v ELECTROMOTIVE ACTION OF EPITHELIAL AND GLAND CELLS 499 



by a plain negative variation, followed by a weaker, positive 

 swing bleed it, and expose the nerves letting it then lie 

 in the chamber, and occasionally testing the effects of ex- 

 citation ; the ingoing skin current then declines gradually, the 

 negative variation grows weaker, the positive effect stronger, and 

 at last (after a brief interval of total failure of effect) it takes 

 complete possession. 



As Hermann pointed out, there is always a fairly protracted 

 after-effect from the positive, in contradistinction to the negative, 

 variation the deflection increasing for some time after the 

 excitation is over ; the decline, too, follows more slowly than 

 with the negative variation. 



An interesting reaction appears in frogs (temporaries), fresh 

 caught in the latter half of February. Any point of the skin 

 at first gives a vigorous electrical variation, often far beyond 

 the scale ; excitation of the sciatic produces a corresponding 

 and monophasic negative variation in the skin of the leg; this 

 only declines very gradually and incompletely, so that a strong 

 persistent diminution of the original current results, which 

 again disappears almost entirely after a second short excitation. 

 A third experiment is followed by a positive effect, preceded by 

 a short, negative fore-swing. In another frog of the same group, 

 the first excitation produced such a marked diminution of the 

 original ingoing skin current that even at the second excitation 

 the negative was replaced by a positive variation. Here we have 

 proof of the extent to which the character of the variation is 

 conditioned by the strength of the rest current. And we also 

 learn from these experiments that Engelmann's attempted ex- 

 planation of the positive effect is fallacious. He supposes that 

 the positive increment produced by the free tension at the 

 surface of the epidermis, in consequence of surface moisture from 

 the skin glands emptied during excitation, over-compensates the 

 negative effect originating in the decline of glandular energy. 

 This theory assumes that the surface cell-layers form a compara- 

 tively non-conducting layer in consequence of dehydration, " only 

 a small fraction of the electrical tensions derived from glandular 

 activity being apparent at its surface." Yet such could not be 

 the case, either in the instances quoted above, or in the dehy- 

 drated frogs. For in the first case, no water had been drawn off, 

 and the vigorous primary skin current shows that the glands were 



