48 ELECTRO-PHYSIOLOGY CHAP. 



Petromyzon to draw any direct conclusion at first hand from the 



diameter of the axis-cylinder as to bulk of conducting substance. 



The distance between two fibrils is always greater than the 

 diameter of the fibrils ; which are there- 

 fore separated by a comparatively large 

 mass of axoplasm. The fibrils of Petro- 

 myzon are excessively unstable, and are 

 visible only during life ; as soon as they 

 begin to die they break up, even when 



FIG. i56.-T.s. of axis-cylinders examined in the serum of the same 



from trigeminal nerve of . . 



Petromyzon fluviatiiis. animal, into fine granules of high refractile 



(Schiefferdecker.) 



ponding with the fibrils of which they are the disintegration product. 

 With advancing dissolution the axial bundle flows away in a viscid 

 mass, along with the firmer substance of the axoplasma." The 

 fibrils appear less capable of resistance than the axoplasm. 

 " Shortly after death nothing remains of the fibrils ; in their place 

 there is a knotty string, which has often been figured." (Schieffer- 

 decker's words, thus quoted, are confirmed by the observations of 

 Biedermann.) 



It is far more difficult to discover the structure of the axis- 

 cylinder in the nerve-fibres of the higher vertebrates, which are 

 surrounded with a thick medullary sheath ; and this no doubt 

 accounts for the current divergences of opinion. We should 

 a priori assume that the structural relations of the axis-cylinder 

 would coincide in all essential points throughout the animal 

 kingdom. When the existence of a fibrillated structure has been 

 determined in one case, it may almost be postulated that fibrils 

 are everywhere the proper constituents of the cylinder-axis. And 

 this presumption of Eemak and Max Schultze has in fact been 

 confirmed by the remarks of Engelmann, Kupffer, Maley, Boveri, 

 Kb'lliker, Jacobi, Joseph, and others neither v. Fleischl's theory, 

 that the axis-cylinder is a column of fluid, nor that of Kuhnt, 

 that the axial space is filled with "a soft, somewhat elastic, 

 homogeneous mass, finely or coarsely granulated," and that the 

 fibrillar longitudinal striae are folds of the supposed " axis- 

 cylinder sheath," having any foundation. 



As in the non-medullated fibres of Petromyzon and certain 

 invertebrates (crayfish), so in medullated fibres, the axis-cylinder 

 is composed of a soft ground-substance rich in water, and of 



