STRUCTURE OF NERVE-FIBRES. 469 



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 extremities 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 distinguished on microscopic 

 examination, the former being indicated by a bright red 

 carmine-coloured point, the latter by a pale ring sur-" 

 rounding 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 main- 

 tained by Stilling. 



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

 do not preserve the same diameter through their whole 

 length, being largest in their course within the trunks 

 and branches of the nerves, in which the majority measure 

 from -goW ^ sinnr ^ an ^ nc ^ * n diameter. As they ap- 

 proach the brain or spinal cord, and generally also in 

 the tissues in which they are distributed, they gradually 

 become smaller. In the grey or vesicular substance of 

 the brain or spinal cord, they generally do not measure 

 more than from lo ; oo to TT i^- of an inch. 



The fibres of the second kind (fig. 127), which constitute 



