466 ELECTRO-PHYSIOLOGY CHAP. 



electromotive action, the jaw was exarticulated and divided 

 by a sharp cut below -the apex of the tongue. It is true that 

 muscles are injured by this method, and that current may diffuse 

 from their stumps into the galvanometer circuit ; but the effect 

 of this is practically negligible against the force of the lingual 

 current, as shown by control experiments in the same preparation 

 when the tongue has been removed. The lead-off is accomplished 

 as follows : the lower jaw, with its under surface, is placed on a 

 block of salt clay of corresponding proportions, which admits of 

 a lead-off' from the lower side of the tongue, by the floor 

 of the mouth, when one brush-electrode is brought into contact 

 with it, while the other is applied to any given point of the 

 upper lingual surface. It should be noticed that the floor of the 

 mouth on which the tongue is lying is itself invested with a 

 irmcosa rich in goblet cells, and therefore gives electromotive 

 reaction. But if the tongue is cut out at the root, and the lead- 

 off effected as before from the clay block and surface of the 

 mucous membrane, which had previously been covered by the 

 tongue, there will usually be very slight deflections only, in one 

 or the other direction, so that the disturbance is negligible. 



There can thus be no doubt that, however the lead-off' from 

 the tongue is effected, the results observed are produced respect- 

 ively by the electromotive activity of the surface epithelium in its 

 widest sense (epithelium of glands and papilla?), even if the 

 absolute intensity of this latter is modified to a degree that 

 cannot always be exactly determined, by the unavoidable in- 

 clusion, on leading off, of other electromotive elements. This is 

 most clearly shown in the fact, that (in every experiment arranged 

 as above) the current, which sometimes makes a very vigorous 

 entrance, driving the scale far out of the field of vision, dwindles 

 after destruction of the surface epithelium into irregular traces, 

 though neither the epithelium of the under surface of the tongue, 

 nor that of the floor of the mouth, can be perceptibly affected by 

 it. On the other hand, it is easy to ascertain that even little 

 scraps of mucosa, snipped out with a flat scissor-cut from the surface 

 of the frog's tongue, and examined, after a short rest, on a bed of 

 salt clay, are, just as much as the tongue, the seat of a strong- 

 ingoing current. Thus we may regard it as certain that the 

 normal current of rest in the tongue is essentially due to the 

 electromotive action of the surface epithelium, its glands included. 



