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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



short-circuiting key (/S), by which the closing shocks can be prevented from 

 reaching the nerve. 



If, with this arrangement, a breaking induction shock of medium strength 

 be given, the nerve will be excited, and the height of the muscular contraction 

 which results may be taken as a test of the irritability of the nerve at /. 



FIG. 27. Method of testing anelectrotonic and katelectrotonic alterations of irritability in nerves. 



Now send the polarizing current through the nerve, in the ascending direction, 

 that is, with the anode nearer the muscle. At the moment the current is 

 closed, if it be of medium strength, a closing contraction will be observed ; 

 then comes a period during which the muscle is not contracting and the polar- 

 izing current is apparently producing no effect on the nerve ; if, however, after 

 the current has acted a short time, the irritability of the nerve at the point 

 I be again tested with a breaking induction shock, it will be found to be de- 

 creased, on account of the condition of anelectrotonus which has been induced. 

 If the key in the polarizing current be then opened, the usual opening con- 

 traction will be recorded. After the polarizing current has been removed, the 

 condition of the nerve at I can be again tested, and it will be seen that the 

 irritability has returned to the normal, or is even greater than it was at the 

 start. 



The effect of the kathode on the irritability may be tested in a similar way, 

 by reversing the polarizing current and again sending it into the nerve. This 

 time the current will be descending, i. e. the kathode nearest the muscle. As 

 before, a closing contraction will be seen when the circuit is made, but on test- 

 ing the irritability at /with an induction shock of the same strength as before, 



