RESPONSE OF ANIMAL AND VEGETAL SKINS 289 



supppsed, in the preparations made for these experiments, 

 that it is one surface only which is electro-motive. I shall 

 show, however, that the responsive effects are brought about 

 by the differential excitability of the two. This response, 

 again, is modified by relative changes induced in the two 

 surfaces. And, in addition to these, still further compli- 

 cations are introduced when the stimulus is indirect that is 

 to say, applied through the nerve. In this case, the relative 

 excitations of the two surfaces will be determined by the 

 particular distribution of the nerve-endings. Again, we 

 shall see that, in an isolated preparation, the nerve itself 

 is liable to undergo certain changes by which its trans- 

 mitted effect may be modified even to reversal (p. 530). 

 Thus, so long as it remains highly excitable, the transmitted 

 effect is one of true excitatory galvanometric negativity. But 

 with physiological depression, the conductivity of the nerve 

 is very much lowered, and the effect transmitted becomes 

 reversed to positivity. 



For all these reasons, if we wish to study the specific 

 reactions of skin, epithelium, and gland preparations, it is 

 better to do so by observing them under direct stimulation. 



Engelmann, in studying the responsive reactions to 

 direct stimulation of frog's skin, found a negative variation of 

 the current of rest. Since the latter was naturally ' ingoing,' 

 as regards the upper or 'epidermal surface, this meant that 

 the responsive current was 'outgoing.' Reid, again, work- 

 ing on the skin of the eel, obtained ingoing response, or 

 positive variations of the resting-current, by single induction 

 shocks in either direction. Biedermann, in the mucous 

 membranes of the tongue and stomach, obtained both 

 positive and negative variations of the current of rest. 

 Waller, using single induction shocks in either direction, 

 found in the digestive mucosa both ingoing and outgoing 

 responses, the former being much predominant. 



Even under the simple conditions imposed by direct 

 stimulation, then, the results obtained are seen to be in- 

 consistent. They would appear to show both that the 



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