136 EFFECTS OF CONSTANT CURRENT. [BOOK i. 



of a sensory nerve in which violent impulses are being generated, 

 giving rise in the central nervous system to sensations of pain, the 

 impulses are toned down or wholly abolished, and the pain ceases. 

 So on the other hand we may at pleasure heighten the irritability 

 of a part by throwing it into katelectrotonus. In this way the 

 constant current, properly applied, becomes a powerful remedial 

 means. 



We said just now that probably every stimulus produces its 

 effect on a nerve by doing what the constant current does when it 

 acts as a stimulus, viz. suddenly raising the irritability of the 

 nerve to a higher pitch. At any rate the stimulus so often 

 employed in experiments, the induction-shock, acts exactly in the 

 same way as the constant current. The induction-shock is a 

 current of short duration, developed very suddenly but disappear- 

 ing more gradually, and this is true both of a making induction- 

 shock, a shock due to the making of the primary current, and of a 

 breaking shock, a shock due to the breaking of the primary 

 current. The two differ in direction (hence if the making shock 

 be ascending, the breaking shock will be descending and vice 

 versa) and in the fact that the breaking shock is more suddenly 

 developed and hence more potent than the making shock; but 

 otherwise they act in the same way. In each case, since the 

 induced current is developed rapidly but disappears more slowly, 

 there is a sudden development of electrotonus, of katelectrotonus 

 at the kathode and of anelectrotonus at the anode, and a more 

 gradual return to the normal condition. Now there are many reasons 

 for thinking that in all cases the passing from the normal condition 

 to katelectrotonus at the kathode is a more potent stimulus than 

 the return from anelectrotonus to the normal condition at the 

 anode, and this will be still more so if the return to the normal 

 condition be much slower than the entrance into electrotonus, as 

 is the case in an induction-shock. And it would appear that in 

 an induction-shock, which as we have said disappears much more 

 slowly than it is developed, we have to deal not with two stimuli, 

 one at the shock passing into a nerve and one at the shock leaving 

 the nerve, but with one only, that produced at the shock passing 

 into the nerve. Hence when an induction-shock is sent into a 

 nerve, one stimulus only is developed and that at the kathode 

 only, the establishment of katelectrotonus. This is true whether 

 the shock be a making or a breaking shock, i.e. due to the making 

 or breaking of the primary current, though of course owing to the 

 change of direction in the induced current what was the kathode 

 at the making shock becomes the anode at the breaking shock. 



Lastly, though we are dealing now with nerves going to muscles, 

 that is to say, with motor nerves only, we may add that what we 

 have said about electrotonus and the development of nervous 

 impulses by it appears to apply equally well to sensory nerves. 



78. In a general way muscular fibres behave towards an 



