ELECTROTONUS. 467 



and cease when the current ceases. They exist in living medul- 

 lated nerves only, or if at all in degenerated nerves but to a slight 

 degree. 



At the time of the passage of the current there is an increase 

 in the excitability or irritability of the nerve in the kathodic 

 region, and a decrease in the anodic region ; the increase is katelec- 

 trotonus, and the decrease, anelectrotonus. After the opening of 

 the current the conditions are reversed, the excitability being 

 temporarily increased in the anodic, and decreased in the kathodic 



FIG. 269. Diagram of changes of excitability and conductivity produced in a 

 nerve by a voltaic current: E, changes of excitability during the flow of the 

 current, according to Pfliiger. The ordinates drawn from the abscissa axis to cut 

 the curve represent the amount of the change. C (1), changes of conductivity 

 during the flow of a moderately strong current; conductivity greatly reduced 

 around kathode; little affected at anode. C (2), changes of conductivity during 

 flow of a very strong current ; conductivity reduced both in anodic and kathodic 

 regions, but less in the former. C', changes of conductivity just after opening a 

 moderately strong current ; conductivity greatly reduced in region which was 

 formerly anodic; little affected in region formerly kathodic (Stewart). 



area. The indifferent point is the point at which there is no 

 change in the excitability of the nerve, where the katelectro- 

 tonic and anelectrotonic effects counteract each other; this will 

 change its position according to the intensity of the current, 

 approaching the kathode as the intensity increases. Fig. 268 

 shows the changes which take place according as the intensity 

 of the current is increased. 



The conductivity of a nerve is also affected by the constant 

 current, the changes being shown in Fig. 269. 



