CHAP, iv.] THE VASCULAR MECHANISM. 163 



of the mean arterial pressure; at the next moment the pulse 

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

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

 it falls again until it is once more at rest, the wave having 

 completely passed by. 



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

 gradual, and is generally marked with some irregularities. The 

 suddenness of the rise is due to the suddenness with which the 

 sharp stroke of the pump expands the tube; the fall is more 

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

 returns to its former condition after the expanding power of the 

 pump has ceased, is gradual in its action. 



2. The size and form of each curve depend in part on the 

 amount of pressure exerted by the levers on the tube. If the 

 levers only just touch the tube in its expanded state, the rise 

 in each will be insignificant. If on the other hand they be pressed 

 down too firmly, the tube beneath will not be able to expand 

 as it otherwise would, and the rise of the levers will be proportion- 

 ately diminished. There is a certain pressure, depending on the 

 expansive power of the tubing, at which the tracings are best 

 marked. 



3. If the points of the two levers be placed exactly one under 

 the other on the recording surface, it is obvious that, the levers 

 being alike except for their position on the tube, any difference in 

 time between the movements of the two levers will be shewn by 

 an interval between the beginnings of the curves they describe, if 

 the recording surface be made to travel sufficiently rapidly. 



If the movements of the two levers be thus compared, it will be 

 seen that the far lever (Fig. 27, II.) commences later than the near 

 one (Fig. 27, 1.), the farther apart the two levers are, the greater is 

 the interval in time between their curves. Compare the series 

 I. to VI. (Fig. 27). This means that the wave of expansion, the 

 pulse-wave, takes some time to travel along the tube. By exact 

 measurement it would similarly be found that the rise of the near 

 lever began some fraction of a second after the stroke of the pump. 



The velocity with which the pulse-wave travels depends chiefly 

 on the amount of rigidity possessed by the tubing. The more 

 extensible (with corresponding elastic reaction) the tube, the slower 

 is the wave ; the more rigid the tube becomes, the faster the wave 

 travels. The width of the tube is of much less influence, though ac- 

 cording to some observers the wave travels more slowly in the wider 

 tubes. 



The rate at which the normal pulse- wave travels in the human 

 body has been variously estimated at from 10 to 5 metres per 

 second. In all probability the lower estimate is the more correct 

 one ; but it must be remembered that in all probability the rate 

 varies very considerably under different conditions. According to 

 all observers the velocity of the wave in passing from the groin to 



112 



