472 ELECTRO-PHYSIOLOGY CHAP. 



mucosa current is always considerably increased if after the 

 deflection has been rendered approximately constant by long 

 immersion in ordinary physiological saline a semi-normal (i.e. 

 G'2-0-3 %) Nad solution is applied: still more so if spring 

 water or distilled water is employed. 



A single drop of distilled water applied with the leading- 

 off electrode to the surface of a tongue previously treated with 

 physiological salt solution is sufficient to produce a strong positive 

 variation of the mucous current, although the resistance in the 

 circuit of course increases considerably. Even long immersion in 

 spring water not merely fails to weaken the normal current, but 

 may even maintain it at a permanently greater E.M.F. than 

 0'6 % salt solution. It is therefore out of the question that 

 the antagonistic effects above quoted should be due to the action 

 of the water of liquefaction, when the mucosa is cooled by the 

 application of snow or ice. Such solutions as contain salt enough 

 to cause dehydration, to a greater or less degree, of the tissues in 

 contact with them, produce a reverse effect from water or highly 

 dilute salt solution. With such we always find (e.g. with O'S- 

 1'5 / o NaCl solution) a comparatively rapid fall of E.M.F. 

 in the ingoing tongue current, which, between certain limits, 

 rises again with equal rapidity when water is added. 



It is very remarkable that in this case also, as on ener- 

 getic cooling of the tongue, there may be a true reversal of the 

 normal ingoing current, although the strength of the opposite 

 current is generally far behind that produced by the action of 

 cooling. It is possible in the same preparation, by alternate 

 immersion in 1 c / o and 2 / Q salt solution, to give a success- 

 ively incoming and outgoing direction to the current many 

 times over. As a rule, a few minutes are sufficient to initiate 

 these changes. Engelmann had already found in the frog's skin 

 that very low differences in concentration of the salt solutions 

 produced extraordinary alterations in electromotive response, 

 from which we may conclude a singular sensibility of the active 

 elements concerned, with regard to changes in the water content. 

 It is well known that even while the frog's tissues are living, 

 water may be drawn out of them vigorously by injecting strong 

 solutions of common salt or glycerin at the back of the head. 

 Half a cc. of the latter injected into the dorsal lymph-sac of a 

 curarised frog is sufficient in a short time (1-2 hours) to draw 



