314 



STRUCTURES IN THE MEDULLARY SHEATH. 



shown by McCarthy that after a nerve has been hardened with chromate of 

 ammonium (picric acid is still better adapted for the purpose) the medullary sheath 

 appears pervaded with minute rod-like structures which pass radially between the 

 axis-cylinder and the primitive sheath in such a manner as to give the cross-section 



Fig. 362. SECTION ACROSS PART OP A NERVE-TRUNK, SHOWING 



THE SECTIONS OF THE NERVE- FIBRES (E. A. S.). ~-- 



(From a photograph.) 



The nerve was hardened in picric acid and 'stained with 

 picrocarmine. The radial striation of the medullary sheath is 

 very apparent. In one fibre the axis-cylinder is shrunken and 

 the medullary striations are broken. The fibrils of the axis- 

 cylinder are clear in section and suggest a tubular structure. 



of a nerve-fibre the appearance of a wheel. The 

 rods stain with carmine and heematoxylin, which 

 do not colour the fatty substance of the medullary 

 sheath. In nerves stained with osmic acid (in 

 which these structures were first detected by Lantermann) they are far less easily 

 seen, in consequence of the dark colouration of the fatty substance in which they are 

 embedded. The apparent rods are not distinct from one another, but are united, for 

 it is not possible to isolate them as separate elements. 



If a nerve is first placedin strong alcohol and then subjected to the action of 

 staining fluids, it will be found that the rod-like structures are not visible as such, but 



A B Fig. 363. NERVE-FIBRES SHOWING RETICULAR APPEAR- 



ANCES IN THE MEDULLARY SHEATH (Gedoelst). 



A. A nerve-fibre from the toad, showing different 

 appearances of the network at different parts of its course. 



B. A nerve-fibre from the guinea-pig, showing a much 

 coarser network, n, nucleus of fibre. 



the medullary sheath appears pervaded instead 

 by a finely reticular structure, which like the 

 rods, becomes readily stained by most dyes. The 

 network thus obtained is considered by Kuhne 

 and Ewald, who first drew attention to it, to be 

 chemically of a horny nature, on account of the 

 resistance it offers to reagents, and especially to 

 digestive ferments, and they have accordingly 

 designated it the horny ret iculum or neurokeratin 

 network. They further describe it as continuous 

 with two delicate membranes of a similar nature, 

 one immediately investing the axis-cylinder and 

 the other lining the primitive sheath. The 

 pre-existence of a neurokeratin network is, how- 

 ever, at least questionable. Of the presence both 

 in the nerves and in the white matter of the 



nerve-centres of the material known as neurokeratin there can be no doubt ; it 

 appears probable, however, that its reticular arrangement is a product of the reagent 

 (alcohol) employed to demonstrate it. This at least has been the opinion of most 

 authors who have investigated the subject (Hesse, Pertik, Waldstein and Weber), 

 whereas others, whilst admitting its variability, consider its pre-existence beyond 

 doubt. Gedoelst, who maintains this view of the neurokeratin network, regards it 

 as the reticulum of the original protoplasmic cell from which the medullary sheath 

 of the nerve-segment has been formed. It must be admitted, however, that its 



i 



