230 



THE CIRCULATION OF THE BLOOD 



it is seen in some cases of nephritis where the arteries are rigid and the periph- 

 eral resistance is high. 



The dicrotic wave is the most important of the secondary waves, and 

 has been the subject of much discussion. It is constantly present in pulse 

 tracings, but varies in height. In point of time the dicrotic wave occurs 

 immediately after the closure of the aortic semilunar valves. In certain 

 conditions, generally of disease, it becomes so marked as to be quite plain 

 to the unaided finger. Such a pulse is called dicrotic. The generally ac- 

 cepted explanation of the cause of the dicrotic wave is that it represents a 



B 



FIG. 200. A, Normal Pulse Tracing from Radial of Healthy Adult Obtained by the 

 Sphygmometer; B, from same artery, with the same extra-arterial pressure, taken during 

 acute nasal catarrh. 1-2 Anacrotic limb; 2-8 Catacrotic limb; 3 Predicrotic notch; 

 5 Dicrotic crest; 6 Postdicrotic notch; 7 Postcrotic crest. 



rebound of the overdistended artery at the time of the closure of the aortic 

 valves. During systole, as the blood is forcibly injected into the aorta, 

 there is an overdistention of the artery. The systole suddenly ends, the 

 aorta by reason of its elasticity tends to recover itself, the blood is driven 

 back against the semilunar valves, closing them and at the same time giv- 

 ing rise to a wave, the dicrotic wave, which begins at the heart and travels 

 onward toward the periphery like the primary wave. According to Foster, 

 the conditions favoring the development of dicrotism are: i, a highly ex- 

 tensible and elastic arterial wall; 2, a comparatively low mean blood press- 

 ure, leaving the extensible reaction free scope to act; 3, a vigorous and rapid 

 stroke of the ventricle discharging into the aorta a considerable quantity 

 of blood. The other secondary waves are probably due to the oscillations 



