64 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



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 1 with an induction shock of the same strength as before, 

 it will be found to be increased, the shock causing a larger contraction. On 

 opening the polarizing current the usual opening contraction will be seen, and 

 if alter the current has been removed the irritability be again tested, it will 

 be found to have returned to the normal, or to be decreased. The changes 

 in irritability described can be ascertained by using mechanical or chemical 

 stimuli as well as induction shocks. Alterations of the irritability induced by 

 anelectrotonic and katelectrotonic changes of the nerve-substance are to be 

 found not only in the part of the nerve between the point to which the polar- 

 izing current is applied and the muscle, but in the extrapolar region at the 

 central end of the nerve, and in the intrapolar region. The experimental 

 evidence of this is not so readily obtained, but there is no doubt of the fact. 



The effect of the polarizing current is the greater, the better the condition 

 of the nerve ; moreover, the stronger the current employed, the more of the 

 nerve influenced by it. Of course, in the intrapolar region there is a point 

 where the effect of the anode to decrease the irritability comes into conflict with 

 the effect of the kathode to increase it, and where, in consequence, the irrita- 

 bility remains unchanged. This indifferent point may be observed to approach 

 the kathode as the strength of the current is increased. The following schema 

 is given by Prliiger to illustrate the way in which the irritability is changed in 

 the anelectrotonic and katelectrotonic regions as the strength of the current is 

 increased : 



Pig. 30.— Electrotonic alterations <>f irritability caused by weak, medium, and strong battery 

 currents: A and Bindicate the points of application of the electrodes to the nerve, A being the anode, 

 Bthe kathode. The horizontal line represents the nerve at normal irritability; the curved lines illus- 

 trate how the irritability is altered al differenl parts of the nerve with currents of different strengths. 

 curve v 1 Bhows the effect ofs weak current, the part below the line indicating decreased, and that above 

 the line increased Irritability, at z* the curve crosses the line, this being the indifferent point at which 

 atelectrotonic effects are compensated for by anelectrotonic ctl'ects: //-gives t lie effect of a stronger 

 f a 8 tm stronger current. As the strength of the current is increased the effect becomes 

 r and extend- farther Into the extrapolar regions. In the intrapolar region the indifferent point is 

 to advance <n ith increasing strengths of current from the anode toward the kathode. 



As in the case of the muscle, so of the nerve, the constant current leaves 

 behind it important after-effects. In general it may be stated that wherever 

 during the flow of the current the irritability is increased, there is a decrease 



