28 STUDIES IN LIFE AND SENSE. 



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 disten- 

 sion 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 elastic 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. Distension 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 onwards 

 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 over-distended on the near side of the 

 resistance; while these elastic tubes, so treated, discharge themselves 

 in turn onwards, and at a rate which corresponds to that with which 

 the force-pump action of the heart charges them from behind. And 

 so, tracing the hydraulics of the circulation through its phases, we see, 

 firstly, the heart over-distending the elastic arteries. We witness the 

 arteries emptying themselves into their minute continuations, the 

 capillaries, and through these latter into the veins or return vessels. 

 The economy is witnessed here in the easy means adapted for con- 

 verting without complications a spasmodic flow of blood into a con- 

 tinuous stream ; insuring also that the amount of blood which flows 

 from the arteries to the veins during the heart's stroke and pause 



