488 NER VE. 



that this is the cathode of an exciting galvanic or induced current, 

 then, in the case of strong NaCl, the gradual alteration in excitability 

 which precedes the appearance of these twitches may be readily 

 observed. It will be found that an intensity of current, just inadequate 

 to excite immediately after the electrode is placed in position, is in a 

 very few minutes adequate, and that the passage of either the galvanic 

 current or the induced current now evokes a maximal contraction. With 

 a little longer time for the operation of the salt, the closure of the 

 galvanic current evokes a closing continuous muscular contraction. 

 The increased excitability thus renders the closing cathodic excitation 

 fully adequate. In a similar way, if the anodic electrode is the salt 

 contact, the opening of a current is rendered effectual, and an opening 

 continuous contraction is evoked in the muscle, with an intensity of 

 current which was inadequate under normal conditions to excite the 

 nerve sufficiently to evoke even a twitch. The salt, NaCl, has thus 

 raised the local excitability. The continuous contraction is due to the 

 salt throwing the nerve into a state of continuous feeble excitation, but 

 at first it stimulates it so slightly that the resultant transmitted nerve 

 effect is unable to evoke the muscular response. The cathodic closure 

 or anodic opening, with its rise of excitability, is sufficient to render 

 this " latent excitation " adequate. 



The most plausible view of the mode in which the nerve excitability 

 is augmented by the salt is the following. The molecular equilibrium 

 of the nerve is upset by the salt in the same manner as it is by any 

 stimulating agent. The disturbance, however, is not in the first instance 

 of sufficient magnitude to constitute an evident excitation, but it gradu- 

 ally becomes more pronounced, and may finally attain the requisite 

 magnitude. In the earlier stages the equilibrium of the molecular state 

 being upset, and thus rendered less stable, the action of any additional 

 external disturbing agent readily causes the necessary magnitude of 

 disturbance which constitutes an excitation. We have here an illus- 

 tration of a fundamental characteristic of nerve. All stimulating 

 agents, when too weak to be able to evoke an adequate excitation, 

 operate upon the nerve so as to disturb it in a direction which, if 

 more intense, would constitute an excitation, and they thus render it 

 easier for an external stimulus to evoke the true excitatory effect, that 

 is to say, they increase local excitability. 



In the case of NaCl it is most probable that the modus operandi 

 largely consists in the withdrawal of water from the nerve fibre, and 

 that glycerin acts in a similar way. The withdrawal of water which 

 takes place in drying, produces the same rise of excitability, passing on 

 into a continuous excitation. It is difficult to localise its operation 

 with the precision which the use of NaCl permits, but the careful 

 experiments of Harless, Griinhagen, and others show that a nerve is 

 more excitable to such stimuli as galvanic currents, when it is gradually 

 dried. 1 The demonstration of this is carried out by experiments similar 

 in character to that described with NaCl, but the drying of the nerve 

 is effected far more slowly than the salt effect, even when, with a dry 

 surrounding of high temperature, evaporation is favoured. The results 

 of both NaCl and drying are most clearly brought forward by Bieder- 

 mann in his Elektrophysiologie. 2 



1 Harless, Ztschr. /. rat. Med. (3), Bd. vii. ; Griinhagen, ibid., Bd. xvi. 

 2 Biedermann, "Elektrophysiologie," 1895, S. 582. 



