STIMULUS BY ELECTRIC CURRENT. 103 



positive pole nearest the muscle, as in Fig. 32, B, the irritability of the nerve 

 at x would have been found to be diminished, instead of increased, by the polar- 

 izing current ; the contraction obtained during the passage of the constant 

 current would be less than before the passage of the current, or might be 

 absent altogether, and the contraction after the current had been shut off 

 would be as great, or perhaps greater, than before. That is to say, when a 

 constant current is applied to a nerve, the irritability of the nerve between 

 the polarizing electrodes and the muscle is, during the passage of the 

 current, increased when the kathode is nearest the muscle (and the polariz- 

 ing current descending) and diminished when the anode is nearest the 

 muscle (and the polarizing current ascending). The same result, mutatis 

 mutandis, and with some qualifications which we need not discuss, would be 

 gained if x were placed, not between the muscle and the polarizing current, 

 but on the far side of the latter. Hence, it may be stated generally that 

 during the passage of a constant current through a nerve the irritability of 

 the nerve is increased in the region of the kathode, and diminished in the 

 region of the anode. The changes in the nerve which give rise to this 

 increase of irritability in the region of the kathode are spoken of as kat- 

 electrotonus, and the nerve is said to be in a katelectrotonic condition. Simi- 

 larly the changes in the region of the anode are spoken of as anelectrotonus, 

 and the nerve is said to be in an anelectrotonic condition. It is also often 

 usual to speak of the katelectrotonic increase, and anelectrotonic decrease of 

 irritability. 



This law remains true whatever be the mode adopted for determining 

 the irritability. The result holds good not only with a single induction- 

 shock, but also with a tetanizing interrupted current, with chemical and 

 mechanical stimuli. It further appears to hold good not only in a dissected 

 nerve-muscle preparation but also in the intact nerves of the living body. 

 The increase and decrease of irritability are most marked in the immediate 

 neighborhood of the electrodes, but spread for a considerable distance in 

 each direction in the extrapolar regions. The same modification is not con- 

 fined to the extrapolar region, but exists also in the intrapolar region. In 

 the intrapolar region there must be, of course, a neutral or indifferent point, 

 where the katelectrotonic increase merges into the anelectrotonic decrease, 



Diagram illustrating the Variations of Irritability during Electrotonus, with Polarizing Cur- 

 rents of Increasing Intensity. (From Pfluger ) The anode is supposed to be placed at A, the 

 kathode at B ; AB is consequently the intrapolar district. In each of the three curves, the por- 

 tion of the curve below the base line represents diminished irritability, that above, increased ir- 

 ritability. y\ represents the effect of a weak current; the indifferent point x\ is near the anode A. 

 In 2/2, a stronger current, the indifferent point x* is nearer the kathode B, the diminution of irri- 

 tability in.anelectrotonusandthe increase in katelectrotonus being greater than In y\ ; the effect 

 also spreads for a greater distance along the extrapolar regions in both directions. In y s the same 

 events are seen to be still more marked, 



and where, therefore, the irritability is unchanged. When the polarizing 

 current is a weak one, this indifferent point is nearer the anode than the 



