482 



COMPARATIVE ELECTRO-PHYSIOLOGY 



are opposed, and when these are equal, and balance each 

 other, the galvanometer indication is then reduced to zero. 

 E and E' are usually at a distance of about 4 cm. from each 

 other. When the stimulator is brought too near to the left 

 contact E', the excitatory effect of 

 galvanometric negativity which is 

 induced there is relatively greater 

 than at E. The balance is thus dis- 

 turbed, and the resultant responsive 

 deflection is then, say, downwards. 

 When the stimulator is placed, on 

 the other hand, too near the contact 

 E, to the right, the resultant galvano- 

 metric deflection will be up. 1 By 

 suitable movement of the stimulator, 

 to and fro between these two ex- 

 tremes, a point may be found where 

 the excitatory effects at E and E' 

 will exactly balance each other. I 

 give here (fig. 290) a record taken 



FIG. 290. Photographic Re- 

 cord made during Pre- 

 liminary Adjustment for 

 Balance of Nerve of Fern 



The first two down-responses 

 show over-balance, when 

 s is too near the left, 

 E' being relatively more 

 excited. The up-responses 

 indicate over - balance 

 caused by s being too 

 much to the right. The 

 horizontal record shows 

 attainment of exact 

 balance. 



FIG. 289. Diagrammatic Representa- 

 tion of the Conductivity Balance 



s, thermal stimulator ; c and c', the 

 conducting arms of the balance ; 

 E and E', responding points. Dif- 

 ferential excitatory electrical effects 

 at E and E' recorded by galvano- 

 meter, G. 



during this preliminary stage of adjustment. The first two 

 down-responses were obtained when the stimulator was too 

 far away from the balancing-point to the left. The next two 



1 It is to be understood that what is said here refers to nerve in a normal 

 condition of conductivity. 



