56o 



A MANUAL OF PHYSIOLOGY 



of the recording plate opens, at a definite point in its 

 passage, a key in the primary coil of an induction machine, 

 and so causes a shock to be sent through the muscle or 

 nerve, which is connected with the secondary. The precise 

 point at which the stimulus is thrown in can be marked 

 on the tracing by carefully bringing the plate to the position 

 in which the key is just opened, and allowing the lever to 

 trace here a vertical line (or, rather, an arc of a circle). The 

 portion of the time-tracing between this line and a parallel 



FIG. 167. SPRING MYOGRAPH. 



A, B, iron uprights, between which are stretched the guide-wires on which tne 

 travelling plate a runs ; k, pieces of cork on the guides to gradually check the plate at 

 the end of its excursion, and prevent jarring ; b, spring, the release of which shoots the 

 plate along ; h, trigger-key, which is opened by the pin d on the frame of the plate. 



line drawn through the point at which the contraction 

 begins gives the latent period. 



Helmholtz measured the length of the latent period by 

 means of the principle of Pouillet, that the deflection of a 

 magnet by a current of given strength and of very short 

 duration is proportional to the time during which the current 

 acts on the magnet. He arranged that at the moment of 

 stimulation of the muscle a current should be sent through 

 a galvanometer, and should be broken by the contraction of 

 the muscle the moment it began. In this way he obtained 



