XIV ] BLOOD PRESSURE. 149 



a. Open the clamps marked c, c and c", so that 

 as little resistance as possible intervenes be- 

 tween the arterial and venous tubes. Bring 

 the manometers to mark on the revolving 

 cylinder, placing V about an inch under A, 

 in the same vertical line. Work the pump 

 steadily, regulating the time with the metro- 

 nome. 



In A, the mercury rises at each stroke, and 

 in the interval between each two strokes 

 falls again to its previous level. (The mo- 

 mentum of the mercury frequently carries it 

 below this level, and the descent may be 

 followed by one or more oscillations.) 

 In V, a similar rise and fall is observed, of 

 nearly if not quite the same extent. 



b. Close the clamps c, c' and c", so that the 

 capillary resistance becomes very considerable. 

 In A, the mercury rises rapidly at the first 

 stroke, and at the end of the stroke begins to 

 fall again, but more slowly than was the case 

 in a. It has not fallen far before the second 

 stroke raises it to a higher level than before. 

 On falling still again, it is once more raised 

 to a yet higher level, but the increase is not 

 so great as before. Each succeeding stroke 

 has a similar effect. Thus at the end of a 

 few strokes, the mean arterial pressure is 

 reached, marked only by comparatively small 

 oscillations corresponding to the strokes of 

 the pump. 



