34 CHANGES OF EXCITABILITY IN NERVES 



While the current is closed, the change of excitability is greatest 

 at the positive pole, and falls off in the direction of the spinal 

 cord. The stronger the polarising current the greater is the 

 diminution of excitability, and the greater also is the tract of 

 nerve over which it extends. If the experiment is repeated in 

 such a way that the polarising current is closed and opened in 

 regular succession while the strengths of the stimulus and of the 

 current remain the whole time unmodified, the muscular con- 

 tractions in each group become smaller and smaller, while the 

 contractions elicited by the mechanical stimulus alone retain all 

 the time their original height, and do not decline nearly so 

 quickly as they usually do in consequence of nerve exhaustion. 



EXAMPLES. 

 Experiment 17. 



+ p = 33 5 - p = 7 ; ^ = 41 ; 3 Meidinger, Eh. = 160 ; y = 42. 



The mechanical stimulus occasions strong contractions ; the constant 

 current make- but no break-contractions. Under its influence the 

 contractions drop to half their initial amount. The experiment is 

 repeated time after time, and now the contractions produced under 

 the influence of the constant current grow always slighter, while 

 those occasioned by the mechanical stimulus alone retain the whole 

 time near upon their original height. Finally, they cease completely 

 when the current is passed. In several series it is observed that 

 the first stimulation after closure of the current produces a greater 

 effect than any of its successors, but that, on the other hand, later on, 

 with the continued flow of the current along the nerve, the contrac- 

 tions increase quite slowly. 



Experiment 18. -\-P = 28 ; P = 14 ; 3 Meidinger ; y = 20. 



a. E = 35 ; Rh. = 40. The mechanical stimulus produces tolerably 

 strong contractions ; the polarising current make- but no break- 

 contractions. Under its' influence the contractions elicited by the 

 mechanical stimulus diminish to half their original strength. As 

 the experiment proceeds the contractions elicited under the influence 

 of the constant current fall off more and more, and at last disappear, 

 while those occasioned by the mechanical stimulus alone retain all 

 the time their initial magnitude. 



6. E = 38 ; Rh. = 100. Under the influence of a stronger current 

 than that in a, which yet produces only make-contractions, the 

 contractions caused by the mechanical stimulus fall to half their 



