44 CHANGES OF EXCITABILITY IN NERVES 



At the positive pole the muscular contractions in many cases rise 

 from the minimum to which they had at first sunk, but this rise is 

 exceedingly trifling and very slowly takes place. 



These phenomena admit of but one explanation, namely, that the 

 strength of the polarising- current, when it has traversed the nerve 

 for a certain time, is lowered by internal polarisation of the nerve *. 

 When the change of excitability produced by the polarising current 

 is so great that the excitation of the nerve becomes in conse- 

 quence stronger than is required to evoke maximal contraction, or 

 weaker than the minimal strength required to produce a response, 

 the diminution in strength of the polarising current produced by 

 polarisation is of course unobservable. Accordingly the phenomena 

 in question are not always exhibited, only appearing when the 

 excitation of the nerve resulting from the change of excitability has 

 either not risen above the strength required to generate maximal 

 contractions, or not fallen below the strength necessary to call 

 forth minimal contraction. 



These phenomena do not present themselves with very minute 

 strengths of the polarising current, presumably because in that case 

 the internal polarisation cannot rise to an observable degree. 



Finally, the following statements hold good with respect to the 

 repeated action of a polarising current passing constantly in the 

 same direction along the nerve. 



The muscular contractions originating at the negative pole remain for 

 a long time at nearly the same height, even when, for other reasons, the 

 contractions elicited by the test-stimulus would have fallen off. 



The muscular contractions originating at the positive pole become 

 gradually smaller, even when those produced by the test-stimulus alone 

 remain throughout at nearly the same height. 



In the foregoing exposition I have spoken of the increased and 

 diminished excitability of the nerve in accordance with the faulty 

 mode of expression in general use, instead of keeping closely to 

 the actual results, and speaking of the occurrence under particular 

 circumstances of an increase or decrease in the size of the muscular 

 contractions. 



If the view propounded by Hermann is right, the diminution 

 or increase of the muscular contractions does not depend on a 



1 See du Bois-Reymond, Gesammelte Abhandlungen, II. p. 171. 



