MUSCLE AND NERVE 



181 



some point between the electrodes the region of increased irri- 

 tability merges into that of decreased irritability. This point 

 is nearer the anode, but with increase of strength of current it 

 approaches the kathode. The facts relating to the effect of 

 direction of current on the resulting contraction constitute 

 what is known as Pfliiger's law. 



Ascending current. 



Descending current. 



Medium current. Contraction j Contraction Contraction Contraction 

 Strong current I Contraction Contraction 



When a nerve intact within the tissues of an animal is sub- 

 jected to an electrical current it is impossible to prevent the 

 spread of the latter through the surrounding tissues. At the 

 point of entrance of the current it spreads out brush-like, to be 

 concentrated again at the point of exit. Directly under the 

 physical anode the current enters the nerve, flows through it 

 at varying angles, and so forms in the nerve a physiological 

 anode and kathode. The same thing happens at the point of 

 exit of the current. Therefore, there are four points from 

 which an impulse may be generated. There may be: 



1. An anodic closing contraction, which is the result of an 

 impulse generated at the physiological kathode under the physical 

 anode. 



2. An anodic opening contraction, which is the result of a 

 change developed at the physiological anode under the physical 

 anode. 



3. A kathodic closing contraction, which is the result of an 

 impulse generated at the physiological kathode under the physical 

 kathode. 



4. A kathodic opening contraction, which is the result of an 

 ^ impulse generated at the physiological anode under the physical 



kathode. 



These are abbreviated, respectively, ACC, AOC, KCC, 



