, as we have seen ; but immediately on the first stroke 

 of the heart succeeds a second. Hence the blood accumulates on the 

 heart's side before that propelled by the first stroke has been com- 

 pletely disposed of. Distention and strain of the vessel succeed, and 

 one of two results must follow. Either the circulating arrangements 

 must collapse, or the elasticity of the tubes into which the blood is 

 being perpetually forced will acquire power sufficient to overcome the 

 resistance, and to propel onward the amount of blood with which 

 each stroke of the heart charges the circulation. Here the true mean- 

 ing of the rapid work of the heart and of the elasticity of the arteries 

 becomes apparent. The otherwise intermittent flow of blood is con- 

 verted into a continuous stream. The heart keeps the arteries over- 

 distended on the near side of the resistance, while these elastic tubes, 



