CHAP, iv.] THE VASCULAR MECHANISM. 275 



tube returning after the expansion to its previous calibre. The 

 curve is therefore the curve of the expansion (and return) of 

 the tube at the point on which the lever rests. We may call 

 it the pulse-curve. It is obvious that the expansion passes 

 by the lever in the form of a wave. At one moment the lever 

 is at rest : the tube beneath it is simply distended to the normal 

 amount indicative of the mean pressure which at the time obtains 

 in the arterial tubes of the model ; at the next moment the pulse 

 expansion reaches the lever, and the lever begins to rise ; it 

 continues to rise until the top of the wave reaches it, after which 

 it falls again until finally it comes to rest, the wave having 

 completely passed by. 



It may perhaps be as well at once to warn the reader that the 

 figure which we call the pulse-curve is not a representation of the 

 pulse-wave itself; it is simply a representation of the movements, 

 up and down, of the piece of the wall of the tubing at the spot on 

 which the lever rests during the time that the wave is passing 

 over that spot. We may roughly represent the wave by the 

 diagram Fig. 61, in which the wave shewn by the dotted line is 



FlG. 61. A KOUGH DIAGRAMMATIC REPRESENTATION OF A PULSE-WAVE PASSING 



OVER AN ARTERY. 



passing over the tube (shewn in a condition of rest by the thick 

 double line) in the direction from H to C. It must however be 

 remembered that the wave thus figured is a much shorter wave 

 than is the pulse-wave in reality (that being, as we shall see, 

 about 6 meters long), i.e. occupies a smaller length of the arterial 

 system from the heart H towards the capillaries C. Moreover the 

 actual pulse-wave has secondary features, which we are neglecting 

 for the present and which therefore we do not attempt to shew in 

 the figure. 



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

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



182 



