59 6 



A MANUAL OF PHYSIOLOGY 



excitability of which is not altered. This indifferent point, as 

 it is called, shifts its position when the intensity of the 

 current is varied. 



These statements have been made on the strength of 



experiments in which 

 the height of the mus- 

 cular contraction was 

 taken as the index 

 solely of the excita- 

 bility of the nerve at 

 any given point. But 

 it is now known, partly 

 from observations on 

 muscular contraction 

 in which changes of 

 excitability of the 

 nerve were eliminated 

 by proper choice of the 

 point of stimulation, 

 and partly from obser- 

 vations on the action 

 stream (p. 642), that 

 very striking altera- 

 tions of conductivity 

 are also produced by 

 the constant current, 

 which even outlast its 

 flow. For all currents 

 except the weakest the 

 conductivity at the 

 kathode and in its im- 

 mediate neighbour- 



FIG. 183. DIAGRAM OF CHANGES OF EX- 

 CITABILITY AND CONDUCTIVITY PRODUCED 

 IN A NERVE BY A VOLTAIC CURRENT. 



E, changes of excitability during the flow of the 

 current, according to Pfliiger. The ordinates drawn 

 from the abscissa axis to cut the curve represent the 

 amount of the change. C(i), changes of conduc- 

 tivity during the flow of a moderately strong current. 

 Conductivity greatly reduced around kathode ; little 

 affected at anode. C(2), changes of conductivity 

 during flow of a very strong current. Conductivity 

 reduced both in anodic and kathodic regions, but 

 less in the former. C, changes of conductivity just 

 after opening a moderately strong current. Con- 

 ductivity greatly reduced in region which was 

 formerly anodic ; little affected in region formerly 

 kathodic. 



hood is diminished, 



and with currents still only moderately strong ( 100 1 000 

 ampere, e.g.) the block deepens into utter impassability. 

 The conductivity at the anode is, during all this stage, but 

 little affected, and is at any rate much higher than at the 

 kathode, so that at the time of full kathodic block the nerve- 

 impulse still freely passes through the region around the 

 positive pole. With still stronger currents the conduc- 



