268 DICROTISM. [BOOK i. 



a little later on in referring to the predicrotic wave and shall see 

 that, under the view we have just given, the closing of the semi- 

 lunar valves is to be regarded rather as the effect than the cause 

 of the dicrotic wave. Many authors however give an interpre- 

 tation of the dicrotic wave different from that detailed above. 

 Thus it is held that the primary shrinking, from A onwards, being 

 brought to bear on the column of blood already come to rest, in 

 face of the great pressure in front, drives the blood back against 

 the semilunar valves, thus closing them, and that the impact of 

 the column of blood against the valves starts a new wave of ex- 

 pansion, which reinforcing the natural tendency of the elastic 

 walls to expand again after their primary shrinking produces the 

 dicrotic wave C. On this view it is the blood driven back from 

 the valves which expands the artery; on the view given above 

 it is the expanding artery which draws the blood back towards 

 the valves. 



Moreover, quite other views have been or are held concerning 

 this dicrotic wave. According to many authors it is what is called 

 a ' reflected ' wave. Thus, when the tube of the artificial model 

 bearing 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. 45, VI. a'), but at the near lever is at some distance from it 

 (Fig. 45, 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 points 

 of bifurcation of the larger 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. 45, VI. 6 a. a), but becomes more and 

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

 it (Fig. 45, I. 1 a. a'). Now this is not the case with the dicrotic 

 wave. Careful measurements shew that the distance between 

 the primary and dicrotic crests is either greater or certainly not 

 less in the smaller or more distant arteries than in the larger 

 or nearer ones. This feature indeed proves that the dicrotic 

 wave cannot be due to reflection at the periphery or indeed in 

 any way a retrograde wave. Besides the multitudinous peripheral 

 division would render one large peripherically reflected wave im- 

 possible. Again, the more rapidly the primary wave is obliterated 

 or at least diminished on its way to the periphery the less con- 

 spicuous should be the dicrotic wave. Hence increased extensi- 



