168 



ARTIFICIAL MODEL. 



[Book i. 



the base line ; no mean arterial pressure, or very little, is estab- 

 lished. The contents of the ventricle (syringe) thrown into the 



Fig. 31. Tracings taken from an artificial scheme with the peripheral 



resistance slight. 



A, Arterial. V, Venous Manometer. This figure, to save space, is on a smaller 

 scale than the corresponding Fig. 32. 



arterial system distend it, but the passage through the peri- 

 pheral region is so free that an equal quantity of fluid passes 

 through to the veins immediately, and hence the mercury at 

 once falls. But the fluid thus passing easily into the veins 

 distends these too, and the mercury in their manometer rises 

 too, but only to 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 screw- 

 ing 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. 32, A', 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 ventricular 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 discharge 

 through the narrowed peripheral region as much fluid as it 

 received from the ventricle, a second stroke drives more fluid into 

 the arteries, distending them this time more than it did before, 

 and raising the mercury to 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 



