INSTRUMENTS. 



[CH. X. 



stimulate a nerve more rapidly, the length of reed allowed to 

 vibrate can be shortened. 



In Ewald's modification of the coil there is another simple method 

 of modifying the rate of the interrupter. But an hour spent 

 in the laboratory with an induction coil and cell will teach the 



Fig. 140. Myograph of von Helmholtz, shown in an incomplete form, a, forceps for 

 holding frog's femur ; 6, gastrocnemius ; c, sciatic nerve ; d, scale pan ; e, marker 

 recording on cylinder; /, counterpoise. (M'Kendrick.) 



student much more easily all these facts than any amount of 

 reading and description. 



We can pass now to the myograph. There are many different 

 forms of this instrument. Fig. 140 shows Helmholtz' instru- 

 ment. 



The bony origin of the gastrocnemius is held firmly by forceps, 

 and the tendo Achillis tied to a weighted lever ; the end of the 

 lever is provided with a writing-point such as a piece of pointed 

 parchment ; when the muscle contracts it pulls the lever up, and 

 this movement is magnified at the end of the lever. The writing- 

 point scratches on a piece of glazed paper covered with a layer of 

 soot ; the paper is wrapped round a cylinder. When the lever 

 goes up the writing-point will mark an up-stroke ; when it falls it 



