284 THE DICROTIC WAVE. [BOOK i. 



But this secondary expansion in turn gives place in a similar 

 manner to another shrinking, and indeed may, in a similar manner, 

 be followed by still other oscillations. And, though the predi- 

 crotic wave when it occurs presents difficulties which we cannot 

 now discuss, the dicrotic wave may on this view be regarded as 

 the main secondary expansion so originating. 



As we urged however in 134, the arguments which led to the 

 view that the ventricle, in a normal beat, discharges the whole of 

 its contents before it has finished its contraction do not appear to 

 be valid. We saw reason to think that the flow from the ventricle 

 into the aorta ceases because the contraction of the ventricle 

 ceases, and not because there is no more blood to be discharged. 

 Hence there is no need to appeal to a suddenly developed 

 negative pressure, such as that upon which the foregoing 

 explanation is based, and that explanation in consequence falls to 

 the ground. 



On the other hand, the simultaneous curves of endocardiac and 

 aortic pressure (Fig. 55 and others) shew us that the end of the 

 systole is, in a normal beat, coincident with the dicrotic notch as it is 

 called, with the depression immediately preceding the dicrotic wave. 

 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 adopt the following 

 explanation of the dicrotic wave. The flow from the ventricle 

 into the aorta ceases because the systole ceases; the cessation 

 takes place while the two cavities 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 lessening 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, pressing upon its con- 

 tents. 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 to- 

 wards 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 

 not necessary that the valves should act perfectly, and the dicrotic 

 wave may occur in cases where the valves are more or less 

 incompetent ; all that is required for its production is an adequate 



