ORIGIN AND PROPERTIES OF THE DICROTIC ELEVATION. 



artery. The most conspicuous features of the sphygmographic tracing 

 are the two entirely distinct elevations in the descending limb of the 

 curve. The more prominent of the two occupies approximately the 

 center of the descending limb, where it appears as a distinct elevation 

 (R); it is known as the dicrotic after-beat or, with reference to its origin, 

 as the recoil-elevation. 



The sphygmographic tracing reproduces the chronological course of the 

 pressure exerted by the undulatory movement of the blood on the arterial wall, 

 the pad of the sphy gmograph , which is supported on a spring, rising and falling 

 with the variations in pressure; the instrument therefore records "pressure- 

 pulse." 



ORIGIN AND PROPERTIES OF THE DICROTIC ELEVATION. 



The recoil-elevation (also designated secondary or dicrotic) is pro- 

 duced in the following manner: After the column of blood propelled 

 into the arterial system by the ventricular 

 systole has generated a positive wave, 

 which, beginning at the aorta, extends 

 rapidly to all of the arteries, even to the 

 minutest arterial branches, in which it 

 disappears, the arteries contract as soon as 

 closure of the semilunar valves prevents 

 the further entrance of blood. The elasti- 

 city and the active contraction of the 

 blood-vessels thus exerts a counter pressure 

 on the blood-column. The blood is forced 

 to seek an outlet. In its progress toward 

 the periphery it finds no obstacle in its 

 path, but the portion that escapes toward 

 the center of the circulation recoils from 

 the already closed semilunar valves. The 

 impact of the blood sets up another posi- 

 tive wave, which is again propagated into 

 the arteries and disappears as before in 

 the remotest minute branches. If, how- 

 ever, there is sufficient time for the com- 

 plete development of the sphygmographic 

 tracing, a second reflected wave is pro- 

 duced in the proximal arteries (especially 

 in the short course of the carotids, but 

 also in the arteries of the upper ex- 

 tremities, but not in those of the lower extremities because of their 

 great length) in the same way as the first. Just as the pulse appears 

 somewhat later in the more peripheral arteries than in those nearer the 

 heart, so the secondary wave, produced by the recoil of the blood from 

 the aortic valves, also appears later in the more distant arteries. Both 

 kinds of waves, the primary and the secondary pulse-wave, and possibly 

 also the tertiary recoil -wave, originate at the same point and are propa- 

 gated in the same way. The longer the distance to be traveled before 

 they reach a given point in the artery, the later will be their arrival 

 at that point. 



The following laws with regard to the recoil-elevation have been 

 determined experimentally : 



FIG. 49. Hemautographic Tracing from 

 the Posterior Tibial Artery of a Large 

 Dog: P, primary pulse-wave; R, re- 

 coil-elevation; e e, elasticity-eleva- 

 tions. 



