264 VETERINARY PHYSIOLOGY 



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 

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

 last 1-5 seconds. The pulse wave takes 1*5 seconds to pass 

 any place, and it travels at 10 metres per second ; its length 

 then is 15 metres. 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 caus- 

 ing it, but the character of the arterial wall modifies it very 

 largely. Thus the true height of the pulse wave in the great 

 arteries near the heart is masked by the thickness of the 

 arterial wall. 



Speaking generally, however, we may say that the pulse 

 wave is highest near the heart, and becomes lower and lower 

 as it passes out to the periphery, where it finally disappears 

 altogether (fig. 128). This disappearance is due to its force 

 becoming expended in expanding the arterial wall. 



4. The Form of the Wave. Waves at sea vary greatly in 

 form, and the form of the wave might be graphically recorded 

 on some moving surface such as the side of a ship by some float- 

 ing body. If the ship were stationary a simple vertical line 

 would be produced, but if she were moving a curve would be 

 recorded, more or less abrupt according to her speed. From 

 this curve the shape of the wave might be deduced, if the 

 speed of the vessel is known. 



The same method may be applied to the arterial pulse. 



