608 COMPARATIVE ELECTRO-PHYSIOLOGY 



sent through these two coils, in a proper direction, two equal 

 north-polar impulses will be generated simultaneously, and 

 travel, one to the right, towards E, and the other to the left, 

 towards E'. In order to obtain a balance of the excitatory 

 effects at E and E', we keep E r at a fixed distance, and move 

 E backwards and forwards till the balance is found. This 

 process of balancing will be found graphically illustrated in 

 the records given in fig. 373. E was placed at first too near 

 to S, and the over-balance is seen as up-responses. The coil 

 was then moved away very gradually, and the response of 

 over-balance is seen at each step to undergo a diminution or 

 approach towards balance. We next note the attainment 

 of exact balance, where the record is seen to be horizontal. 

 The coil is now moved still further to the right, and the con- 



FlG. 373. Process of Balancing illustrated by Photographic Record 

 of Responses 



sequent increasing under-balance is exhibited by the gradually 

 increasing reversed down-responses. 



Having thus obtained balance, we are able to record the 

 variations induced in conductivity by a given agent. This 

 is applied on the right arm of the balance, the subsequent 

 upset of which, in one direction or the other, indicates the 

 enhancement or depression of conductivity. Resulting up- 

 responses will indicate enhancement, and down-responses 

 depression. A well-known agent for the enhancement or 

 depression of the conductivity of the nerve is the polar 

 action of the kathode and anode. Moderate kat-electro- 

 tonus enhances conductivity, whereas the anode depresses 

 or inhibits it. The explanation which I have already offered, 

 regarding anodic and kathodic effects on excitability, will 

 also be found applicable in the case of conductivity. An 

 excitatory or kathodic effect will be facilitated in trans- 



