BLOOD VESSELS 437 



distance from one another may be taken, and the time which 

 the wave takes to pass between them may be measured. 



In this way it is found that the pulse wave travels at 

 about 9 or 10 metres per second — about thirty times as 

 fast as the blood flows in the arteries (p. 465). 



2. Length of the Wave. — To determine this in a wave at 

 sea is easy, if we know its velocity and know how long it 

 takes to pass any one point. The same method may be 

 applied to the pulse wave. We know its velocity, and, by 

 placing the finger on an artery, we may determine that one 

 wave follows another in rapid succession, so that there is no 



a h c a c 



A. B. 



Fig. 184. — ^Two Pulse Tracings — A. with a relatively sluggish heart and 

 relatively high arterial pressure ; B. with a relatively active heart 

 and relatively low arterial pressure. Both show the primary crest 

 exaggerated by the inertia of the sphygmograph. 



pause between them. Each wave lasts the length of a 

 cardiac cycle. There are about 40 cycles per minute — i.e. 

 per 60 seconds; hence, each must last 1'5 second. The 

 pulse wave takes I'o second to pass any place, and it 

 travels at 10 metres per second ; its length then is 15 

 metres, or about five times the length of the body. It is 

 then an enormously long wave, and it has disappeared at 

 the periphery long before it has finished leaving the aorta. 



3, The Height of the Wave. — The height of the pulse 

 wave, as of a wave at sea, depends primarily on the pressure 

 causing it. It is really the difference between the maximum 

 systolic pressure (fig. 190) and the minimum diastolic 

 pressure. It may be most accurately measured by determin- 



