60 



THE SIGNS OF LIFE 



LECT. 



We shall study the skin-currents of the frog, of the cat, 

 and of man, also the currents of mucous membranes and the 

 currents of vegetable " skins." To-day we shall confine our- 

 selves to the case of the frog, examining : (a) the normal 

 current, (ff) the effects of indirect excitation, (V) the effects of 

 direct excitation. 



37. TJie normal current. A piece of skin of the back, or 

 indeed of any part of the body, carefully excised and placed 

 between impolarisable electrodes, gives current, directed from 

 outer to inner surface (through the skin). This "ingoing" 

 " negative " or centripetal current increases during observation. 

 A piece of skin spread upon a glass plate with a central hole, 



and placed between the elec- 

 trodes as figured, gives current 

 that reflects the galvanometer 

 spot to your left. And, as you 

 notice, the spot is creeping 

 further to the left. Two re- 

 flections arise from this obser- 

 vation : clearly the current 

 cannot be due to injury of the 

 internal surface, since in that 

 case it would be outgoing ; 

 probably the increasing nega- 

 tive current is due to the sub- 

 sidence of what in the case of 

 the eyeball we referred to as 

 the manipulation blaze. It 

 looks as if resting skin were 

 the seat of an ingoing current 

 of rest, and as if the increasing 

 negative deflection occurring on the galvanometer scale were in 

 reality a decreasing positive effect caused by the previous 

 disturbance by manipulation. We shall find confirmation of this 

 view later on, when we have learned that direct mechanical and 

 electrical excitation of the skin gives almost invariably a positive 

 or outgoing electrical effect 



FlG. 26. Frog's skin on a perforated 

 glass or ebonite plate, between unpolar- 

 isable electrodes. The external surface 

 of the skin is uppermost. The arrows 

 signify the direction of " normal cur- 

 rent " " ingoing," from external to 

 internal surface. 



