ix ELECTRICAL EXCITATION OF NERVE 159 



tion of current corresponds with the above conditions, the 

 exclusive effect of excitation will be the appearance of the 

 closure twitch with descending direction of current. In regard 

 to the action of weak ascending currents, there is a perceptible 

 difference from the cases of partial death of the nerve as previously 

 described. For while in these last even weak ascending 

 currents (and an arrangement of the electrodes at which the same 

 currents when descending produce only a closure twitch) elicit 

 unmistakable opening twitches hardly less in size than the latter, 

 these fail altogether with local application of ammonia, or appear 

 merely as a trace when the drug begins to act, or with marked 

 strengthening of the current. 



When Pfliiger formulated his law of contraction in 1859, it 

 seemed hardly doubtful that the effectuation of the opening 

 stimulus in the motor nerve depended in first degree upon 

 the momentary intensity of current. This view was confirmed 

 by the majority of later workers in this department. Yet 

 Eitter, and subsequently ISTobili, had already pointed out as a 

 second factor the differing " states of excitability " in the nerve. 

 Eosenthal and v. Bezold (32) accordingly drew up a law of con- 

 traction for moribund nerve, which conforms perfectly with 

 Pfliiger's law, and by which, with unaltered (low) strength of 

 current at the same point of nerve in three successive stages of 

 dying, the same alternating effects of excitation are observed as 

 appear in fresh nerve (according to Pfliiger's law) with weak, 

 medium, and strong currents. These changes may be simply 

 explained on Pfiiiger's theory from the course of the alterations 

 of excitability, which (according to prevailing views) characterise 

 the single points of nerve that is in process of dying, and which 

 appear at the more central points before the peripheral. 



Pfliiger in the first instance deduced his law from observations 

 taken almost exclusively from the isolated nerve-muscle prepara- 

 tion of the frog, where the conditions of excitability are not 

 essentially different from the normal. For this preparation, 

 therefore, his law is practically uncontested. 



Yet there have been objections even here to its validity in 

 the case in which the excited nerve is still connected with the 

 central organs of the living animal. 



Bernard, Schiff, and Valentin (33) all agree that the electrical 

 excitation of undivided nerve " produces contraction of the muscle 



