v ELECTROMOTIVE ACTION OF EPITHELIAL AND GLAND CELLS 471 



in preparations of the lower jaw, which by alternate freezing and 

 thawing had been rendered perfectly currentless, and had lain for 

 some time in salt solution warmed to the temperature of the room, 

 even repeated application of snow or ice leaves hardly any trace 

 of an outgoing current. 



From these observations we may take it as proved that the 

 regular entering current of the mucous membrane of the frog's 

 tongue is not merely diminished to zero with extreme rapidity 

 when sufficiently cooled, but may also lie reversed when the 

 reversed current, under some conditions, reaches the same pro- 

 portions as those of the original " normal " current. 



In the experiments last quoted, the surface of the lingual 

 mucosa was moistened with the water of the melting ice, so that 

 it became necessary to consider whether the results of the experi- 

 ment were not due, at least in part, to this factor. That it was 

 certainly not the main cause is amply proved by the facts above 

 stated, but the strikingly rapid reversal of the current, as well as 

 its E.M.F., might be partially due to a water effect. We accordingly 

 examined the effect of the varying bulk of water on the electro- 

 motive properties of the lingual mucosa during " rest." Engel- 

 mann (72) had already made an excellent series of observations 

 with the same object on the skin of the frog, to which we shall 

 return later. These relate to the action of water, and of different 

 concentrations of salt solution, upon the equally ingoing current 

 of rest in the skin. Since, as we shall find, there is in every 

 respect almost complete agreement between the electromotive 

 action of the external skin, and the tongue, of the frog, it might 

 be assumed a priori that the same would be the case with regard 

 to the effects of addition and subtraction of water. Owing 

 to the extraordinary sensitivity of the lingual mucosa (infra) to 

 the slightest mechanical stimulus, the fluid to be tested must 

 not simply be poured on, or applied with the brush (which would 

 easily lead to the worst fallacies), but the preparation must be 

 dipped into watch-glasses containing the required solutions. 

 After a shorter or longer bath, the lingual current is tested again, 

 as described above, by leading off from a clay bed and the 

 surface of the mucosa. While normal 0'6 NaCl solution 

 is indifferent for the tongue also, in so far that the power of 

 giving electromotive reaction will persist for hours and even days 

 if the temperature is not too high, the E.M.F. of the ingoing 



