CHAP, iv.] THE VASCULAR MECHANISM. 129 



the transmission of the shock is so small, that it may be neglected. 

 This result remains the same when any resistance to the now is 

 introduced into the system. The force of the pump remaining the 

 same, the introduction of the resistance undoubtedly lessens the 

 quantity issuing at the distal end at each stroke, but it does so 

 simply by lessening the quantity entering at the proximal end; 

 the income and outgo remain equal to each other, and occur at 

 almost the same time. And what is true of the two ends, is also 

 true of any part of the course of the system, so far, at all events, as 

 the following proposition is concerned, that in a system of rigid 

 tubes, either with or without an intercalated resistance, the flow 

 caused by an intermittent force is, in every part of the tubes, 

 intermittent synchronously with that force. 



In a system of elastic tubes in which there is little resistance to 

 the progress of the fluid, the flow caused by an intermittent force 

 is also intermittent. The outgo being nearly as easy as the 

 income, the elasticity of the walls of the tubes is scarcely at all 

 called into play. These behave practically like rigid tubes. 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 system from the pump, tends to accumulate on the 

 proximal side of the resistance. This it is able to do by expanding 

 the elastic walls of the tubes. At each stroke of the pump a 

 certain quantity of fluid enters the system 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 011 the proximal side of the resistance, the elastic tubes 

 having expanded to receive it. During the interval between this 

 and the next stroke, the distended elastic tubes, striving to return 

 to their natural undistended condition, press on this extra quantity 

 of fluid which they contain and tend to drive it past the resistance. 

 Thus in the rigid system (and in the elastic system without 

 resistance) there issues, from the distal end of the system, 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 system 

 with resistance, on the contrary, the quantity which passes the 

 resistance is only a fraction of that which enters the system from 

 the pump, the remainder or a portion of the remainder continuing 

 to pass during the interval between the strokes. In the former 

 case, the system 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 dis- 

 tension of that part of the system, at the close of each stroke an 

 accumulation and distension, however, which go on diminishing 

 until the next stroke comes. 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 tubes ; 

 and the amount which passes the resistance before the next stroke 



F. 



