304 MANUAL OF PHYSIOLOGY. 



the wave travels (10 m. per sec.) and the time it takes to pass 

 any given point ( sec.), its length may be calculated to be about 

 three metres, or about twice as long as the longest artery. Thus 

 the pulse wave reaches the most distant artery in one-sixth of a 

 second, or about the middle of the ventricular systole, and when 

 the wave has quite passed from the arch of the aorta, the summit 

 of it has only just reached the arterioles. 



Numerous instruments have been invented for the demonstration 

 and graphic representation of the pulse in the human being. Of 

 these the one most commonly used is Marey's Sphygmograph 



FIG. 139. 



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

 the straps at B, n, 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 R 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. 



(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 



