CHAP, ii.] THE CONTRACTILE TISSUES. 129 



a ' making ' contraction ; this we may neglect. 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 

 now shut off, a ' breaking ' contraction will probably be produced ; 

 this also we may neglect. If now the point x after a short 

 interval be again tested with the same induction-shock as before, 

 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. 21 B, the irritability of the nerve at 

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

 the polarizing 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 polar- 

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

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

 the polarizing 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 



F. 9 



