304 ELECTRO-PHYSIOLOGY CHAP. 



one who has accepted the point of view that living animal or 

 vegetable cells are alone excitable, could subscribe to the con- 

 clusions deduced from them. This is another example of the 

 danger of generalising from observations on any one object, and of 

 criticising vital manifestations from a one-sided physical point of 

 view, without regard to differences of structure. 



Without denying that further investigation may perhaps 

 bring to light other analogies between the conduction of excita- 

 tion on the one hand, and that of the undulatory electrotonus in 

 the core-model on the other, it must be borne in mind that 

 excitation and conductivity of excitation may be observed in 

 objects, and under conditions, where Boruttau's physical assump- 

 tions are certainly not present. 



Even for "fixed polarisation" however, it is questionable 

 whether the persistent electrotonic currents that appear in the 

 immediate vicinity of the area traversed in medullated nerve can 

 be explained entirely on Hermann's principle of interpretation, 

 although they are no doubt partly physical in origin. That 

 structural constitution of the medullated fibres which here comes 

 more especially (and perhaps solely) under consideration, i.e. the 

 investment of the axis-cylinder with the medullary sheath, exhibits 

 prima facie few characteristics integral to the original core-model 

 of metal and fluid. In the first place, there is the enormous 

 difference of conductivity between moist sheath and metal core. 

 Any such marked disparity in conductivity between axis-cylinder 

 and medullary sheath is obviously excluded ab initio ; it may, 

 indeed, be asked whether any perceptible difference exists. A 

 second question is, whether any polarisation at all is present at 

 the interface of these two elementary constituents of medullated 

 nerve, and if so, whether such a polarisation at the surface of two 

 electrolytes can, as regards its effect upon current diffusion, be 

 compared with that taking place at the junction between metal 

 and fluid ? 



With regard to the first point, Hermann long ago gave ex- 

 perimental proof that the very considerable difference between 

 longitudinal and transverse resistance in the nerve is essentially 

 to be referred to an E.M.F., heterodromous to the current and clue 

 to polarisation. With transverse passage of current this seenis 

 to occur mainly at the boundary between neurilemma (sheath of 

 Schwann) and medullary sheath, so that with Hermann we must 



