292 



THE CIRCULATION IN THE BLOOD-VESSELS [oil. XXII. 



inscribe the magnified record of the arterial movement on a travelling 

 surface. 



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

 the numerous modifications of which is represented in figures 255, 

 256, and 257, and of Dudgeon (fig. 258). 



FIG. 257. The Sphygmograph applied to the arm 



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 



FIG. 258. Dudgeon's Sphygmograph. The dotted outline represents the piece of blackened paper on 

 which the sphygmogram is written. 



sphygmogram; this is specially noticeable in Dudgeon's Sphygmo- 

 graph. But these defects may be overcome by the use of some 

 form of sphygmometer. (See later, p. 295.) It is also important 

 to remember that the pad or button placed upon the artery rests 

 partly on the venae comites, so that not only arterial tension but any 



