22 CHANGES OF EXCITABILITY IN NERVES 



strength with that of the stimulus remains all the time unaltered, 

 the muscular contractions evoked under the influence of the polar- 

 ising current remain of uniform amount through the whole of 

 the long series. The effect due to exhaustion of the nerve in 

 consequence of stimuli following each other in close succession 

 is by no means so pronounced when the current remains closed 

 as when the nerve is exposed to mechanical stimulation only. 



The increase of excitability occurs also with perfect distinctness 

 even when the current produces make- and break- contractions. 

 I have not made experiments with stronger currents, as no ob- 

 server has questioned that with stimulation in the presence of 

 a powerful ascending current the muscular contractions fall off 

 and disappear. It is only the theoretical explanation of the 

 phenomenon about which there is disagreement. 



EXAMPLES. 

 Experiment 1. 



+ P = 24 ; P = 40 ; E = 44 ; 3 Meidinger, Eh. = 45. 



The mechanical stimulus by itself generated no contractions. The 

 constant current produced a strong make- but no break-contraction. 

 Under its influence the mechanical stimulus called forth strong 

 contractions of uniform height, which disappeared after the current 

 was opened, but returned in even greater strength when it was again 

 closed. 



Experiment 2. 



+ P = 14 ; P = 30 ; E = 35 ; i Meidinger, Eh. == 160 ; y = 30. 



a. The mechanical stimulus evoked only small contractions. The 

 constant current produced a make- but no break-contraction. Under 

 its influence the muscular contractions elicited by the mechanical 

 stimulus at once gained double strength, and during the whole time 

 the current was passing kept at nearly the same height. When the 

 current was opened, the contractions were very slightly stronger at 

 first, but soon declined to their former strength. 



6. If the nerve was now stimulated somewhat nearer to the pole 

 (.#=34), other conditions being the same, the mechanical stimulus 

 evoked no muscular contractions, but, under the influence of the current, 

 these showed themselves much stronger than in a, though not of the 

 same height all the time. When the current was interrupted they 

 at once disappeared. 



