CH. XXI.] 



SPHYGMOGRAPHS 



The instruments most frequently used are those of Marey, one of 

 the numerous modifications of which is represented in figures 286, 

 287, and 288, and of Dudgeon (fig. 289). 



FIG. 289. Dudgeon's Sphygmograph. The dotted outline represents the piece of blackened paper on 

 which the sphygmogram is written. 



Each instrument is provided with an arrangement by which the 

 pressure can be adjusted so as to obtain the best record. The 

 measurement of the pressure is, however, rough, and both instruments 

 have the disadvantage of giving oscillations of their own to the 

 sphygmogram ; this is specially notice- 

 able in Dudgeon's sphygmograph. 

 But these defects may be overcome 

 by the use of some form of sphyg- 

 mometer. (See later, p. 292). It is 

 also important to remember that the 

 pad or button placed upon the artery 

 rests partly on the vense comites, so 

 that not only arterial tension but any 

 turgidity arising from venous conges- 

 tion, will affect the height and form 

 of the sphygmographic record. 



Fig. 290 represents a typical sphyg- 

 mographic tracing obtained from the radial artery. It consists of 

 an upstroke due to the expansion of the artery, and a downstroke 

 due to its retraction. The descent is more gradual than the up- 

 stroke, because the elastic recoil acts more constantly and steadily 



T 



FIG. 290. Diagram of pulse-tracing. A, up- 

 stroke ; B, downstroke ; c, pre-dicrotic 

 wave; D, dicrotic; E, post-dicrotic wave. 



