ix ELECTRICAL EXCITATION OF NERVE 211 



that there is at every intensity of current a minimum direction 

 below which it cannot fall without abolishing the contraction ; 

 while, if the duration of the current increases beyond this limit, 

 the twitch rises steadily from zero, and gradually reaches 

 the maximum possible at that strength of current. The 

 time-values involved are very low in excitation of the nerve. 

 A duration of 0*002 sec. is the maximum. Tick finds that the 

 increase of twitch does not proceed constantly with increasing 

 duration of a current of uniform strength descending in the nerve, 

 but that the rise is intermittent : the twitches increase again after 

 an initial maximum, if the passage of the current is prolonged. 



If, e.g., a descending current of given strength discharges a 

 maximal twitch with a closure of 0*003 0*004 sec., this will not 

 increase with further increase of current intensity, provided the 

 current continues to pass for a very short time only. But if the 

 same current is persistently closed, a twitch results which is con- 

 siderably in excess of the ultimate maximum from the momentary 

 action of current, i.e. is in a certain sense a supramaximal con- 

 traction. This can only mean that the kathodic make excitation 

 exhibits the greatest possible maximum in consequence of the 

 prolonged passage of the current, as is expressed in both direct 

 and indirect excitation of the muscle by the fact that no in- 

 duction shock, however energetic, can effect the same degree of 

 contraction as the closure of even a moderate constant current. 

 Single induction shocks never elicit more than the relative maxi- 

 mum, that is not exceeded in brief constant currents also (Fick, 

 I.e. p. 25). Between these extremes of brief impact of current, 

 and persistent closure, it is quite possible that the irregular increase 

 in height of twitch with increasing closure depends partly upon 

 an anodic break excitation, since the effects of the closure and 

 immediately succeeding opening excitation are summated in the 

 muscle. In favour of this interpretation we have in the first 

 place the fact that (as Fick discovered later) the same phenomena 

 appear with descending induction currents, of increasing intensity, 

 since after reaching a first maximum the twitches rise again to a 

 second. 



Along with these data we have the still more weighty 

 observations of Fick (54). He found that with ascending 

 impacts of constant currents the twitches declined after the 

 first maximum to zero (the so-called " breach," Liicke), so soon as 



