ix ELECTRICAL EXCITATION OF NERVE 131 



Von Kries agreed with v. Fleischl that the twitches discharged 

 by " time -stimuli" (i.e. linear variations of current) are not 

 usually distinguishable from those due to " instantaneous stimuli." 

 In single cases, however, we meet with notable exceptions, the 

 twitches from time-stimuli being much more extended. Yet it 

 must be remembered that the mechanical changes of form in the 

 indirectly excited muscle-twitch give a very incomplete repre- 

 sentation of the true time-distribution of the excitation at the 

 point where the nerve is directly excited. So that when v. 

 Kries concludes from his experiments that a linear variation of 

 current in one direction excites the nerve for a brief period only, 

 this would seem to be as little justified as the postulate of a 

 universal law of excitation, based upon observation of the make 

 and break twitch. 



As a rule, in order that a time-stimulus may evoke as high 

 a twitch as an instantaneous stimulus, the intensity (i s } finally 

 reached at a given pitch (D) must exceed the intensity of the 



momentary stimulation (i m ) for the same effect. This different 



( \ 

 - ) for each value (D) is termed by v. Kries the stimula- 

 m/ 



tion quotient. It increases of course with increasing values of 

 (D), and affords a direct gauge of the diminution of excitatory 

 effect caused by the extension in time of the oscillation. In the 

 frog's nerve-muscle preparation v. Kries found it almost invariably 

 greater than 1. In other cases, however, strong time-stimuli 

 yielded larger twitches than were usually produced by momentary 

 closures. This seems to be the rule in sluggishly reacting 

 excitable substances. A stimulation quotient is naturally not to 

 be obtained in such cases. The integral dependence of the nerve 

 upon the nature or mobility of its constitution is clear from the 

 observation of v. Kries, that cooled nerve reacts better to lower, 

 warmed nerve to higher oscillation-pitch. 



It is noteworthy, as first pointed out by v. Fleischl, and con- 

 firmed by v. Kries, that rheonome twitches do not readily evoke 

 secondary contractions. Secondary action only appears with very 

 strong supramaximal stimuli. V. Kries also observed stronger 

 effects in the capillary electrometer with time-stimuli, along with 

 simultaneous failure to excite the secondary preparation. It is 

 plain that the wave of oscillation is differently distributed in 

 instantaneous and in linear excitation, the latter being characterised 



