30 CHANGES OP EXCITABILITY IN NERVES 



the condition of diminished excitability extended more and more 

 over the intrapolar tract of the nerve. Whether the decrease 

 of the contractions which took place with a given strength of 

 current on stimulation of a given spot (A) depended on a hindrance 

 occurring at the positive pole to the conveyance of excitation to 

 the muscle, or on an actual decrease of excitability at this spot, 

 I thought might be decided by stimulating a spot () higher up, 

 which, supposing the last hypothesis confirmed, would necessarily 

 manifest a heightened or unaltered excitability. The justness of 

 this assumption was established in the most convincing manner. 



With regard then to the changes of excitability in the intrapolar 

 tract, in so far as these depend on the strength of the polarising 

 current, Pfliiger's law holds absolutely. The more the strength 

 of the polarising current is raised, the greater is the portion of 

 the intrapolar tract over which the condition of diminished ex- 

 citability extends, and the shorter is that portion of the tract which 

 exhibits an increase of excitability ; or, in other words, with in- 

 creasing strength of the polarising current the indifference-point 

 moves from the positive toward the negative pole. With quite 

 weak currents the condition of diminished excitability shows itself 

 only in the immediate neighbourhood of the positive pole, while 

 far the larger portion of the intrapolar tract manifests an increased 

 excitability. With stronger currents, on the other hand, the ex- 

 citability is lowered over the greater part of the intrapolar tract, 

 and only the smaller part exhibits enhanced excitability. 



In consequence of the inhibition which sets in at the positive pole 

 with strong polarising currents, the investigation into the increase 

 of excitability at the negative pole can, of course, not be pursued 

 beyond certain strengths of the polarising current. 



EXAMPLES. 

 Experiment 12. 4- P = n ; P = 32 ; 2 Meidinger ; y = 10. 



a. Rh. = 220 ; E = 13. The mechanical stimulus produces weak 

 currents. The constant current excites a make- but no break-con- 

 traction. Under its influence the magnitude of the contractions 

 elicited by the mechanical stimulus is unaltered. 



b. Rh. = 220 ; E = 1 8. Here the mechanical stimulus excites only 

 small contractions, but these increased considerably under the influ- 

 ence of a current of equal strength with that in a. 



