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230 



THE INTELLIGENCE SERVICE 



Electrotonic Currents. I'liat negative })()larisation oecurs while 

 the polarisation eurrent is running may be shown by an experiment 

 as indicated in Fig. 58. The centre circuit is supplying the 

 polarising current, x being the anode, and y the cathode. The two 

 lower circuits are merely leads to galvanometers, Gj and ^3. 



t 1 



y 



I 

 c 



G2 



Fig. 58. — Diagram showing electrotonic currents. P. polarising circuit ; 6'', G-, galvanometers. 



When the polarising circuit is closed and the polarising current 

 passes in the nerve from x to y, the galvanometers will both 

 indicate currents passing in the same direction, from a to b and 

 c to d respectively. These extra-polar, or as Du Bois-Reymond 

 called them, electrotonic currents, are due to the same causes as 

 negative polarisation. Consider Fig. 59, where the positive charges 

 are shown gathering on the surface of the axon at a-b. That is, 

 a-b will have a higher + potential than in the nerve at c or to the 

 left of c. The result will be a flow of current from left to right 



riG. 59. — Diagram to show polarisation at the surface between conducting core and 

 electrolyte sheath. 



along the surface of the axon to reduce this difference of potential. 

 In the same way, under the cathode at d-e the accumulation of 

 electrons in excess of what is at /and to the right of/ will cause the 

 passage of some of the excess towards the right. It is clear, then, 

 that all three currents, left to b {anelectrotonic), a to d (polarising), 

 and e to right {cateIectroto7iic) flow in the same direction. 



The model referred to above responds in the same way (Fig. 57), 



