484 



COMPARATIVE ELECTRO-PHYSIOLOGY 



conductivity-variation induced by two different agents which 

 are applied simultaneously, one on the arm C and the other 

 on C'. 



It is possible again to compare the variation of con- 

 ductivity with that of responsivity, by applying one agent at 

 a responding region, say E, and the other on the opposite arm 

 of the balance at C'. The mode of investigation of receptivity 

 changes will be described presently. 



In fig. 291 we have the complete apparatus. The animal 

 or vegetal nerve, N N, rests on non-polarisable electrodes of 



FIG. 291. Complete Apparatus of Conductivity Balance 



The nerve N supported on electrodes E 2 , E 3 . The two other electrodes 

 E, E 4 are not used in this experiment, but are employed for experiments 

 on electrotonus ; T, thermal stimulator, the relative lengths of the 

 arms of the balance being adjusted by the slide s. 



a U -shape. For the present experiments, two electrodes, E 2 

 and E 3 , are sufficient, their mutual distance being capable of 

 any variation by movement along a sliding-bar. The same 

 apparatus might be used for experiments on electrotonus, in 

 which two additional electrodes would be required. The 

 position of the electrothermic stimulator T is capable of very 

 careful adjustment for purposes of balance, by means of the 

 sliding-rod S. A glass cover, not shown in the figure, fits into 

 the groove which is represented by a double dotted line sur- 

 rounding the apparatus, and thus enables the chamber con- 

 taining the nerve to be kept in a properly humid condition. 



