78 ELECTROTONUS. [BOOK i. 



as in Fig. 13^1. If while the current is passing, the same 

 induction-shock as before be sent through x, the contraction which 

 results will be found to be greater than on the former occasion. 

 If the polarizing current be shut off, and the point x after a 

 short interval again tested with the same induction-shock, the 

 contraction will be no longer greater, but of the same amount, 

 or perhaps not so great, as at first. During the passage of 

 the polarizing current, therefore, the irritability of the nerve at 

 the point x has been temporarily increased, since the same shock 

 applied to it causes a greater contraction during the presence than 

 in the absence of the current. But this is only true so long as the 

 polarizing current is a descending one, so long as the point x lies on 

 the side of the kathode. On the other hand, if the polarizing 

 current had been an ascending one, with the anode or positive pole 

 nearest the muscle, as in Fig. 135, the irritability of the nerve at 

 x would have been found to be diminished instead of increased by 

 the polarizing current. That is to say, when a constant current is 

 applied to a nerve, the irritability of the nerve between the polar- 

 izing electrodes and the muscle is, during the passage of the current, 

 increased when the kathode is nearest the muscle (and the polar- 

 izing 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 katelectrotonus, and the nerve is said to be 

 in a katelectrotonic condition. Similarly 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 inter- 

 rupted current, with chemical and with 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 neighbourhood of the electrodes, but spread for a 

 considerable distance in either direction in the extrapolar regions. 

 The same modification is not confined to the extrapolar region, 

 but exists also in the intrapolar region. In the intrapolar region 

 there must be of course an indifferent point, where the katelectro- 



