288 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXI. 



individual heart-beats. The blood-pressure is really twice as great 

 as that indicated by the height of the tracing above the abscissa, 

 because if the manometer is of equal bore throughout, the mercury 

 falls in one limb the same distance that it rises in the other ; 

 the true pressure is the difference of level between a and a' 



_; * i / 



Fig. 275 shows a more complete view of the manometer, and 



Fig. 276. Diagram of mercurial kymograph. A, revolving cylinder, worked by a clock- 

 work arrangement contained in the box (B), the speed being regulated by a fan above 

 the box ; the cylinder is supported by an upright (&), and is capable of being raised or 

 lowered by a screw (a), by a handle attached to it; D, c, B, represent the mercurial 

 manometer, which is shown on a larger scale in flg. 275. 



fig. 276 is a diagram of the arrangement by means of which it is 

 made into a kymograph. 



Fig. 277 shows a typical normal arterial blood-pressure tracing 

 on a larger scale. 



In taking a tracing of venous blood-pressure, the pressure is 

 so low and corresponds to so few millimetres of mercury, that a 

 saline solution is usually employed instead of mercury. If the 

 vein which is investigated is near the heart, a venous pulse 



