IX 



ELECTRICAL EXCITATION OF XERYE 



127 



the iron ring from the point H between the poles of the magnet, 



the magnetic force of which is correspondingly diminished. 



This slow decline in the magnetism corresponds to a magnetic 



induction current in the coil, 



which rises the more slowly as 



the iron ring increases more 



gradually in thickness, and as the 



disc is the more slowly rotated. 



On the other side the part M 



of the ring induces, under similar 



conditions, by its sharp edge, an 



almost instantaneous ascending 



current. In the frog-preparation 



the current from the sharp tooth 



(like the break shock) is always 



more effective than that from the 



obtuse tooth, while this is reversed in the toad, where the slowly 



ascending current invariably excites more effectually than the 



sharp rise (Fig. 180). 



It is a question whether this is due merely to the known 



FIG. 179. 



FIG. ISO. Gastrocnemius of toad. Twitches on excitation with a, obtuse tooth ; b, sharp tooth. 



sluggishness of toad-muscle as compared with that of the frog, 

 or whether there are actual differences in the nerve-fibres im- 

 plicated. In any case du Bois-Eeymond's dictum, that current 

 excites not in virtue of its actual density, but from the rapidity 

 of its variations, is not applicable to all locomotor apparatus. 

 The rapidly twitching muscles of the frog correspond with the 



