248 HUMAN PHYSIOLOGY 



and to accommodate this the aortic wall expands. But since 

 the aorta communicates with the other arteries this increased 

 pressure passes outwards along them expanding their wall as 

 it goes. 



The pulse wave may thus be compared to a wave at sea, 

 which is also a wave of increased, pressure, the only differ- 

 ence being that, while the waves at sea travel freely over the 

 surface, the pulse wave is confined in the column of blood, 

 and manifests itself by expanding the walls of the arteries. 



It greatly simplifies the study of the pulse to regard it in 

 this light, and to study it just as we would study a wave 

 at sea. 



1. Velocity. To determine how fast a wave is travelling 

 we might select two points at a known distance from one 

 another, and with a watch note how long the wave took to 

 pass from one to the other. So with the pulse wave, two 

 points on an artery at a known distance from one another 

 may be taken and the time which the wave takes to pass 

 between them may be measured. 



It is thus 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. 



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. Suppose it is travelling at 

 50 feet per second, and that it takes 1 second to pass a par- 

 ticular point, obviously it is 50 feet in length. 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 pause between them. Each wave corre- 

 sponds to a ventricular systole, and therefore each wave must 

 last, at any point, just the time between two ventricular 

 systoles just the time of a cardiac cycle. There are about 

 70 cycles per minute i.e. per 60 seconds ; hence each must 

 last - 88 second. The pulse wave takes 0'8S second to pass 

 any place, and it travels at 30 feet per second ; its length then 

 is 26*4 feet, or about Jive 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. 



