THE CIRCULATION 249 



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

 as of the 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 we pass out to the periphery, where, as already seen, it 

 finally disappears altogether. 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 like 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 we 

 knew the speed of the vessel. 



The same method may be applied to the arterial pulse. 

 By recording the changes produced by the pulse wave as it 

 passes any point in an artery the shape of the wave may 

 be deduced. 



This may be done by any of the various forms of sphygmo- 

 graph. (Practical Physiology, Chap. XI.) 



Such a tracing is not a true picture of the wave, but 

 simply of the effect of the wave on one point of the arterial 

 wall. Its apparent length depends upon the rate at which 

 the recording surface is travelling and not on the length of 

 the wave. 



Its height depends in part upon the length of the recording 

 lever, in part upon the resistance offered by the instrument, 

 in part upon the degree of pressure with which the instru- 

 ment is applied to the artery, and in part on the thickness of 

 the arterial wall. 



Such a trace shows (Fig. 120) 



1st. That the pulse waves follow one another without any 

 interval. 



2nd. That the rise of the wave is much more abrupt than 

 the fall. 



