On Skin Currents. 



489 



state that the normal current of the frog's skin is directed from with- 

 out inwards. All subsequent observers have confirmed this point. 



Indirect Excitation. — Roeber,* acting upon a suggestion of Rosenthal, 

 was the first to make nerve-skin preparations of the sciatic nerve and 

 skin of the leg, and to show that excitation of the sciatic nerve gave 

 rise to an electromotive variation of the skin. He observed in the 

 great majority of instances " a negative variation of the gland currents 

 in consequence of non-electrical as well as of electrical excitation of the 

 sciatic nerve." He mentions as an exceptional phenomenon, p. 644, 

 a positive variation of the normal current. 



Engelmann,f using the same method, comes to a similar conclusion, 

 viz., that the usual effect of indirect excitation is negative variation of 

 an ingoing current. He gives measurements of the effect (p. 130), 

 from which may be gathered that a good response in his hands had the 

 value 0*025 Daniell. The latent period is given as being from \ to 

 4 seconds. He describes the course of the variation as being very 

 usually triphasic ( - + - ), which in the terminology used in the 

 present communication reads + - + . 



He considers that skin currents are " myogenic," the effects of the 

 muscular investment of skin glands. He studies with particular care 

 the influence upon the currents of variations in moisture of the skin, 

 (imbibition and concentration currents). 



Hermann]: contradicts Engelmann's theory, and, to a certain extent 

 his statement of fact as regards the action current. He gives the usual 

 and principal effect as being a positive variation of the normal current. 



He states, however,, that such positive variation is sometimes preceded 

 by a negative effect, and that, in rare cases, a pure positive effect is 



* Du Bois-Reyinond, < Archiv,' 1869, p. 633. 

 f ' Pfliiger's Archiv,' vol. 6, p. 97, 1872. 

 % ' Pfliiger's Archiv,' vol. 17, p. 291, 1878. 



