ix ELECTRICAL EXCITATION OF NERVE 191 



Fig. 197, a, shows, after an insignificant fall of the curve 



beginning at the moment of closure, a marked rise, in consequence 



of the now developing make excitation, leading at I and c to 

 a permanent reinforcement of the initial tonus. With the less 

 favourable descending direction, the same weak current produces 

 excitation only at closure, without previous inhibition, i.e. acts 

 as a weaker stimulus. The same gradation of effect with the 

 two directions of current is in most cases more or less plainly 

 visible on stimulating with alternately ascending and descending 

 currents. With increasing strength of excitation, the primary 

 inhibition becomes more and more conspicuous, the curve falling 

 deeper on the one hand at closure of the current, and on the 

 other rising again the more slowly to the abscissa, or passing 

 beyond it, in proportion with the intensity of the current. 



Seeing that with indirect stimulation of the adductor muscle 

 by not unduly weak constant currents, inhibition with stimula- 

 tion of the abductor muscle by strong currents, on the other 

 hand, excitation makes a more or less delayed entrance after 

 closure (as expressed in the corresponding changes of form in 

 the muscle), insufficient duration of closure in either case will 

 give the impression of single, or negative, effects of excitation. 

 This is more especially the case in preparations of the abductor 

 muscle, where, owing to want of tonus, the inhibitory effect, 

 as expressed directly in changes of form in the muscle, is 

 absent. 



Such inhibitory action is then apparent only in a retardation 

 of the latent period, which may, under certain conditions, last for 

 several seconds a fact which gives a characteristic form to these 

 curves, and indicates their origin from the abductor muscle (Fig. 

 198, a, b). That this is really no more than the effect of an 

 inhibition, antecedent to the excitatory action of the current, is 

 most plainly seen in cases in which the muscle is excited once 

 with uniform strength of current, while there is still a perceptible 

 tonus, and again later in the relaxed condition. 



In both cases the make contraction is retarded in about the 

 same degree, but while 'in the one, closure of the circuit produces 

 a visible diminution of tonus, inhibition is expressed in the other 

 solely by the lengthening of the latent period. 



It follows that the inhibitory action of the constant current 

 antecedent to excitation may be demonstrated in almost every 



