130 INTERMITTENT FLOW. [BOOK T. 



will depend on the degree of elastic reaction of which the tubes 

 are capable. Thus, if the resistance be very considerable in 

 relation to the force of the stroke, and the tubes very distensible, 

 only a small portion of the fluid will pass the resistance, the 

 greater part remaining lodged between the pump and the re- 

 sistance. If the elastic reaction be great, a large portion of this 

 will be passed on through the resistance before the next stroke 

 comes. In other words, the greater the resistance (in relation to 

 the force of the stroke), and the more the elastic force is brought 

 into play, the less intermittent, the more nearly continuous, will be 

 the flow on the far side of the resistance. 



If the first stroke be succeeded by a second stroke before its 

 quantity of fluid has all passed by the resistance, there will be an 

 additional accumulation of fluid on the near side of the resistance, 

 an additional distension of the tubes, an additional strain on their 

 elastic powers, and, in consequence, the flow between this second 

 stroke and the third will be even more marked than that between 

 the first and the second, though all three strokes were of the same 

 force, the addition being due to the extra amount of elastic force 

 called into play. In fact, it is evident that, if there be a sufficient 

 store of elastic power to fall back upon, by continually repeating 

 the strokes a state of things will be at last arrived at, in which the 

 elastic force, called into play by the continually increasing dis- 

 tension of the tubes on the near side of the resistance, will be 

 sufficient to drive through the resistance, between each two strokes, 

 just as much fluid as enters the near end of the system at each 

 stroke. In other words, the elastic reaction of the walls of the 

 tubes will have converted the intermittent into a continuous flow. 

 The flow on the far side of the resistance is in this case not the 

 direct result of the strokes of the pump. All the force of the 

 pump is spent, first in getting up, and afterwards in keeping up, 

 the over-distension of the tubes on the near side of the resistance ; the 

 cause of the continuous flow lies in the over-distension of the tubes 

 which leads them to empty of themselves into the far side of the 

 resistance, at such a rate, that they discharge through the resistance 

 during a stroke and in the succeeding interval just as much as 

 they receive from the pump by the stroke itself. 



This is exactly what takes place in the vascular system. The 

 friction in the minute arteries and capillaries presents a considerable 

 resistance to the flow of blood through them into the small veins. 

 In consequence of this resistance, the force of the heart's beat is 

 spent in maintaining the whole of the arterial system in a state 

 of over-distension, as indicated by the arterial pressure. The over- 

 distended arterial system is, by the agency of its elastic walls, con- 

 tinually emptying itself by overflowing through the capillaries into 

 the venous system, overflowing at such a rate, that just as much 

 blood passes from the arteries to the veins during each systole 

 and its succeeding diastole as enters the aorta at each systole. 



