148 



THE VASCULAR MECHANISM. 



fall again, as a corresponding quantity issues from the ends of the veins into 

 the basin, which serves as an artificial auricle. Now introduce " peripheral 

 resistance " by screwing up the clamps on the connecting tubes, and set the 

 pump to work again as before. With the first stroke the mercury in the 

 arterial manometer (Fig. 44, A 1 ) rises as before, but instead of falling rapidly 

 it falls slowly, because it now takes a longer time for a quantity of fluid 

 equal to that which has been thrust into the arterial system by the ventricu- 

 lar stroke to pass through the narrowed peripheral region. Before the curve 

 has fallen to the base line, before the arterial system has had time to dis- 

 charge through the narrowed peripheral region as much fluid as it received 

 from the ventricle, a second stroke drives more fluid into the arteries, dis- 

 tending them this time more than it did before, and raising the mercury to 



FIG. 44. 



Tracings taken from an Artificial Scheme, with the Peripheral Resistance Considerable: A l t 

 arterial ; V 1 , venous manometer. 



a still higher level. A third, a fourth, and succeeding strokes produce the 

 same effect, except that the additional height to which the mercury is raised 

 at each stroke becomes at each stroke less and less, until a state of things is 

 reached in which the mercury, being on the fall when the stroke takes place, 

 is by the stroke raised just as high as it was before, and then beginning to 

 fall again is again raised just as high, and so on. With each succeeding 

 stroke the arterial system has become more and more distended ; but the 

 more distended it is the greater is the elastic reaction brought into play ; this 

 greater elastic reaction more and more overcomes the obstacle presented by 

 the peripheral resistance and drives the fluid more and more rapidly through 

 the peripheral region. At last the arterial system is so distended, and the 

 force of the elastic reaction so great, that during the stroke and the succeed- 

 ing interval just as much fluid passes through the peripheral region as enters 

 the arteries at the stroke. In other words, the repeated strokes have estab- 

 lished a mean arterial pressure which, at the point where the manometer 



