ELECTRO-PHYSIOL OGY 643 



and an apparent negative variation is caused in the extrapolar 

 regions by stimulation. Or it may be that the excitation renders 

 the envelope which interposes the chief resistance to the passage of 

 ions into and out of the core more permeable. But there is no 

 direct evidence for either view. 



After the opening of the polarizing current, electromotive changes 

 can, as we have seen, be recognised for a short time in the intrapolar 

 area. This is also true of both extrapolar regions. The main after- 

 current in the anodic region is in the opposite direction to the 

 polarizing stream ; but this is, under certain circumstances, preceded 

 by a very short kick of the galvanometer magnet in the same direc- 

 tion. The kathodic after-current is in the same direction as the 

 polarizing stream, and is, except with strong currents and a compara- 

 tively long time of closure, much weaker than the main anodic. 

 The latter is to be looked upon as having the same origin as the 

 positive polarization current of the intrapolar region, a state of open- 

 ing excitation around the anode ; in other words, it is an action 

 current. The kathodic and the preliminary anodic after-currents are 

 probably due to negative polarization. 



Stimulation of the nerve after opening the polarizing current causes 

 well-marked effects ; 

 in the intrapolar 

 region the stimula- 

 tion effect is in the 

 opposite direction 

 to the polarizing 

 current ; in the ex- 

 trapolar anodic 

 area, in the same 

 direction as the 

 polarizing stream. 

 In the extrapolar FIG. 211. DIAGRAM SHOWING THE DIRECTION OF 

 kathodic region it THE STIMULATION EFFECTS AFTER OPENING THE 



"in thP nnnnQil-P POLARIZING CURRENTS IN THE ANODIC AND 



is in the opposite KATHODIC EXTRAPOLAR REGIONS (A AND K), AND 



direction, and, ex- IN THE I NT RAPOLAR REGION Ej, E 2 . 



cept with strong 



polarizing currents, and a more than momentary time of closure, less 



in amount than the stimulation effect in the anodic region. 



All these cases are readily explained by the fact that immediately 

 after opening the polarizing current the conductivity of the nerve is 

 more depressed in the anodic than in the kathodic region, although 

 with strong currents it is depressed in both. An excitation reaching 

 the extrapolar anodic area from S will pass over E 3 in greater intensity 

 than over E 4 (Fig. 211). E 4 will therefore be positive to E 3 , and the 

 action current will go through the nerve in the direction of the arrow. 

 An excitation reaching the kathodic extrapolar area from S' will 

 arrive at E 6 in greater intensity then at E 5 . The resultant action 

 stream will therefore have the direction in the nerve from Eg to E 5 . 

 And the effects in the intrapolar region can be similarly e*j|ained. 



