128 ELECTROTONUS. [BOOK i. 



75. Electrotonus. The marked feature of the electrotonic 

 condition is that the nerve though apparently quiescent is changed 

 in respect to its irritability ; and that in a different way in the 

 neighbourhood of the two electrodes respectively. 



Suppose that on the nerve of a muscle-nerve preparation are 

 placed two (non-polarizable) electrodes (Fig. 21, a, k) connected 

 with a battery and arranged with a key so that a constant current 

 can at pleasure be thrown into or shut off from the nerve. 

 This constant current, whose effects we are about to study, may be 

 called the ' polarizing current.' Let a be the positive electrode or 

 anode, and k the negative electrode or kathode, both placed at 

 some distance from the muscle, and also with a certain interval 

 between each other. At the point x let there be applied a pair of 

 electrodes connected with an induction-coil. Let the muscle 

 further be connected with a lever, so that its contractions can 

 be recorded, and their amount measured. Before the polarizing 

 current is thrown into the nerve, let a single induction-shock 

 of known intensity (a weak one being chosen, or at least not 

 one which would cause in the muscle a maximum contraction) be 

 thrown in at x. A contraction of a certain amount will follow. 



A. 



TT -* 



x 



B. 



FIG. 21. MUSCLE-NERVE PREPARATIONS, with the nerve exposed in A to a descending 

 and in B to an ascending constant current. 



In each a is the anode, k the kathode of the constant current, x represents the 

 spot where the induction-shocks used to test the irritability of the nerve are sent in. 



That contraction may be taken as a measure of the irritability of 

 the nerve at the point x. Now let the polarizing current be 

 thrown in, and let the kathode or negative pole be nearest the 

 muscle, as in Fig. 21 A, so that the current passes along the 

 nerve in a direction from the central nervous system towards the 

 muscle; such a current is spoken of as a descending one. The 

 entrance of the polarizing current into the nerve will produce 



