208 THE CIRCULATION OF THE BLOOD 



The origin of the secondary waves is to some extent a matter of uncer- 

 tainty. The anacrotic wave occurs when the peripheral resistance is high; 

 that is, when, for some time during the systole, the flow from the aorta toward 

 the periphery is slower than the flow from the ventricle into the aorta. Thus 

 it is seen in some cases of nephritis where the arteries ase 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- 



FIG. 198. Diagram of Pulse Tracing. A , upstroke or anacrotic limb; B, downstroke or kat- 

 acrotic limb; C, pre-dicrotic wave; D, dicrotic; E, post-dicrotic wave. 



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 view of the cause of the dicrotic wave is that it represents a rebound 

 from the closed aortic valves. During systole, as the blood is forcibly in- 

 jected 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, 



FIG. 199. Sphygmogram from the Radial Artery Taken with Marey's Sphygmograph. 



(Langendorff.) 



the blood is driven back against the semilunar valves, closing them and at 

 the same time giving rise to a wave, the dicrotic wave, which begins at the 

 heart and travels onward toward the periphery like the primary wave. Ac- 

 cording to Foster, the conditions favoring the development of dicrotism are: 

 i, a highly extensible and elastic arterial wall; 2, a comparatively low mean 

 blood pressure, leaving the extensible reaction free scope to act; 3, a vigorous 

 and rapid stroke of the ventricle discharging into the aorta a considerable 



