24 CHANGES OF EXCITABILITY IN NERVES 



slight increase of excitability, which, however, is not very great and 

 in many cases is quite absent. It persists for a longer or shorter 

 time and then gradually disappears. 



During the passage of the current the diminution of excitability 

 is greatest at the positive pole, and falls off as we move from this 

 point lower down. The stronger the polarising current, the greater 

 also is the diminution of excitability, and the greater the nerve- 

 tract over which it extends. 



If the experiment is repeated in such a manner that the polaris- 

 ing current is closed and opened in regular succession, while the 

 strength of the current and that of the stimulus remain all the 

 time constant, the contractions evoked under fhe influence of the 

 polarising current in most cases get weaker and weaker, while those 

 produced by the mechanical stimulus alone keep throughout at 

 their original height, and do not decline in consequence of ex- 

 haustion of the nerve or muscle nearly so quickly as under ordinary 

 circumstances. 



EXAMPLES. 

 Experiment 4. 



+ P = 26 ; P = 37 ; E = 9 ; 3 Meidinger, Eh. 4500 ; y = 30. 



a. The mechanical stimulus produces very strong muscular con- 

 tractions : the polarising current both make- and break-contractions. 

 Directly after the current is closed the contractions fall to less than 

 half their previous magnitude, and rise somewhat with the con- 

 tinuance of the current, but to a very trifling extent and quite slowly. 

 When the current is opened they return to their former strength, 

 which they retain until it is again closed, when once more a 

 diminution occurs. In the course of the experiment the contractions 

 produced under the influence of the polarising current become feebler 

 and feebler gradually, though not quite regularly, while those evoked 

 by the mechanical stimulus alone retain their original magnitude, 

 and this although the number of stimulations in this experiment 

 amounts to 120. 



b. Under conditions otherwise quite similar, but with a weaker 

 strength of the polarising current (3 Meidinger, fch. 1500), which 



however still elicits make- and break-contractions, the nerve is 

 stimulated nearer to the positive pole (E = 15). The contractions 

 produced by the mechanical stimulus, which are of the same strength 

 as in a, disappear completely under the influence of the current, and 

 again reach their original height as soon as the current is opened. 



