5i4 



NER VE. 



due to a weak opening excitation of the inhibitory fibres, as in 

 1,6. 



The results show that, as far as the closing excitation is concerned, 

 this is of a twofold character, one set of nerve fibres being excited 

 strongly on the instant of closure, whilst the other set is excited by the 

 persistence of such closure, and that by altering the intensity of the 

 current either set may be excited during the whole time that the 

 circuit is closed. 



Experiments with the abductor muscle prepared in a similar way 

 give, as might be anticipated, different results ; if the muscle still 

 possesses tonicity, then the following facts are observed when the nerve 

 is excited by weak and strong currents respectively (Fig. 268) : — 



Weak currents (abductor muscle). — Closure — (a) An initial re- 

 laxation due to excitation of the inhibitory fibres ; (b) a prolonged 

 persistent augmentation of tonicity (i.e. contraction) succeeding (a), 

 lasting during closure, and due to the excitation of the augmentor 

 fibres. Opening — (c) The return of the muscle to its normal state, this 

 being fused with a more prolonged relaxation, which slowly passes away, 



Strong rn- 



I I I L 



I I I 



JL_L 



./" 



Fig. 268. 



due to the weaker opening excitation of the inhibitory fibres, evoking a 

 repetition of (a). 



With stronger currents (Fig. 268, right-hand figure) the effects are 

 changed — (a) The initial relaxation on closure becomes fused with a more 

 prolonged effect of the same kind ; (b) a conflict occurs during closure 

 between the prolonged relaxation now evoked and a succeeding augmen- 

 tation, due to simultaneous prolonged excitation of both sets of fibres, in 

 which conflict the relaxation has the advantage for some time, but is 

 finally overpowered ; (c) on opening, an initial relaxation occurs suc- 

 ceeded by a prolonged augmentation, due to the overpowering effect of 

 excitation of the augmentor fibres. 



These results show that both at closure and at opening a twofold 

 excitation occurs, those at the opening of a strong current being similar 

 in character to such closure effects as are evoked by a weak one. 



The interpretation here given rests on the supposition that the 

 effects are all due to the excitation of augmentor and inhibitory nerve 

 fibres supplying each muscle. It might, however, be urged that the 

 second prolonged effect is in each case an actual polar inhibition ; thus 

 the closure effects in the abductor muscle might be excitation of the 

 inhibitory set of fibres at the cathode, succeeded by overpowering 

 inhibition of such excitation at the other pole, anode. This would 



