IX 



ELECTRICAL EXCITATION OF NERVE 



135 



ing direction. We cannot linger over the earlier discussions 

 of the matter, which for the rest have been admirably and 

 exhaustively summed up by du Bois-Eeymond. It is sufficient 

 to state that (after Pfaff had pointed out certain regular differ- 

 ences in the action of ascending and descending currents) Eitter 

 was the first to formulate a " law of contraction " as subse- 

 quently confirmed by Nobili. It will be seen from the following 

 table that, apart from direction of current, the temporary 

 excitability of the preparation plays a great part in the effects of 

 stimulation. 



ElTTER-NOBILl's LAW OF CONTRACTION 



II. (Ritter) 



III. (Ritter) 

 I. (Nobili) 



IV. (Ritter) 

 II. (Nobili) 



M. contraction 



B. weak contraction 



M. weak contraction 

 B. contraction 



M. contraction 

 B. contraction 



M. weak contraction (0) 

 B. contraction 



M. contraction 

 B. contraction 



M. contraction 



B. weak contraction 



Eitter distinguishes six, Nobili four, stages of excitability. The 

 completely contrary effect of homodromous currents at the first 

 (highest), and later (Eitter's fifth), stages of excitability is very 

 striking. Nobili denies it and asserts that there is only one 

 marked twitch for each direction of current, the opening twitch of 

 ascending, the closure twitch of descending currents. More 

 recently there have been various attempts to determine the facts 

 which underlie the law of contraction, as well as a theoretical 

 explanation of them. An important step was taken almost 



