226 



ARTIFICIAL PULSE. 



[Boon 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 Kthe 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. 59. 

 123. 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. 58, 61 &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 in 

 Fig. 61, in which are given three tracings taken from the same 



FIG. 61. PULSE TRACINGS FROM THE 

 SAME RADIAL ARTERY UNDEK DIF- 

 FERENT PRESSURES OF THE LEVER. 



The letters are explained in a later 

 part of the text. Taken with 

 .Dudgeon's sphygmograph. 



