CHAPTER XII 

 Polar Effects of a Galvanic Current ; Electrotonus 



A POLARISING current, as already explained, produces changes of excita- 

 bility not only at its poles but also in the adjacent parts of the nerve, 

 and even some distance from them. This is due to the fact that owing 

 to spread of current in the extra-polar regions changes of potential are 

 manifested in those regions during the passage of the current between 

 the poles, and are accompanied by physiological changes, viz., increased 

 excitability near and beyond the kathode, and diminished excitability 

 near and beyond the anode. Such a condition is known as electrotonus ; 



FIG. 49. Paradoxical contraction shown by induced currents. 



that produced by the kathode being termed katelectrotonus ; that pro" 

 duced by the anode, anelectrotonus. 



The spread of the electrical changes beyond the poles is illustrated 

 by the following experiment. 



Paradoxical contraction. Dissect out the sciatic nerve of a frog, 

 cutting all the branches save that to the calf muscles, but leaving the 

 cut branch to the peronei muscles as long as possible. Place the cut 

 end of this branch upon non-polarisable electrodes connected with a 

 battery and rheochord and have a mercury key in the circuit. On 

 making or breaking the circuit the gastrocnemius will contract. 

 Owing to the electrotonic spread of the current in the fibres of the 

 peroneus nerve the fibres to the gastrocnemius in the trunk of the 



