496 ELECTRO-PHYSIOLOGY CHAP. 



If every diminution of the normal ingoing skin current is 

 thus favourable to the appearance of homodromous (positive) ex- 

 citation effects, it is a priori possible that even where the " current 

 of rest " is altogether wanting, " an outgoing secretion current " 

 may be present. In fact, Bach and Oehler showed that whereas 

 after quite a short (6-8 sees.) action of saturated solution of sub- 

 limate the skin betrayed hardly any electrical activity, excitation 

 from the nerve still produced tolerably pronounced effects, always 

 in the direction of an entering current. We cannot agree with 

 Hermann when he finds in this fact a convincing proof that the 

 epithelial layer alone is intrinsically the seat of electromotive 

 activity in the unexcited skin, only those effects which occur with 

 nerve-excitation (" secretion currents ") being true gland functions. 

 For apart from the fact that even from the histological point of 

 view this theory is highly improbable, it is also quite conceivable 

 that in spite of the short duration of the sublimate bath, traces 

 of the substance may penetrate into the gland cells, and reduce 

 their normal electromotive activity, i.e. entering current, almost 

 to zero, without abolishing it completely. We have, however, 

 found ample proof in the preceding discussion, that under 

 circumstances in which any kind of injury has weakened the 

 entering current of the mucous secreting cells to a greater or 

 less degree, homodromous effects may appear with direct or in- 

 direct excitation. 



Since, as shown by Engelmann, the degree of moisture in the 

 .skin is far the most important factor in determining the strength 

 of its normal electrical activity, as is also the case in true mucoste, 

 we should a priori presume that it would be possible, by altering 

 the bulk of water, to alter the direction of the galvanic effects 

 occurring when the skin is excited as in the foregoing. Indeed 

 the experiments on the mucous coat of the tongue, throat, and 

 cloaca, described above, points in this direction. 



It is well known that frogs, when kept dry, gradually lose a 

 large amount of water through the skin, but it would take a long- 

 time before they were sufficiently dehydrated by this method to 

 be fit for experiment. This is effected much more quickly with 

 the aid of dehydrating substances. A quantity of water can be 

 drawn out of the frog's tissues in the shortest possible time by 

 the simple injection of strong salt solution, or glycerin of sufficient 

 density, under the skin of the back. 



