112 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. X. 



clamp which fixes the reed, so that different lengths of it can be 

 made to vibrate. If a long piece of reed vibrates, it does so slowly, 

 and thus successive induction shocks at long intervals can be sent 

 into the nerve. But if one wishes to 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 student 



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.) 



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's instrument. 



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 parch- 

 ment; 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 will mark a 



