NERVOUS CONDUCTION AND EXCITATION 145 



identical whether the impulse traverses a given distance of 

 narcotised nerve (A), or double the distance (A+B) when 

 there is an intervening region of unaffected nerve between 

 A and B. This indicates that the impulse emerges from the 

 region of decrement with fully recovered intensity. 



The second point can be demonstrated by observing the 

 interval between the galvanometer deflections obtained by 

 the use of non-polarisable electrodes, separated by a measured 

 distance of nerve. The electrical variation recalls what has 

 already been observed in muscle ; the excited part at a given 

 instant is electronegative to a non- excited region. The 

 electrical variation is an invariable accompaniment of the 

 transmission of the nervous impulse ; and its existence may 

 be taken as indicative that reversible movements of ions occur 

 along the track of the nervous impulse. This suspicion is 

 strongly reinforced by the fact that the propagation of a nervous 

 impulse is completely blocked by a region through which an 

 ascending (but not a descending) constant current is passed 

 (Bernstein's experiment). 



Taking into consideration the fact that the energy of the 

 impulse is distributed along the whole length of the fibre, we 

 might picture the propagation of the nervous impulse in one 

 of two ways : either by comparing it to the ignition of a train 

 of gunpowder, regarding it as a process which is in the thermo- 

 dynamic sense irreversible, or interpreting it as a succession 

 of local reversible changes of colloidal equilibrium along the 

 course of the neurone. In the first case, it would be predicted 

 that the nerve would be fatiguable, since it could not have 

 inexhaustible supplies of the material sources of its energy 

 of transmission. Conduction would be accompanied by in- 

 creased metabolism . In the second case , the bioelectric currents 

 propagated along the course of the nervous impulse would 

 be the only physically measurable accompaniments of its 

 passage. The facts that nerves are not fatiguable ; and that 

 nervous conduction is not, according to the extremely delicate 

 determinations of Hill, associated with any measurable increase 

 of heat-production, favour the second alternative. At the 

 same time certain observers, Tashiro among others, have 



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