STRUCTURE OF NERVE-FIBRES. 467 



substance of Schwann, being that to which, the peculiar 

 white aspect of cerebro-spinal nerves is principally due. 

 The whole contents of the nerve-tubules appear to be ex- 

 tremely soft, for when subjected to pressure they readily 

 pass from one part of the tubular sheath to another, 

 and often cause a bulging at the side of the membrane. 

 They also readily escape, on pressure, from the ex- 

 tremities of the tubule, in the form of a grumous or 

 granular material. 



That there is an essential difference in chemical com- 

 position between the central and circumferential parts of 

 the nerve-fibre, i,e., between the axis-cylinder and the 

 medullary sheath, has of late been clearly shown by Messrs. 

 Lister and Turner. Their observations, founded on Mr. 

 Lockhart Clarke's method of investigating nervous sub- 

 stance by means of chromic acid and carmine, have shown 

 that the axis-cylinder of the ' nerve-fibre is unaffected by 

 chromic acid, but imbibes carmine with great facility, 

 while the medullary sheath is rendered opaque and brown 

 and laminated by chromic acid, but is entirely untinged 

 by the carmine. From this difference in their chemical 

 behaviour, the central and circumferential portions of 

 the nerve -fibres are readily distingushed on microscopic 

 examination, the former being indicated by a bright red 

 carmine-coloured-point, the latter by a pale ring surround- 

 ing it. The laminated character of the medullary sheath 

 after treatment with chromic acid is believed by Mr. 

 Lockhart Clarke to be due to corrugations effected by 

 the acid, and not to its having a fibrous structure, as 

 maintained by Stilling. 



The size of the nerve-fibres varies, and the same fibres 

 ! do not preserve the same diameter through their w r hole 

 length, being largest in their course within the trunks 

 and branches of the nerves, in which the majority measure 

 from -jo-L-. to -g-tfVjj- of an inch in diameter. As they ap- 

 proach the brain or spinal cord, and generally also in 



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