PRODUCED BY A CONSTANT CURRENT. 27 



4. The intrapolar changes of excitability at the positive pole 

 of an ascending current. 



The excitability of the nerve is considerably diminished with all 

 strengths of the current, even with the weakest. Frequently this 

 diminution does not show itself on the instant, but the muscular 

 contractions gradually fall off under the influence of the current, 

 so that the first are somewhat greater than those which follow. 

 This diminished excitability is exhibited all the time the current 

 is closed. It pretty often happens that meanwhile the muscular 

 contractions begin gradually to rise, but this rise is always 

 inconsiderable, and in certain cases is imperceptible. With 

 the current open the contractions instantaneously increase. At 

 the same time no inhibition can be demonstrated. On the contrary 

 there occurs a trifling increase of excitability, which is however 

 sometimes quite unnoticeable. It lasts a longer or shorter time, 

 and then gradually disappears. 



The diminution of excitability when the current is closed is 

 greatest at the pole itself. It increases with the strength of the 

 polarising current. 



If, under the same external circumstances, repeated experiments 

 are made, the diminution of excitability generally increases, though 

 very inconsiderably. Sometimes, too, it may be observed that 

 the diminution falls off, but at the same time the contractions 

 evoked by the mechanical stimulus alone have become stronger. 

 In general these maintain their original height or undergo a very 

 slight increase ; now and then it may happen that they fall off, 

 but this is very unusual. 



EXAMPLES. 

 Experiment 8. 



+ P= 8 ; P = 34 ; JE = 9 ; 2 Meidinger, Eh. = 50 ; y = 20. 



a. The mechanical stimulus produces weak contractions. The 

 polarising current evokes neither make- nor break-contractions. 

 Under the influence of the current the contractions instantly vanish 

 and reappear only when the current is opened. They are then a 

 trifle weaker. 



6. The same nerve is stimulated somewhat higher up, E = 10, with 

 the same strength of mechanical stimulus, and the polarising current 

 is made still weaker (2 Meidinger, Eh. = 42). Even then the 

 contractions cease under its influence and resume their previous 

 strength only when the current is again opened. 



