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 introduced 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 accommodates 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 circulation 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 completely 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 meaning of the rapid work of the heart and of the elasticity of the arteries becomes apparent. The otherwise intermittent flow of blood is converted into a continuous stream. The heart keeps the arteries overdistended on the near side of the resistance, while these elastic tubes,
