PRACTICAL PHYSIOLOGY 



29 



jects from it, by striking against a spring fixed out- 

 side, instantaneously makes and breaks the circuit 

 at each revolution. Although two induction shocks 

 are thereby produced in the secondary circuit, they 

 follow one another so closely as to act as a single 

 stimulus. 1 Either attach a pair of electrodes to the 

 muscle itself or lay the nerve upon them. The elec- 



Muscle. 



Spring. 



. Drum. 



FIG. 26. GRAPHIC RECORD OF MUSCLE-TWITCH. THE HYOGLOSSUS (TONGUE OF FROG) 

 is REPRESENTED IN THIS DIAGRAM INSTEAD OF THE GASTROCNEMIUS. THE MUSCLE 

 AND LEVER SHOULD BE ON THE RIGHT SIDE OF THE DRUM (WHICH MUST REVOLVE 

 FROM RIGHT TO LEFT) ; NOT ON THE LEFT SIDE, AS HERE REPRESENTED. 



trodes are connected through a short-circuit key 

 with the secondary coil. Before the lever is allowed 

 actually to touch the cylinder determine that the 

 apparatus is all in working order, and at what dis- 



J It is also possible to employ a single induction shock as the stim- 

 ulus by introducing a break key into the primary circuit and mak- 

 ing the pin open this key as the drum revolves ; but this is not neces- 

 sary, since it is easy with the arrangement above described to slide 

 the secondary coil at such a distance from the primary that, even if 

 the drum is revolving slowly, only the break shock is effective. 



-l B.R A'i 



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