494 ELECTRO-PHYSIOLOGY CHAP. 



diminution of the skin current in consequence of nerve excitation, 

 whether this were produced electrically, chemically, or mechanic- 

 ally. Even with the application of a single, vigorous, make 

 induction shock to the peripheral stump of the sciatic, he observed 

 a preliminary fall of E.M.F. to 25-30 / Q , which is naturally 

 even more considerable with tetanising excitation. Accord- 

 ing to Engelmann, the course of an " elementary " variation in 

 nerve excitation through a single momentary stimulus is as 

 follows : " After a latent period, lasting with weak excitation for 

 4 sees., with stronger excitation for less than ^ sec., the E.M.F. 

 falls at first with increasing, and later with diminishing, rapidity 

 reaching its minimum after a few seconds with weak excitation, 

 in 1020 sees, with stronger stimuli; it rises again imme- 

 diately, at first with increasing and subsequently with diminishing 

 rapidity, eventually reaching its original proportions." " But it 

 does not often remain stationary at this point, especially when 

 the skin has been resting for a long time before excitation. More 

 frequently, during the next few minutes it rises higher in propor- 

 tion with the strength of the previous excitation " (positive after- 

 variation), " sinking down again slowly afterwards. If the excita- 

 tion is repeated frequently, the positive after-variation fails to 

 appear, and there will each time be a negative variation only. 

 With prolonged tetanising excitation of the nerve, the depression 

 of E.M.F. continues much longer, and outlasts the excitation. 

 If the excitation is subsequently very vigorous, the positive after- 

 variation may be wanting, even where it appeared unmistakably 

 after a short excitation. The E.M.F. then remains permanently 

 depressed, and renewed excitation only produces an insignificant 

 diminution of it." 



Hermann (82), on the contrary, finds in the skin of the lower 

 limb, and more particularly in the skin of the frog's back, in con- 

 sequence of nerve excitation, either a pure positive variation, or the 

 same preceded by a negative forc-su: 'iiiy, " which, however, is usually 

 much weaker than the positive variation itself " the latter being 

 " the true main effect." Hermann only found a pure negative 

 variation twice in the skin of the back, and in the leg " in a 

 negligible number of cases only " (three out of eighty frogs). 

 The order of the deflection (in skin of back with tetanising ex- 

 citation) is, according to Hermann, as follows : " At first the scale 

 remains at rest for several (2-4) sees. ; after this latent period 



