CHAP, iv.] THE VASCULAR MECHANISM. 213 



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 re- 

 sistance) 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 resist--^ 

 ance, 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 begin- 

 ning ; in the latter case there is an accumulation 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 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 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, 



