2 9 6 



COMPARATIVE ELECTRO-PHYSIOLOGY 



from inner to outer. The degree of this diminution of 

 excitatory negativity in the outer surface, moreover, cul- 

 minating as this may in actual positivity, will depend upon 

 the extent of its transformation. In connection with this 

 it should be remembered that, in order to bring out the 

 differential excitabilities of the two surfaces, it is necessary 

 to apply localised stimuli of an intensity not too excessive. 

 For, if the stimulus be very strong, there is always a 

 possibility of its affecting deeper layers of the tissue and 

 thus causing complications in the resultant excitatory 

 changes. The intensity of stimulus which may be safely 



used without bringing about 

 such complications will depend 

 on the conductivity of the 

 tissue. Epidermal cells are, 

 generally speaking, feeble con- 

 ductors, but in this matter it 

 must be understood that the 

 differences in this respect be- 

 tween different tissues are not 

 absolute, but a question of 

 degree, and may to a certain 

 extent be modified under dif- 

 ferent circumstances. Thus a 

 feebly conducting tissue, under 

 a favourable condition of tem- 

 perature and strong intensity of stimulation, will become to 

 a certain extent conducting. Highly conducting tissues like 

 nerve, on the other hand, under unfavourable circumstances 

 may, as I shall show later, be converted into very feeble 

 conductors. 



Returning now to the question of the responsive reactions 

 of skin, we see the theoretical possibility of the following 

 typical reactions. Let the scale of excitabilities be repre- 

 sented by diagram to the left of fig. 177. Now, if the trans- 

 formation of the outer epidermal surface, A, be maximum, the 

 sign of its reaction will exhibit the greatest extent of deviation 



FIG. 177. Diagram Representing 

 Different Levels of Excitability, 

 Plus, Zero, and Minus 



Diagram to right of figure shows 

 how resultant up response (inner 

 to outer) may be obtained when 

 induced change at A is plus, and 

 at B minus, or when induced 

 change at A is less negative than 



