328 



CIRCULATORY SYSTEM. 



arteries. It is attached to the wrist, and the point of the lever 

 makes a sphygrnogram upon the card previously smoked. It 

 is an instrument which needs great care in its use in order to 

 make its records of practical value. Fig. 172 shows a sphygmo- 

 gram of the radial pulse. It represents five complete pulsa- 

 tions of the artery and the beginning of a sixth. The upstroke 

 is caused by the expansion of the artery due to the arrival of the 

 pulse-wave, while the downstroke is due to the retraction of the 

 vessel. The upstroke, the primary or percussion-wave, is abrupt, 

 because the systole of the left ventricle is abrupt, but the down- 



FIG. 174. Plethysmograph for arm: p, float attached by A to a lever which 

 records variations of level of the water in B, and therefore variations in the vol- 

 ume of the arm in the glass vessel c. Or the plethysmograph may be connected 

 to a recording tambour. The tubulure at the upper part of C is closed when the 

 tracing is being taken (Stewart). 



stroke is gradual, because of the fact that the return of the 

 artery to its former condition is gradual by virtue of its elas- 

 ticity. The downstroke is seen to be made up of a number 

 of waves: First, the predicrotic or tidal wave; second, the 

 dicrotic wave; and third, the post-dicrotic wave. These sec- 



FIG. 175. Plethysmograph tracing from arm. The tracing was taken by means 

 of a tambour connected with the plethysmograph ; the dicrotic wave is distinctly 

 marked (Stewart). 



ondary waves of the downstroke are termed katacrotic. There is 

 sometimes a secondary wave in the upstroke, called anacrotic. 

 The predicrotic and post-dicrotic waves are supposed to be due 



