On Skin Currents. 171 



cate that in sodium vapour we have a medium which approaches more 

 nearly to the ideal absorbing medium, with but a single natural period 

 of vibration, than any substance heretofore investigated. 



Of course I am speaking here only with reference to the natural 

 vibration which appears to influence the dispersion. Strictly speaking, 

 there are two natural periods, of course, which influence the velocity of 

 the light in the medium, but when the medium is very dense the con- 

 dition certainly approaches very nearly to that of a single period 

 medium. As I have said before, the fluted absorption bands are 

 without influence on the dispersion, at least their influence is too 

 slight to be detected by the methods that have been employed thus 

 far. 



"On Skin Currents. Part II. Observations on Cats." By 

 AUGUSTUS D. WALLER, M.D., F.K.S. Keceived October 7, 

 Read November 21, 1901. 



In the first part of the present investigation* I have stated, as one 

 of the principal conclusions with regard to the frog's skin, that the 

 normal electrical response of the excited skin is of outgoing direction. 



The chief object of the following observations was to ascertain 

 whether or no similar effects of outgoing direction are manifested by 

 the pad of the cat's foot, this having been, since the first observations 

 of Hermann and of Luchsinger, the chosen object upon which to 

 demonstrate the cutaneous (epithelial and glandular) currents aroused 

 by nerve-stimulation. 



I have examined the cutaneous currents, A indirectly aroused by 

 nerve-stimulation, B directly aroused by electrical excitation of the 

 skin itself in the manner described and figured in my previous com- 

 munication (loc. cit., p. 481). 



A. INDIRECT EXCITATION. The animals were decapitated, and used 

 in the first instance for the observation of indirect effects and their 

 possible modification in consequence of arrested circulation. In every 

 case, without exception, the electrical effect of excitation of the sciatic 

 nerve upon the pads proved to be an ingoing current, as described by 

 Luchsinger and by Hermann.! The effect gradually declined with 

 lapse of time, and disappeared within 1 hour after decapitation, with- 

 out exhibiting any change of sign or other modification. 



Although it was not my purpose to pay particular attention to this 

 point, I may take the opportunity of stating that the experiment, as 



* "On Skin Currents. Part I. The Frog's Skin," ' Roy. Soc. Proc.,' June 6, 

 1901, vol. 68, p. 480. 



t Kendall and Luchsinger, " Zur Theorie der Seoretionen," ' Pfliiger's Archiv,' 

 Tol. 13, 1876, p. 212 ; Hermann and Luchsinger, " Ueber die Secretionsstrome der 

 Haut bei der Katze," ' Pfliiger's Archiv,' vol. 17, 1878, p. 310. 



