ix ELECTRICAL EXCITATION OF NERVK 175 



tage of enabling us without otherwise altering the experimental 

 conditions to follow exactly, on one preparation, the nature and 

 course of the gradual changes developed at break of the current 

 during the chemical action. We thus learn directly that the two 

 forms of twitch may exist simultaneously, and must, therefore, be 

 regarded as distinct effects of current. This cannot be determined 



^ 



with equal certainty from the opening effects of excitation in 

 drying or in " salt " nerves, since the prompt appearance of spon- 

 taneous tetanus prevents any prolonged observations ; nor would 

 those cases be decisive in which break twitch I alone makes its 

 appearance, since the initial increase of excitability is either want- 

 ing altogether, or finds insufficient expression. 



Ranke (39) states that "the excitability of the nerve is in 

 the first instance raised by the action of potash salts. It is only 

 later, and with very strong potash, that excitability is depressed, 

 and the nerve dies." He reckons neutral salts of potash among 

 the " fatigue products " of the nerve, giving as characteristic of 

 " nerve fatigue " that " it presents two different stages : the 

 primary stage is a heightening, the secondary a depression of 

 excitability," passing finally into the death of the nerve. The 

 order of alterations of excitability in the different stages of 

 potash effect is therefore the same as in treatment with alcohol- 

 ised saline ; and a similar reaction towards weak opening stimuli 

 might be expected. Experimentally, however, these anticipations 

 are only partially realised. 



If the nerve of a nerve-muscle preparation, separated from 

 the central organ, is thoroughly bathed in a very dilute solution 

 (1 %) of KN0 3 , a highly characteristic alteration in the reaction 

 from the muscle to excitation of the nerve by weak battery 

 currents will shortly (510 minutes) be observed. An opening 

 twitch (of the character of break twitch I) now appears not 

 merely, as before, with the descending direction of current only, 

 so soon as the anode is applied to the cross -section but is 

 discharged, independently of position and distance of electrodes, 

 with both descending and ascending direction of exciting 

 current, without any apparent increase of response to closure 

 stimuli. In most cases indeed the height of the closure twitches 

 is less than at the beginning of the potash treatment. The same 

 results appear with undivided nerves, still in connection with the 

 spinal cord, and treated with 1 / Q KNO S solution (Fig. 194). 



