306 



COMPARATIVE ELECTRO-PHYSIOLOGY 



histological modifications, the outer surface is reduced in 

 excitability. The epidermal layers have little protoplasmic 

 contents, and may be transformed in various ways, becoming 

 corneated or cuticularised. The extent of such transforma- 

 tion may be small or great, but the external layer will as 

 a general rule become less excitable than the inner tissue. 

 Hence, under normal conditions, we have a current of rest 

 from without to within. 



If the inner layer be only moderately excitable, or if its 

 power of recovery from excitation be great, then the dis- 

 turbance caused by the prepara- 

 tion of the specimen will be 

 slight, or will pass off quickly. 

 It is to be remembered that as 

 the inner surface is the more 

 excitable, the responsive current 

 due to the mechanical stimulus 

 of preparation will be from 

 inner to outer ; and therefore 

 its after-effect, proving in certain 

 cases persistent, may give rise 

 to a current apparently the re- 

 verse of the true normal current 

 Thus, the direction of the current 

 of rest, which we should have 

 inferred theoretically to be from 

 the less to the more excitable, 

 may occasionally be found re- 

 versed, owing to the excitatory 

 after-effect of preparation. The 



current of rest, moreover, is liable to autonomous periodic 

 variation, as we have seen. 



The most satisfactory method of determining the relative 

 excitabilities of two surfaces, then, lies in subjecting them to 

 simultaneous excitation, and observing the direction of the 

 responsive current. In the skin, unless the tissue was 

 fatigued, I have always found this to be from the more 



1 86. Photographic Record 

 of Electrical Responses in Skin 

 of Neck of Tortoise to Stimulus 

 of Equi-alternating Electrical 

 Shocks at Intervals of One 

 Minute 



The direction of the responsive 

 current was from inner to 

 outer. The so-called current 

 of rest was also in this case, 

 owing to the excitatory after- 

 effect of preparation, from inner 

 to outer. 



