6 4 2 



A MANUAL OF PHYSIOLOGY 



an action stream. Hermann has explained its direction on the 

 assumption that the excitation diminishes in intensity as it approaches 

 the kathode or recedes from the anode, and increases in intensity as 

 it passes towards the anode or away from the kathode (law of 

 polarization increment). But the fact that during the flow of a 

 current the conductivity of the nerve is far more depressed around 

 the kathode than near the anode affords a sufficient explanation. 



The nerve-impulse, starting from the stimulating electrodes S 

 (Fig. 209), will pass over E, the anode, in greater intensity than over 

 EI, the kathode j and therefore, upon the whole, during tetanus E 



will be negative to E lf and 

 a current of action will be 

 developed in the same 

 direction as the polarizing 

 current, and reinforcing it. 

 When the kathodic block 

 is complete, and the excita- 

 tion has to pass over the 

 kathode before reaching 

 FIG. 209. DIAGRAM SHOWING DIRECTION the intrapolar region, no 

 OF THE STIMULATION EFFECT IN THE e ffgct is oroduced bv stimu- 

 INTRAPOLAR REGION DURING THE FLOW j * 



OF THE POLARIZING CURRENT. lation. 



The stimulation effects 



in the extrapolar regions are probably due partly to action currents, 

 as is shown by the fact that when the polarizing current is strong 

 enough to markedly depress the conductivity in the neighbourhood 

 of the anode, the variation becomes positive instead of negative 

 when one of the galvanometer electrodes lies near the anode. For 

 here the excitation coming from S passes E 2 in far less intensity than E 3 

 (Fig. 2 TO). E 3 is therefore, on the whole, during tetanus negative to E 2 , 

 and the direction of the action current in the nerve is from E 3 to E 2 . 

 The negative variation in the extrapolar kathodic region could 

 also be explained as an action current due to diminished conduc- 

 tivity in the neighbourhood of the kathode. But the negative anodic 



variation cannot be an 

 action current, unless we 

 suppose that with the 

 weaker polarizing currents 

 the conductivity is in- 

 creased around the anode ; 

 and for this there is not 

 sufficient proof. It is pro- 

 bable, therefore, that there 



FIG. 210. DIAGRAM TO SHOW DIRECTION is another factor mixed up 

 OF THE POSITIVE STIMULATION EFFECT w ith the currents of action, 

 IN THE ANODIC EXTRAPOLAR REGION j ,. nnnnc:i - ncr thpm 



DURING THE FLOW OF A STRONG POLARI- * 



ZING CURRENT. Some have supposed that 



the capacity for polariza- 

 tion between core and sheath is diminished during excitation, and 

 that, accordingly, less of the current spreads beyond the electrodes, 



