BLOOD VESSELS 441 



in pressure is indicated by the dicrotic notch. But the 

 elasticity of the semilunar valves at once makes them spring 

 up, thus increasing the pressure in the aorta and causing the 

 second crest, the dicrotic wave (fig, 184, c). After this the 

 pressure in the arteries steadily diminishes till the minimum 

 is reached, to be again increased by the next ventricular 

 systole. 



If all the blood does not leave the ventricle in the first 

 gush, e.g. when the intra-aortic pressure is high as compared 

 with the force of the heart (fig. 175, continuous line), there 

 is a residual outflow which, by catching the lever of the 

 sphygmograph on its back-spring from the initial crest, may 

 again raise it, causing the predicrotic wave. 



It is thus manifest that the foron of the pulse wave varies 

 according to the relationship between the arterial pressure 

 and the activity of the heart. It is not the actual activity 

 of the heart or the actual arterial pressure, but their relation- 

 ship to one another which is of importance. Thus, a heart 

 actually weak may, with a low arterial pressure, be relatively 

 active. 



(A) If the heart is active and strong i7i relation to the 

 arterial pressure, the main mass of the blood is expelled in 

 the first sudden outflow, and the residual flow is absent or 

 slight (fig. 175, dotted line). In this case there is a sudden 

 and marked rise of the arterial pressure, followed by a steady 

 fall till the moment of ventricular diastole. The rebound of 

 the semilunar valves is marked and causes a very prominent 

 dicrotic wave, while the predicrotic wave is small or absent 

 (fig. 184, B.). Such a condition is well seen after violent 

 muscular exertion, and in certain fevers. In these con- 

 ditions the dicrotic wave may be so well marked that it can 

 be felt with the finger. 



(B) On the other hand, if the ventricles are acting slowly 

 or feebly in relationship to the arterial pressure, the initial 

 outflow of blood does not take place so rapidly and com- 

 pletely (fig. 175, continuous line), and the initial rise in the 

 pulse is thus not so rapid. The residual outflow of blood is 

 more marked and causes the well-marked secondary rise in 

 the pulse curve — the predicrotic wave. In certain cases, 



