276 



ARTIFICIAL PULSE. 



[BOOK i. 



by levers placed on the points x, y, z. At Z the greater part of 

 the wave has already passed under the lever, which during its 

 passage has already described the greater part of its curve, shewn 

 by the thick line, and has only now to describe the small part, 

 shewn by the dotted line, corresponding to the remainder of the 

 wave from Z to H. At Y the lever is at the summit of the wave. 

 At X the lever has only described a small part of the beginning of 

 the wave, viz. from C to a?, the rest of the curve, as shewn by the 

 dotted line, having yet to be described. 



But to return to the consideration of Fig. 60. 

 141. The rise of each lever is somewhat sudden, but the fall 

 is more gradual, and is generally marked with some irregularities 

 which we shall study presently. The rise is sudden because the 

 sharp stroke of the pump suddenly drives a quantity of fluid into 

 the tubing and so suddenly expands the tube ; the fall is more 

 gradual because the elastic reaction of the walls of the tube, which, 

 after the expanding power of the pump has ceased, brings about 

 the return of the tube to its former calibre driving the fluid 

 onwards to the periphery, is more gradual in its action. 



These features, the suddenness of the rise or up-stroke, and the 

 more gradual slope of the fall or down-stroke, are seen also in 

 natural pulse-curves taken from living arteries (Figs. 59, 62 &c.). 

 We shall see, however, that under certain circumstances this 

 contrast between the up-stroke and the down-stroke is not so 



marked. 



It may here be noted that the actual size of the curve, that is 



the amount of excursion of the 



lever, depends in part (as does also 



to a great extent the form of the 



curve) on the amount of pressure 



exerted by the lever on the tube. 



If the lever only just touches the 



tube in its expanded state, the rise 



will be insignificant. If on the 



other hand the lever be pressed 



down too firmly, the tube beneath 



will not be able to expand as it 



otherwise would, and the rise of the 



lever will be proportionately dimin- 

 ished. There is a certain pressure 



which must be exerted by the lever 



on the tube, the exact amount 



depending on the expansive power 



of the tubing and on the pressure 



exerted by the fluid in the tube, 



in order that the tracing may be 



best marked. This is shewn 



AB p p 



FIG. 62. PULSE TRACINGS FROM THE 

 SAME RADIAL ARTERY UNDER DIF- 

 FERENT PRESSURES OF THE LEVER. 



The letters are explained in a later 

 part of the text. Taken with 

 Dudgeon's sphygmograph. 

 is stiewn in 

 Fig. 62, in which are given three tracings taken from the same 



