THE SPINAL CORD, BY J. GERLACH. 



carmine and ammonia, the difference between the axis-cylinder 

 and medullary sheath is most strikingly displayed. Each 

 nerve fibre then presents the appearance of the sun as given 

 in almanacs, the centre being an intensely red point the axis- 

 cylinder which is surrounded by a broad, strongly refractile, 

 colourless ring the medullary sheath (fig. 219, B a 6). Not un- 

 frequently the medullary sheath exhibits, on transverse section, 

 a more or less well-marked concentric striation ; but I cannot 

 determine whether this appearance is a peculiar phenomenon 

 of coagulation, or is the result of a laminated structure in the 

 axis-cylinder, which, as the history of its development teaches, 

 is formed at an earlier period than the medullary sheath. 



The sheath of Schwann, present in the medullated nerve 

 fibres of the periphery, is absent in the fibres of the central 

 organs. It is impossible to distinguish it in isolated fibres 

 either of the grey or of the white substance, nevertheless it is 

 within the bounds of possibility that it is not entirely deficient, 

 but is only not brought into view in teazed-out preparations, 

 on account of its being intimately fused with the neuroglia. 

 In favour of this view is the circumstance that in sections of the 

 "white substance the boundary between the medullary sheath 

 and neuroglia is indicated by a sharply defined line, which 

 may be regarded as the expression of the sheath of Schwann, 

 An additional argument is derived from the fact that, the 

 micro-chemical reactions of the sheath of Schwann are incon- 

 testably those of elastic tissue, and that the fine elastic fibres I 

 have shown to be present in the neuroglia may be followed as 

 far as to the boundary of the medullary sheath, where they 

 often, as may be clearly seen in very thin slightly compressed 

 longitudinal sections, end abruptly (fig. 219, A). Now, since 

 it is impossible to admit the continuity of the elastic fibres 

 with the medullary sheath, on account of the diversity of their 

 micro- chemical reactions, it suggests itself to the mind as highly 

 probable that the elastic fibres coalesce with the sheath of 

 Schwann, which is likewise elastic, and which is fused with the 

 neuroglia. If this view be well founded, the existence of a 

 plexus of fine elastic fibres in the neuroglia possesses a physio- 

 logical value that can scarcely be misunderstood, constituting, 

 in fact, a protection for the nervous elements, and being ob- 



