274 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XX11. 



mercury, is an ivory float, from which a long steel wire extends 

 upwards, and terminates in a stiff piece of parchment or a bristle 

 which writes on a moving surface covered with smoked paper. When 

 the two limbs of the mercury are at rest, the writing-point inscribes 

 a base-line or abscissa on the travelling surface ; when the pressure 

 is got up by the syringe it writes a line at a higher level. When 

 the arterial clip is removed it writes waves as shown in the diagram 



PIG. 241. 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, which is shown on a larger scale in 

 fig. 240. 



(fig. 239), the large waves corresponding to respiration (the rise of 

 pressure in most animals accompanying 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. 239). 



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

 after we have studied Respiration. 



