PULSE TK AGINGS. 305 



waves of oscillation can be seen as a gradually decreasing series 

 in tracings taken from elastic tubes, but we cannot say positively 

 that they occur in the arteries. When several secondary waves 

 exist in the pulse curve, the smaller ones probably depend on 

 oscillation caused by.the lever of the instrument. 



The dicrotic wave does not depend on the instrument, because 

 the skilled finger laid on the radial artery at the wrist can easily 

 detect it, and it can be directly seen in the vessel when the pul- 

 sation in the arteries is visible, or when a jet of blood escapes from 

 an artery. 



When a new charge of blood is shot into the aorta the elastic 

 wall of the vessel is suddenly stretched. At the same time a 

 shock is given to the column of blood, and the fluid next the 

 valves is moved forwards with great velocity. Owing to its in- 

 ertia the fluid tends to pass onwards from the valves, and thus 



FIG. 140. 



Tracing drawn by Marey's Sphygmograph. The surface moved from 

 right to left. The vertical upstrokes show the period when the shock is 

 given by the systole of the ventricle. The upper wave on the downstroke 

 shows when the blood has ceased to enter the aorta. Then comes the di- 

 crotic depression which is a negative wave produced by the momentary 

 backflow in aorta, and the dicrotic elevation caused by the closure of the 

 valves. 



allows a momentary fall in pressure, which is at once followed by 

 the reflux of the blood and the forcible closure of the valves. 



The first crest or apex of the pulse, curve corresponds to the 

 shock given by the systole, and is greatly exaggerated by the in- 

 ertia of the lever. The crest of the predicrotic wave marks the 

 moment when the blood ceases to flow from the ventricle, and 

 therefore it is the real head of the pulse wave. 



The dicrotic wave has been explained as (1) a wave of oscil- 



26 . 



