MUSCLE- JS'ERVE PH\'bIOLOGY. 



509 



tion coil, and the one employed for physiological purposes is mostly Du Bois 

 Reymond's, the one seen in fig. 323. 



Wires from a battery are brought to the two binding screws d' and d. a key 

 intervening. These binding screws are the ends of a coil of coarse covered wire 



Fig. 321. Du Bois Reymond's Key. 



Fig 322. Mercury Key. 



c, called the primary coil. The ends of a coil of finer covered wire g, are attached 

 to two binding screws to the left of the figure, one only of which is visible. 

 This is the secondary coil, and is capable of being moved nearer to c along a 

 groove and graduated scale. To the binding screws to the left of g, the wires 

 of electrodes used to stimulate the muscle are attached. If the key in the cir- 



Fig. 828. Du Bois Reymond's induction coil. 



cuit of wires from the battery to the primary coil (primary circuit) be closed, 

 the current from the battery passes through the primary coil, and across the 

 key to the battery, and continues to pass as leng as the key continues closed. 

 At the moment of closure of the key, at the exact instant of the completion of 



