CHAP, iv.] THE VASCULAR MECHANISM. 165 



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 con- 

 dition, 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 quantity 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 accu- 

 mulation of fluid between the pump and the resistance, and a 

 corresponding distension of that part of the tube, at the close of 

 each stroke, an accumulation and distension, 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 between 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 conti- 

 nuous, 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 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 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 tube on the near side of the resistance, will be 

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



