rx ELECTRICAL EXCITATION OF NERVE 189 



corresponds with the existing ~tonus), and then, sooner or later, 

 makes a sudden drop below it (Fig. 196), either rising again 

 immediately, or more slowly, after a certain interval, so that the 

 lever often recovers its initial position 

 during closure, in other cases, however, 

 only when the circuit is opened. It 

 not infrequently happens, at a given 

 strength of current, that the shortening 

 of the tonic muscle, on closing the circuit, 

 corresponds both in magnitude and dura- 

 tion with the subsequent relaxation, so 

 that the first section of the curve above 

 the abscissa is almost equal with that of "~ 



A H A k /i 



the lower half (Fig. 196). In current- 

 intensities below this limit, the second 



half of the curve seems generally to preponderate, while beyond 

 it the effects of excitation come more and more into play at the 

 expense of the inhibiting action so that the first section of the 

 curve is highly characteristic. 



The inhibitory effects are often so indefinite, that their 

 existence as independent signs of stimulation might easily 

 be overlooked, without some knowledge of the action of 

 weaker currents ; and they might be viewed merely as fatigue- 

 effects from the immediately preceding persistent excitation. 

 This is indeed contradicted by the fact (as insisted 011 above) that 

 re-entry of the more or less strongly inhibited tonus usually 

 occurs during the passage of the current ; while, moreover, the 

 break of a stronger current not unusually inhibits to the same 

 extent as the closure of a weak current. On opening the exciting 

 circuit a fall of the curve similar to that previously obtained 

 during closure (Fig. 195, c) is apparent. 



We thus learn that the reaction of the tonically contracted 

 adductor muscle, on exciting its nerve with the constant 

 current, is characterised throughout (with increasing strength of 

 current) by depression of inhibitory effects in favour of excitatory 

 action, until the inhibition becomes imperceptible. In the abductor 

 muscle the contrary occurs, owing to its much stronger and 

 more persistent tonus. This is evident from comparison of the 

 curves (Figs. 195, 197), which, though recorded as far as possible, 

 under uniform conditions, are in many respects exactly opposite. 



