ix ELECTRICAL EXCITATION OF NERVE 181 



part. According to Engelmann (4, p. 411), however, it arises 

 here from pre-existing spontaneous stimuli, which were at first 

 inadequate, and now become effective from the positive modifica- 

 tion in excitability at the previously anelectrotonic tract of the 

 nerve, on opening the current thus producing a change of 

 form in the muscle. Engelmann makes special reference to 

 the fact (as is easily confirmed) that " a simple twitch, 

 which cannot be distinguished from the closure twitch, or -the 

 twitch from a single induction -shock, appears in fresh nerve- 

 muscle preparations of normal frogs (when preserved from 

 evaporation) on breaking the current." " On the other hand, the 

 opening tetanus (as also the analogous closure tetanus) makes its 

 appearance with the greatest regularity in cooled preparations," 

 the nerves of which are characterised by peculiar excitability 

 referred by Engelmann to the presence of internal stimuli, that 

 are often so powerful as to induce spontaneous twitches, or even 

 tetanus, when every precaution is taken against evaporation. 

 Further support of Engelmann's views, as to the nature of 

 Eitter's tetanus, is found in an experiment of Griinhagen (40), 

 which shows that " weak tetanising excitation that produced no 

 visible effect before the closure of the polarising current, calls 

 out an unmistakable tetanus when the latter is opened, lasting 

 the longer in proportion with the strength of the polarising 

 current and susceptibility of the nerve." Griinhagen hence 

 deduces the following proposition : " The raised excitability of 

 the nerve at the previously anelectrotonised region, summating 

 with the increased disintegration stimuli, normal to the nerve, 

 results in the opening tetanus of constant currents. These 

 chemical stimuli may be counterfeited by a sub-liminal tetanis- 

 ing excitation." 



We should thus expect the secondary opening twitch only 

 when the nerve is, so to speak, in a state of latent excitation. 

 And the above facts relating to the appearance of break twitch II 

 are in complete agreement with this anticipation. For with loss 

 of water from evaporation, or in treatment with concentrated 

 saline, the nerve falls directly into that state of excitation which, 

 though at first too weak to express itself in twitches of the 

 muscle, appears later on as a vigorous tetanus. Just at the 

 moment at which the excitation is latent, break twitch II, or 

 Eitter's tetanus, may be produced even by weak currents. The 



