260 



ARTIFICIAL PULSE. 



[BOOK i. 



The curves below X, Y, Z represent, in a similarly diagram- 

 matic fashion, the curves described, during the passage of the wave, 

 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 T 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 x, the rest of the curve, as shewn by the 

 dotted line, having yet to be described. 



But to return to the consideration of Fig. 45. 

 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 

 brings about the return of the tube to its former calibre after the 

 expanding power of the pump has ceased, 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. 44, 47, &c.). 



Indeed the difference between the 

 up-stroke and the down-stroke is 

 even more marked in the latter 

 than in the former, the delivery 

 of blood from the ventricle being 

 more rapid than the issue of water 

 from a pump as ordinarily worked. 

 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 diminished. 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 



FIG. 47. PULSE TRACINGS FKOM THE 

 SAME RADIAL ARTERY UNDER DIF- 

 FERENT PRESSURES OF THE LEVER. 



The letters are explained in a later 

 part of the text. 



