CHAP, iv.] THE VASCULAR MECHANISM. 259 



At each stroke of the pump, each lever rises until it reaches 

 a maximum (Fig. 45 la, 2a, &c.) and then falls again, thus 

 describing a curve. The rise is due to the expansion of the part of 

 the tube under the lever, and the fall is due to that part of the 

 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 in the 

 diagram Fig. 46 in which the wave shewn by the dotted line is 



y 



.--1- 



i i 



Y X 



FlG. 46. A ROUGH DIAGRAMMATIC EEPRESENTATION 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. 



172 



