CHAP, iv.] THE VASCULAR MECHANISM. 233 



wave ; careful measurements shew that the distance between 

 the primary and dicrotic crests is either the same 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 probably render one large peripherically reflected wave im- 

 possible. Again, the more rapidly the primary wave is oblite- 

 rated or at least diminished on its way to the periphery the less 

 conspicuous should be the dicrotic wave. Hence increased ex- 

 tensibility and increased elastic reaction of the arterial walls 

 which tend to use up rapidly the primary wave, should also lessen 

 the dicrotic wave. But as a matter of fact these conditions, as we 

 have said, are favourable to the prominence of the dicrotic wave. 



We may conclude then that the dicrotic wave like the primary 

 wave begins at the heart, and travels thence towards the periphery. 

 But even if this be admitted observers are not agreed as to the 

 mechanism of its production. The following view is the one 

 which seems the most satisfactory, though it is not accepted 

 by all inquirers. 



The simultaneous curves of endocardiac and aortic pressure 

 (Fig. 54 and others) shew us that the dicrotic notch as it is called, 

 the depression immediately preceding the dicrotic wave is, in a 

 normal beat, coincident with the end of the systole. The curve 

 of the differential manometer further shews us that this is the 

 point at which the pressure in the ventricle begins to become less 

 than in the aorta. We may therefore reason in the following 

 way. The flow from the ventricle into the aorta ceases because 

 the systole ceases ; the cessation takes place while the two cavi- 

 ties are still open to each other, and probably, in most cases at 

 least, while there is still more or less blood in the ventricle. The 

 pressure in the ventricle tends to become less than that in the aorta, 

 and the blood in the aorta tends to flow back into the ventricle. 

 But the first effect of this is to close firmly the semilunar valves. 

 The expansion of the aorta, (which in many cases had been lessen- 

 ing even during the systole owing to the flow through the periphery 

 of the arterial system being more rapid than the flow from the 

 ventricle, but in some cases, in the anacrotic instances, had not,) 

 lessens with the cessation of the flow ; the aorta shrinks, press- 

 ing upon its contents. But part of this pressure is spent on the 

 closed semilunar valves, and the resistance offered by these starts 

 a new wave of expansion, the dicrotic wave, which travels thence 

 onwards towards the periphery in the wake of the primary wave. 

 If we admit that the blood is flowing from the ventricle during 

 the whole of the systole, we must also admit that the semilunar 

 valves do not close until the end of the systole, and this being, as 

 shewn by the curves, just antecedent to the dicrotic wave, we may 

 attribute this wave to the rebound from the closed valves. It is 



