160 



Dr. N. H. Alcock. 



[Apr. 9, 



other two, C and B (fig. 1). The skin was placed with the outer surface 

 resting on A and C, the movable upper portion of the partition adjusted, and 

 contact effected with the electrode B through a bundle of linen threads soaked 

 in m/10 ISTaCl solution. In this series the electrode C served merely as 

 a support, and the potential differences between A and B were alone 

 considered. 



Thread- 



To key board 



Fig. 1. — Chamber and Electrodes for Series I and II. 



To make a preparation the frog is first killed by pithing, and the skin 

 of the back cut into two longitudinal strips 3 to 5 mm. wide, one on either 

 side of the middle" line. These strips are very gently lifted up and the 

 connections below severed, special care being taken to avoid stretching 

 or other injury. A preparation of this kind gives a " resting " potential 

 difference of from 0'0200 to O'llOO volt,* sometimes exceeding this latter 

 figure, the value varying according to the avoidance of injury and the 

 condition of the frog, freshly caught frogs giving much higher values than 

 those kept in captivity for some time. Any injury diminishes the potential 

 indefinitely, and it is desirable to test the preparations from time to time 

 by examining the skin in situ. If one electrode be placed very gently on the 

 external surface of the skin and another on any portion of the subcutaneous 

 tissue, one obtains the " resting " potential with the minimum of manipula- 

 tion, and the value from the excised strip should not be much below this.f 



* Du Bois-Beymond (' Untersuchungen,' vol. 2, Abth. 2, pp. 9 — 20) gives a maximum 

 value of 0'0561 volt and a minimum of 0'0044 ; Bayliss and Bradford (loc. cit.), 0"25 and 

 0-03 volt. 



t There are many points still remaining to be determined with regard to the resting 

 current in relation to variations in the conditions of observation : some of these are under 

 consideration. 



