ELECTRICAL EXCITATION OF NERVE 167 



repeatedly, on stimulating the -deeper portions of the sciatic of 

 Eana esculenta with weak descending currents, that " the opening 

 twitch responded to the moment of opening the descending 

 current at a very long interval, often lasting for several seconds." 



Such a striking delay has been observed by Biedermann in 

 rare cases only, and never on nerves in which excitability had 

 been raised by loss of water ; and notwithstanding the compara- 

 tively small number of experiments, and the possibility of indi- 

 vidual variations, it is impossible not to suspect that Pfliiger 

 had before him in these cases preparations which were in the 

 first stage of desiccation. We have never observed the pheno- 

 mena in question, save when the excitability of the nerve 

 was artificially raised above the normal by certain influences to 

 be described below. Eckhardt (37), e.g., explained the excita- 

 tory manifestations observed with the action of neutral salts of 

 alkalies (more especially NaCI, as a solid, or in strong solutions), 

 as caused by abstraction of water comparing them directly 

 with the excitatory phenomena concomitant with the drying of 

 the nerve. And there is in fact a great similarity of action in 

 both cases as regards alterations of excitability in general, and 

 also the appearance and character of the opening excitation. 



The use of concentrated solution of NaCl is so far advan- 

 tageous that it is more easy to localise the effect to a definite 

 tract of nerve therewith, than in desiccation ; but, on the other 

 hand, there is the objection that in electrical excitation of a nerve- 

 section treated with NaCl the tendency to tetanic contraction of 

 the muscle is far more pronounced with even the weakest closure 

 or opening stimuli than it is in desiccation, so that this method 

 nearly always results in opening tetanus, and rarely in opening 

 twitches. 



Since in treating a nerve with NaCl in its entire length the 

 closure tetanus (which at once appears, independently of direction 

 of current, provided that, as always, weak currents are employed) 

 would considerably interfere with the recognition of excitatory 

 phenomena at break of the current, it is well to confine the 

 action of the NaCl as far as possible to the region of the anode. 



It is convenient in these experiments to employ the 

 form of unpolarisable tube electrode first described by Engel- 

 mann (38). A little pad of cotton wool soaked in saline 

 is then placed upon one of the electrodes, so that a length 



