x ELECTROMOTIVE ACTION IN NERVE 307 



Matteucci's core-model, this is a metal, or, as in Hering's experi- 

 ments, as well as in an analogous method of Griinhagen (42), and 

 in certain combinations recently employed by Boruttau, & fluid 

 conductor. According to Griinhagen (Fig. 218), we should 

 imagine that in every combination of conductors on the plan of 

 the accompanying schema (Fig. 218), "the branch currents 

 running in the sheath take only one direction towards the better- 

 conducting core, while the recurrent branches, on the contrary, 

 are included within the better-conducting axis." " In consequence 

 of this absorption of all recurrent diffusion-currents by the core- 

 conductor, the sheath is free of them, and wherever the leading- 

 off contacts of the galvanometer circuits are applied, whether at 

 the side near, or opposite to, the leading-in electrodes, there will 

 only be branch currents in the one direction, identical with that 

 of the diverging lines of current as described above " (Griinhagen). 

 If this view is accepted, the axis-cylinder should be a much better 

 conductor than the medullary sheath, which, indeed, from the 

 histo-chemical point of view, seems not improbable. The com- 

 plete failure of any definite physical electrotonus in non-medullated 

 nerve, and in muscle, would accordingly be determined by the 

 absence of badly-conducting sheaths to the single elements ; to 

 which it must further be added that (as has been recently con- 

 firmed Biedermann) electrotonic effects are also absent in cases 

 where, as in many crustacean nerves, the solitary axis-cylinder is 

 surrounded by well -developed stratified sheaths of connective 

 tissue. The physico-chemical constitution of the medullary 

 sheath appears, therefore, to be indispensable to current diffusion. 

 In this connection there are some interesting experiments on the 

 nerves of Palcemo7i, which, according to Eetzius, contain meclul- 

 lated fibres, and thus differ completely from those of most other 

 Crustacea. 



As the outcome of the preceding discussion, it must be 

 admitted that there is in medullated nerve a diffusion, whether 

 produced by " secondary polarisation " or by direct current escape, 

 of the external current over the tract directly traversed, i.e. an 

 electrotonus of physical origin, which is however complicated, as 

 a rule, by homodromous physiological alterations of the nerve. 

 From the physiological standpoint the chief interest lies in the 

 alterations of the nerve, i.e. of its excitability, produced by the 

 diffusion of the exciting current. In muscle, where the entrance 



