288 COMPARATIVE ELECTRO-PHYSIOLOGY 



employed in the investigation of plant phenomena, moreover, 

 have proved so highly satisfactory that records will be given 

 of the results obtained by their means in the case of 

 animal tissues also. And this will, I hope, show the great 

 reliability and simplicity which it is thus possible to intro- 

 duce into the investigation as a whole. 



With regard to the electrical effects in animal skin, 

 epithelium, and glands, the inquiry resolves itself into the 

 determination of, (i) the direction of the current of rest ; (2) 

 that of the excitatory current ; and, lastly, (3) a consideration 

 of theories regarding these. The first of these, the current 

 of rest, was found by Du Bois-Reymond and Engelmann in 

 the skin of frog to be * ingoing ' that is to say, passing from 

 the outer surface to the inner. Hermann also found a 

 similar current in the skin of eel. He regarded the source of 

 electro-motive action as lying in the partial mucin -metamor- 

 phosis of single cells. From the fact that in the toad, where 

 the ingoing current is specially strong, the skin glands are 

 vigorously developed, and from the discovery by Rosenthal 

 that in the mucous glands of the stomach the current is also 

 ingoing, it was assumed that the observed electro-motive 

 forces were due to the glandular nature of the tissues. The 

 skin current of the frog and of the fish, and the glandular 

 current of the stomach, are thus usually regarded as due to 

 the same cause. 



There is, however, a serious discrepancy in this view, 

 inasmuch as, while local stimulation of the upper surface of 

 the frog's skin induces. a positive change, a similar stimula- 

 tion of an unmistakably glandular surface is found to bring 

 about a negative. If then the electrical effect on the skin 

 of frog be the same as on a glandular surface, the dis- 

 crepancy of their responsive reactions becomes inexplicable. 



As regards the excitatory change, very diverse results 

 have been recorded when stimulus has been applied indi- 

 rectlythat is to say, through the nerve. This fact is not to 

 be wondered at, since the responsive effects are subject, as will 

 be shown, to numerous modifying influences. It is generally 



