V ELECTROMOTIVE ACTION OF EPITHELIAL AND GLAND CELLS 477 



to make use of melted snow or ice ; it also depends conspicuously 

 upon certain conditions in the mucosa, which again are due to 

 environment during the life of the frog. In the throat, 

 as in the tongue, the best and most convincing results are 

 obtained when the preparation is taken from a warm frog, and 

 the original entering current not too pronounced. Accord- 

 ingly the frogs (R. temporaria, not curarised) intended for this 

 experiment were usually left two to three days in a warm 

 room near the stove. In order, as far as possible, to avoid 

 mechanical excitation of the mucosa by friction or pressure, it 

 was found best to apply loose melting snow, a lump of which 

 was placed upon the mucosa of the skinned upper jaw, and 

 several times renewed before the current of rest was tested. 

 The water of liquefaction is carefully removed with a brush, and 

 the leading-off electrodes arranged in such a way that they are 

 divided by a not too thick layer of melting snow from the mucous 

 surface below them. Immediately after reading the scale the 

 galvanometer is opened again by removing the electrodes from 

 the mucosa, so as to avoid the development of accidental " thermo- 

 currents " as far as possible. Exactly the same method is employed 

 to investigate the effect of cold upon the cloacal current. In 

 the one case, as in the other, a very rapid fall of the original 

 E.M.F. is visible, accompanied, generally speaking, by reversal of 

 the current, upon which it often reaches such proportions that 

 the spot flies off the scale. When the snow is entirely melted the 

 original E.M.F. of the current usually conies back in consequence 

 of the increasing temperature. The experiment may be repeated 

 many times on the same preparation with identical results. 



"We cannot doubt that the cooling of the surface epithelium 

 here, as in the tongue, leads per se to the appearance of hetero- 

 dronious electromotive force. 



The skin of the leech is another no less favourable object for 

 the study of electromotive action in superficially flattened, uni- 

 cellular, mucous glands. After removing the connective tissue 

 it is easy to free the skin with scissors from all ragged ends of 

 tissue, so that only the cuticular muscle-layer is left. Then, 

 on leading off from external and internal surface, there is in- 

 variably a strong entering current, which reacts under different 

 conditions as described above. 



The well-known homodromous electromotive action of the 



