672 COMPARATIVE ELECTRO-PHYSIOLOGY 



increasing the E.M.F. still more, the reversed effect passes 

 first into neutral and then back to the normal, characteristic 

 of a moderate E.M.F. 



Similar effects are also obtained on employing the 

 stimulus of thermal shocks. The electro-thermic stimulator 

 is now inserted in the Sensimeter, in place of the mechanical 

 tapper. The indifferent and incompletely fused sensation 

 becomes continuous and painful, on making the receptive 

 point kathode, under a moderate E.M.F. of 2 volts. On now 

 making the receptive point anode, the sensation becomes 

 converted into a markedly discrete positive. The reversal of 

 these normal effects under a feeble E.M.F. was obtained in 

 a given experiment by thermal shocks, when the polarising 

 E.M.F. was '03 volt. 



Turning next to the chemical mode of stimulation, it will 

 be remembered that an experiment has already been 

 described (p. 582) where a continuously irritating sensation 

 was caused by the application of salt on a wounded spot, 

 this moderate sensation of pain being rendered soothing 

 when the spot was made normal anode, and intensely painful 

 when normal kathode. Effects precisely opposite resulted 

 from the application of a feeble E.M.F. It will thus be seen 

 that those same conditions which depressed the normal 

 negative response, also acted to obliterate the negative tone 

 from the resulting sensation. Those conditions, on the other 

 hand, which exalted the negativity of the response, would 

 convert the positive tone of sensation into negative or 

 painful. These general facts have been demonstrated by 

 the employment of different forms of stimulation, the neces- 

 sary depression or exaltation of excitability having been 

 brought about by electrotonus. So far we have dealt with 

 the modifications induced in the responsive sensation by 

 the variation of the receptivity of the excited point. We 

 shall next briefly discuss the effects which ensue on the 

 variation of the excitability and conductivity of the nerve by 

 different agents. In order to induce depression, there are 

 various anaesthetic agents which might be used. 



