232 THE DICROTIC WAVE. [Book i. 



characteristic forms of pulse-curve. Were we able with certainty 

 to trace back the several features of the curves to their respective 

 causes, an adequate examination of sphygmographic tracings 

 would undoubtedly disclose much valuable information concerning 

 the condition of the body presenting them. The problems, how- 

 ever, of the origin of these secondary waves and of their variations 

 are complex and difficult; so much so that the detailed interpre- 

 tation of a sphygmographic tracing is still in many cases and in 

 many respects very uncertain. 



§ 128. The Dicrotic Wave. The chief interest attaches to 

 the nature and meaning of the dicrotic wave. In general the 

 main conditions favouring the dicrotic wave are (1) a highly 

 extensible and elastic arterial wall ; (2) a comparatively low mean 

 pressure, leaving the extensible and elastic reaction of the arterial 

 wall free scope to act; and (3) a vigorous and rapid stroke of the 

 ventricle, discharging into the aorta a considerable quantity of 

 blood. 



The origin of this dicrotic wave has been and indeed still is 

 much disputed. 



In the first place, observers are not agreed as to the part of 

 the arterial system in which it first makes its appearance. In 

 such a system as that of the arteries we have to deal with two 

 kinds of waves. There are the waves which are generated at the 

 pump, the heart, and travel thence onwards towards the periphery ; 

 the primary pulse-wave due to the discharge of the contents of 

 the ventricle is of this kind. But there may be other waves 

 which, started somewhere in the periphery, travel backwards 

 towards the central pump ; such are what are called ' reflected ' 

 waves. For instance, when the tube of the artificial model, bear- 

 ing two levers, is blocked just beyond the far lever, the primary 

 wave is seen to be accompanied by a second wave, which at the 

 far lever is seen close to, and often fused into, the primary wave 

 (Fig. 59, VI. a'), but at the near lever is at some distance from it 

 (Fig. 59, I. a'), being the farther from it the longer the interval 

 between the lever and the block in the tube. The second wave is 

 evidently the primary wave reflected at the block and travelling 

 backwards towards the pump. It thus, of course, passes the far 

 lever before the near one. And it has been argued that the 

 dicrotic wave of the pulse is really such a reflected wave, started 

 either at the minute arteries and capillaries, or at the several 

 points of bifurcation of the arteries, and travelling backwards to 

 the aorta. But if this were the case the distance between the 

 primary crest and the dicrotic crest ought to be less in arteries 

 more distant from, than in those nearer to the heart, just as in 

 the artificial scheme the reflected wave is fused with a primary 

 wave near the block (Fig. 59, VI. 6 a. a'), but becomes more and 

 more separated from it the farther back towards the pump we trace 

 it (Fig. 59, I. 1. a. a'). Now this is not the case with the dicrotic 



