THE CIRCULATION OF THE BLOOD. 



217 



and is made to rest lightly on the artery, the pulsations of which it is 

 desired to investigate. The up-and-down movement of the button is 

 communicated to the lever, to the hinder end of which is attached a 

 slight spring, which allows the lever to move up, at the same time that 



Fig. 180. Diagram of the lever of the Sphygmograph. 



it is just strong enough to resist its making any sudden jerk, and in the 

 interval of the beats also to assist in bringing it back to its original 

 position. For ordinary purposes the instrument is bound on the wrist 

 (fig. 181). 



It is evident that the beating of the pulse with the reaction of the 

 spring will cause an up-and-down movement of the lever, the pen of 

 which will write the effect on a smoked card, which is made to move by 

 clockwork in the direction of the arrow. Thus a tracing of the pulse 

 is obtained, and in this way much more delicate effects can be seen than 

 can be felt on the application of the finger. 



Fig. 181. The Sphygmograph applied to the arm. 



Two forms of Sphygmograph are shown in figs. 179, 182, viz. , a modifica- 

 tion of the original instrument of Marey and Dudgeon's. Marey's instrument, 

 and indeed all modifications of it, suffer from the defect that there is no ade- 

 quate method of measuring the pressure exercised by the button of the instru- 

 ment upon the artery, and that it is difficult to be certain of the exact position 

 it occupies over the artery. Dudgeon's Sphygmograph, although very conven- 

 ient to use, is, according to Roy and Adami, even less satisfactory, and the 

 tracings obtained by it are so disfigured by inertia vibrations as to render 

 them more or less worthless. "The mechanical construction of the instrument 

 is such as to render great inertia vibrations unavoidable. " These authors, have 



