RESPONSE OF EPITHELIUM AND GLANDS 



321 



site direction to the natural current, thus demonstrating the 

 fact that the epithelial layer was here, as in the plant, the 

 more excitable of the two. The regularity of this effect will 

 be seen from the series of photographic records given below 

 (fig. 196), in which is exhibited a slight staircase effect. 



We next proceed to deal with the response of the glan- 

 dular organ, the tongue. The tongue of the frog has formed 

 the subject of a very extended series of researches, by 

 Engelmann and Biedermann. On very 

 careful isolation, entailing as little injury 

 as possible, it was found by these workers 

 that the natural current was ' entering ' : 

 that is to say, it flowed across the tongue 

 from the upper surface to the lower. Both 

 electrical and mechanical stimulation was 

 found by these observers to cause a negative 

 variation of this natural current 



As isolation of such a highly excitable 

 organ as the tongue may, however, give rise 

 to unknown excitatory after-effects, it ap- 

 peared to me very desirable that an investi- 

 gation on this subject should be carried out 

 on the intact human tongue. In connection 

 with this, I must point out that both the 

 surfaces of the tongue are excitable. Our 

 inquiry, therefore, is into the relative excit- 

 abilities of its upper and lower surfaces. Here 

 the experimental difficulty lies in this very high excitability 

 of the organ, on account of which except when in a quies- 

 cent state and with a very steady contact the galvanometer 

 spot of light is apt to be erratic in its movements. Much 

 of this difficulty is overcome, however, by holding the pro- 

 truded tongue lightly clamped between the teeth. The upper 

 and lower surfaces may then easily be held in the clip-key 

 already described. From this double support of the clip and 

 the teeth it is, with a little practice, possible to arrange 

 matters in such a way that the galvanometer spot 



Y 



Electrical Response 

 of Intact Human 

 Lip 



Responsive current 

 from epithelial to 

 epidermal surface. 



is 



