252 ELECTRO-PHYSIOLOGY CHAP. 



experiments prove nothing as to the processes that are 

 taking place within the nerve, the conclusion being only formed 

 retrospectively, from the reaction of the peripheral organ. If the 

 motor apparatus exhibits activity when the nerve is stimulated 

 by any means, there can of course be no doubt as to its 

 excitation. Otherwise, however, there are two possible alter- 

 natives : either the nerve is not really excited, or at least the 

 excitatory process is not transmitted ; or the terminal apparatus 

 may not react to the conducted stimulus (Griitzner, I.e.). 



If, then, the negative variation is a true expression of excita- 

 tion in the nerve, it affords a simple and convenient means of 

 testing the excitability of different nerves, independent of the end- 

 organ, towards various stimuli. Here, again, there are two alter- 

 natives: (1) homogeneous stimuli, acting upon different nerves, may 

 produce the same negative variation, and in this case the dis- 

 parity of effect must be referred to the end-organ ; (2) the 

 negative variation itself may differ, in correspondence with the 

 dissimilar effects of excitation in the end-organ. The cause of 

 the heterogeneous effects must then lie within the nerve itself. 

 From this point of view Griitzner (22) in the first place investi- 

 gated the effect of thermal excitation upon the negative variation 

 in different nerves. A similar experiment, but one that is open 

 to criticism, had been made by du Bois-Keymond (23). He 

 placed the nerve (frog's sciatic) upon a layer of moist gunpowder, 

 which, when lighted, charred the nerve from one end onwards. 

 Notwithstanding the indisputably drastic stimulus to the succes- 

 sive sections of the nerve (which, by the way, can hardly be 

 termed " thermal "), the galvanic consequences were very incon- 

 spicuous, and not to be compared with the marked effects of 

 electrical excitation. Obviously, as was pointed out by Griitzner, 

 the negative variation may here be checked ab initio by the un- 

 avoidable shortening of the excitable portions of the nerve. 



Yet, with a more perfect method, Griitzner failed in obtain- 

 ing any effective results. At temperatures of 40 50 C. the 

 demarcation current of frog nerves did indeed decline perceptibly, 

 but only to an inconspicuous degree, and very slowly ; there was, 

 moreover, as a rule, a persistent diminution of the current, so 

 that the effect was hardly comparable with that from electrical 

 stimulation. Experiments on the anterior and posterior roots 

 gave still less certain results, so that the question whether 



