THE MAIN FACTS OF THE CIRCULATION. 145 



each stroke, but it at the same time lessens the quantity entering at the 

 proximal end ; the inflow and outflow remain equal to each other, and still 

 occur at the same time. 



In an elastic tube, such as an India-rubber one (or in a system of such 

 tubes), whose sectional area is sufficiently great to offer but little resistance 

 to the progress of the fluid, the flow caused by an intermittent force is also 

 intermittent. The outflow being nearly as easy as the inflow, the elasticity 

 of the walls of the tube is scarcely at all called into play. The tube be- 

 haves practically like a rigid tube. When, however, sufficient resistance is 

 introduced into any part of the course, the fluid being unable to pass by 

 the resistance as rapidly as it enters the tube from the pump, tends to accu- 

 mulate on the proximal side of the resistance. This it is able to do by ex- 

 panding the elastic walls of the tube. At each stroke of the pump a cer- 

 tain quantity of fluid enters the tube at the proximal end. Of this only a 

 fraction can pass through the resistance during the stroke. At the moment 

 when the stroke ceases, the rest still remains on the proximal side of the 

 resistance, the elastic tube having expanded to receive it. During the 

 interval between this and the next stroke, the distended elastic tube, striving 

 to return to its natural undistended condition, presses on this extra quantity 

 of fluid which it contains and tends to drive it past the resistance. 



Thus, in the rigid tube (and in the elastic tube without the resistance) 

 there issues, from the distal end of the tube at each stroke, just as much 

 fluid as enters it at the proximal end, while between the strokes there is 

 perfect quiet. In the elastic tube with resistance, on the contrary, the quan- 

 tity which passes the resistance is only a fraction of that which enters the 

 tube from the pump at any one stroke, the remainder or a portion of the- 

 remainder continuing to pass during the interval between the strokes. In 

 the former case the tube is no fuller at the end of the stroke than at the 

 beginning ; in the latter case there is an accumulation of fluid between the 

 pump and the resistance, and a corresponding distention of that part of the 

 tube at the close of each stroke an accumulation and distention, however, 

 which go on diminishing during the interval between that stroke and the 

 next. The amount of fluid thus remaining after the stroke will depend on 

 the amount of resistance in relation to the force of the stroke and on the 

 distensibility of the tube ; and the amount which passes the resistance before 

 the next stroke will depend on the degree of elastic reaction of which the 

 tube is capable. Thus, if the resistance be very considerable in relation to 

 the force of the stroke, and the tube very distensible, only a small portion 

 of the fluid will pass the resistance, the greater part remaining lodged be- 

 tween the pump and the resistance. 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 accumula- 

 tion of fluid on the near side of the resistance, an additional distention of 

 the tube, an additional strain on its 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 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 stroke a state of things will 

 be at last arrived at in which the elastic force, called into play by the con- 

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