720 COMPARATIVE ELECTRO-PHYSIOLOGY 



ammonia are the same in the one case as in the other. The 

 effects of various drugs on the receptivity, conductivity, and 

 responsivity of the vegetal nerve are the same as on those 

 of the animal, and finally, in the action of different salts, the 

 acid and basic moieties exhibit the same characteristic effects 

 in plant and animal nerves alike (Chapters XXXII. and 

 XXXIII.). 



In order to study the variation of excitatory effects in 

 nerves I was able to devise a very delicate instrument, the 

 Conductivity Balance (fig. 291). This apparatus not only 

 enables us to study the modifications of conductivity, excit- 

 ability, and responsivity, induced by a given agent separately, 

 but also to compare relative variations as between any two 

 of these, say, for instance, conductivity against excitability, 

 or receptive excitability against responsivity. It also enables 

 us to compare and contrast the action of two different 

 reagents applied simultaneously in different parts of the 

 same nerve. In this way the factor of uncertainty introduced 

 by the unknown individual differences between two nerves is 

 eliminated. 



The principle on which the Method of Conductivity 

 Balance depends is that of applying stimulus at a point 

 which, in the excitatory sense, is exactly midway between 

 the electrodes E and E' ; the excitatory effects at E and E' 

 exactly balance each other, and the galvanometric deflection 

 is then zero. When the excitability of the right hand, E, or 

 the conductivity of the right arm, c, of the balance is 

 enhanced, the balance is upset and the resultant response 

 is, say, up ; depression, on the other hand, upsets it in the 

 opposite direction. Not only may the effects of various 

 chemical agents be determined by this method, but it is 

 easy also to study by its means the effects of temperature 

 on conductivity. Cold is thus found to depress, and warmth 

 to enhance it (figs. 307, 309). Another important investi- 

 gation carried out by this means was on the curious 

 phenomenon presented by the effect of section in enhancing 

 the excitability of adjacent points. It was shown that this 



