310 MANUAL OF PHYSIOLOGY. 



(Fig. 139), by means of which a graphic record of the pulse is 

 made, in the form of a tracing of a series of elevations and depres- 

 sions (Fig. 140). The elevations correspond to the onset of a 

 wave, and the depressions to its departure, or to the temporary 

 rise and fall of the arterial pressure. In the falling part of the 

 curve an irregularity caused by a slight second wave is nearly 

 always seen. This is called the dicrotic wave. Sometimes there 

 are more than one of these secondary waves, the most constant of 

 which is a small wave preceding the dicrotic, called predicrotic ; 

 but the dicrotic is always more marked than any other. Several 

 waves of oscillation can be seen as a gradually decreasing series 



FIG. 139. 



Marey's Sphygmograph. The frame (B, B, B) is fastened to the wrist by the straps at 

 B, B, and the rest of the instrument lies on the forearm. The end of the screw (v) rests 

 on the spring (R), the button of which lies on the radial artery. Any motion of the 

 button at K is communicated to v, which moves the lever (L) up and down. When in 

 position, the blackened slip of glass (p) is made to move evenly by the clockwork (H) so 

 that the writing point draws a record of the movements of the lever. 



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 



