iv GENEEAL PHYSIOLOGY OF NEEVOUS SYSTEM 257 



ductivity of nerve should be remembered. They confirm the 

 preceding results of Helmholtz, Grlitzner, and other authors, and 

 show that conduction must be an effect of excitation because it 

 varies with variations of temperature in correspondence with the 

 rise or fall of excitability. 



An exhaustive theory of nervous activity would have to define 

 in what the physico-chemical alterations of the fibre that we 

 term "excitation" consist, and how they are propagated to 

 adjacent segments, which is the process of " conduction/' We 

 are far from any such theory. We can only affirm that the active 

 state of the point of nerve stimulated is intimately associated 

 with its electrical negativity, and the conduction of excitation 

 with the wave-like propagation of this negativity, i.e. the current 

 of action. Since the electrical stimulus is among the most 

 powerful excitants of neural activity, and is an important factor 

 in excitability, it seems probable that the action current of 

 nerve is no mere accessory pheno- 

 menon, but that it is the im- 

 mediate cause of the conduction 

 of the excitatory impulse. 



According to Hermann this 

 conduction may be explained on 

 the assumption that anelectro- 

 tonus is produced at the excited 

 spot, in consequence of external 

 stimulation, and katelectrotonus 

 in the adjacent segments. Hermann's hypothesis is illustrated 

 in the diagram (Fig. 162), in which KK represents the axial 

 substance, HHHH the sheath of the nerve-fibre, pqrs the segment 

 of nerve stimulated. The lines of demarcation (ps, qr} between the 

 excited segment and the adjacent non-excited segments present two 

 electromotive surfaces, owing to the negative electrical potential of 

 the former, which generate currents in both directions, as indicated 

 by the diagram. According to Hermann these currents must be 

 of enormous strength, seeing the microscopic interval, and conse- 

 quent minimal resistance, between the two electromotive, surfaces. 

 They are therefore capable of producing in the anodal zone (aa, aa) 

 an anelectrotonus which throws the substance of the nerve into 

 rest, and in the kathodal zone (cc, cc) a katelectrotonus of sufficient 

 intensity to excite it. In this way the excitatory impulse is trans- 

 mitted along the nerve. 



Yet even on these assumptions we have, as Hermann confesses, 

 no perfect theory of conduction in the nerve. His hypothesis 

 seems probable from the fact that his core-model, consisting of a 

 platinum wire surrounded by a solution of copper sulphate, is 

 able (according to Hermann and Samways and Boruttau) to 

 exhibit electrotonic currents, which advance in wave form like the 



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