5 i2 NERVE. 



A necessary condition for its production is the increased excit- 

 ability of the nerve. Other chemical agents may be employed for 

 increasing nerve excitability. In cooled nerves it sometimes happens that 

 the opening of an ascending current evokes the single twitch, followed 

 by a prolonged contraction, having the same characters as that seen in 

 the above case. 



The opening of the current may thus produce two excitation 

 phenomena, — one sudden and rapidly developed, indicated by the initial 

 opening twitch ; the other prolonged and more slowly developed, 

 indicated by the continuous contraction. These cannot be identical as to 

 their causation, since it is possible to place the nerve under conditions 

 which favour the production of the one, although they disfavour that of 

 the other. General increase in excitability, such as that produced by 

 immersion in 1 per cent. NaCl (alcoholic solution), favours the produc- 

 tion of both effects ; but a general decrease in excitability can be caused 

 by appropriate reagents, which will disfavour the second whilst favouring 

 the first. The most striking of these reagents are 10 per cent, solution of 

 NaCl and 1 per cent, solution of KNO3. 1 If the nerve is immersed in 

 the latter solution, the following facts are observed. Immediately after 

 its removal a single twitch is evoked in the muscle by the closure of 

 an ascending current, and on opening an initial single twitch is evoked 

 which may, under appropriate conditions, be succeeded by a prolonged 

 muscular response. As the experiment is repeated, the closing excita- 

 tion response becomes feebler, showing gradual decrease in excitability ; 

 the opening initial single twitch becomes larger, whilst the prolonged 

 opening effect becomes small and finally disappears. By placing the 

 nerve in physiological salt solution, all these phenomena are replaced 

 by those obtained in a normal nerve ; they may be again produced by 

 fresh immersion in the KN0 3 solution. 



Hence it appears that the two opening effects differ as regards the 

 condition of nerve most suitable for their manifestation. The single 

 excitation evoking the twitch is favoured by some conditions which 

 cause general lowering of excitability ; the prolonged response is 

 favoured by general rise of excitability. The probable causation of 

 the initial opening effect is the polarisation after effect, aided by the 

 demarcation nerve current, if this is of sufficient intensity and appro- 

 priate direction ; the causation of the second continuous response is a 

 prolonged molecular change, coincident with the gradual disappearance 

 of anelectrotonus, in other words, a rebound of the nerve equilibrium 

 from an enforced state of greater stability into one of instability. 



As already stated, the closing tetanus in the frog appears to be an 

 effect in which a continuous " latent " excitation becomes adequate at 

 the cathode, through the local rise in excitability during closure. There 

 are, however, clear indications that in some nerves there are prolonged 

 excitation phenomena, even at closure. 



The most striking instances of such closing effects are furnished by 

 the results of the elaborate experiments made by Biedermann upon 

 the non-medullated nerves of the muscles which open and close the 

 claw of the crayfish ; these experiments also afford demonstrative 

 evidence of the validity of the above conclusions as to the character 

 of the opening excitation. 2 



1 Biedermann, "Elektrophysiologie," 1895, S. 590-594. 



2 Biedermann, Sitzungsb. d. 1c. Akad. d. Wissensch., Wieri, 1888, Bd. xcvii. Abth. 3, S. 49. 



