Currents on Transmission of Excitation. 



503 



4. After-Effects of Heterodromous and Homodromous Currents. 



On the cessation of a current there is induced in the plant-tissue a transient 

 conductivity change of opposite sign to that induced by the direct current 

 (cf Experiment 3). The same I find to be the case as regards the after-effect 

 of current on conductivity change in animal nerve. Of this I only give a 

 typical experiment of the direct and after-effect of homodromous current on 

 salt-tetanus. 



Experiment 8. — In this experiment sufficient length of time was allowed to 

 elapse after the application of the salt on the nerve, so that the muscle, in 

 response to the transmitted excitation, exhibited an incomplete tetanus. The 

 homodromous current was next applied, with the result of inducing a complete 

 block of conduction, with the concomitant disappearance of tetanus (fig. 10). 



Fig. 10. — Normal transmitted salt-tetanus without current at 0. Enhancement under 

 heterodromous current of 3 microamperes. Reversal at 10 microamperes. 



The homodromous current was gradually reduced to zero by the appropriate 

 movement of the potentiometer slide. The after-effect of homodromous 

 current is now seen in the transient enhancement of transmitted excitation, 

 which lasted for nearly 40 seconds. After this the normal conductivity was 

 restored. Eepetition of the experiment gave similar results. 



5. Phenomenon of Reversal. 



In experiments with Mimosa it was shown that an increase of polarising 

 current above the critical value gave rise to a reversal of the normal 

 conductivity variation (cf. Experiment 4). Even in this matter of reversal 

 I find a very remarkable parallelism between the reactions of the conducting 

 tissue of the plant and the nerve of the animal. The reversal was obtained 



