COMPARATIVE ELECTRO-PHYSIOLOGY 



negativity of the more excitable, will now give rise to a current 

 in the same direction as this resting-current, thus constituting 

 a positive variation of it. It is only when fatigue has set in 

 at the more excitable, and induced great depression of 

 excitability there, that the response-current may undergo a 

 reversal, its direction now being from what was originally 

 the less excitable, to the originally more excitable (p. 177). 

 An example of this I found in the sectioned foot of the large 

 Indian garden-snail. Here the excitatory action on the 

 glandular surface, due to the shock of preparation, was ex- 

 tremely great, as evidenced by the profuse secretion which 

 occurred immediately. Owing to this over-stimulation, 

 fatigue was induced, with consequent great depression of ex- 

 citability. Hence the responsive current was now found 

 to be reversed, having, with reference to the glandular 

 surface, become outgoing instead of ingoing. 



It has been stated above that the ingoing current of rest, 

 observed at the glandular surface under preparation, was not 

 the true natural current, but due to the excitatory after-effect 

 of injury. This I was able to verify by observing the current 

 of rest under natural conditions, without excitation. The 

 snail was allowed to crawl on a glass surface, in the middle of 

 which was a strip of linen moistened in normal saline. This 

 brought the glandular surface into electrical connection with 

 one of two non-polarisable electrodes. When the snail had 

 of its own accord come to a temporary standstill on this piece 

 of linen, the other electrode was quietly placed against the 

 skin of the upper side of the protruding body. The natural 

 current of rest was now found to be outgoing, as regards 

 the glandular surface of the foot, the more excitable being 

 thus galvanometrically positive. The absolute electro-motive 

 difference was found to be + '0013 volt. The foot was now 

 sectioned, and the difference of potential between the same 

 points was found to have undergone a reversal. The 

 supposed resting-current was now ingoing, through the 

 glandular surface. Thus, owing to the excitation consequent 

 on preparation, the more excitable surface, originally positive, 



