CH. XXI.] 



THE KYMOGRAPH 



271 



inspiration),* the smaller ones to the 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 measured by the difference 

 of level between a and a (fig. 270). 



Fig. 271 shows a more complete view of the manometer, and 



FIG. 272. Diagram of mercurial Kymograph. A, Revolving cylinder, worked by a clockwork arrange- 

 ment contained in the box (B), the speed being regulated by a fan above the box ; the cylinder is 

 supported by an upright (b), and is capable of being raised or lowered by a screw (a), by a handle 

 attached to it; D, c, E, represent the mercurial manometer, vrhich is shown on a larger scale in 

 fig. 271. 



fig. 272 is a diagram of the arrangement by means of which it is 

 made into a kymograph. 



Fig. 273 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 



* The explanation of the respiratory curves on the tracing is postponed till 

 after we have studied Respiration. 



