Barh or Douhle-bordered Nerve Fibre. 213 



place at those varicose places, giving rise to the development of 

 gases which finally cause the dilatation ; or it may be attributed to 

 an endosmosis of liquids. These hypotheses would naturally require 

 the supposition of the existence of a iuhular memhrane. The pro- 

 bability of these varicosities being produced by decomposition, or 

 some similar process, may be inferred from the fact that they are 

 especially met with in specimens of the fresh substance of the brain 

 or spinal marrow examined in water, while in others of the same 

 substance, hardened in a solution of chromic acid, they are only 

 rarely observed. 



The fine fibrils of the fibrillous layer I have thus far not suc- 

 ceeded in demonstrating on the finer nerve fibres of the central 

 organs, though a distinct double contour may be always recognized 

 on them. The cause thereof may well be found in the very incon- 

 siderable thickness or even absence of the medullary layer, in conse- 

 quence of which the fibrillous layer would directly surround the 

 axis cylinder. That this is really the case may be inferred from the 

 fact that the inner line of the double contour also forms the border 

 of the axis cylinder. The absence, or even the presence of the 

 medullary layer in an inconsiderable quantity, would render the 

 endosmotic process more difficult, and enable the fibrils to remain 

 in their original parallel position ; only on those varicose places the 

 entering water would accumulate, and cause those oval dilatations 

 of the fibrillous layer and tubular membrane. In many cases the 

 axis cylinder is distinctly recognized in the axis of the dilatation 

 (Fig. 16, a). 



Although, as far as we have seen, no considerable difference 

 seems to exist in the anatomical composition of the double-con- 

 toured nerve fibres of the central organs and that of the peripheral 

 fibres, I believe I have observed that the former are able to resist 

 the action of water or a weak chromic acid solution for a longer 

 time than the latter. In the examination of nervous tissues of the 

 brain and spinal marrow in water, or of such which had remained 

 for a short time in a weak solution of chromic acid, a number of 

 nerve fibres, the double contour of which is unchanged, is always 

 met with ; a circumstance not often occurring with the fibres of the 

 peripheral nerves under the same conditions. The medullary layer 

 of the former also seems to preserve its greyish lustre for a longer 

 time than that of the latter, which may be attributed to a greater 

 amount of fat-hke constituents in their chemical composition, 

 increasing their power of resistance to the solving action of the 

 water. 



The fibrils of the fibrillous layer of the central double- bordered 

 nerve fibres difi'er in nothing from those of the peripheral fibres. 

 In the anterior horns of the grey substance of the spinal marrow, 

 they are seen to escape from the open ends of the peripheral nerve 



