392 



SPECIAL HISTOLOGY. 



fibres (fig. 138), in which the medullary sheath presents some- 

 times, minute moniliform enlargements, sometimes, various sized, 

 irregularly distributed nodosities, or even, in places, complete 

 interruptions. All these forms, in which the sheath frequently 

 participates, but in which the central fibre takes no part, arise 

 artificially, and are developed most readily in the finer fibres 

 with more delicate sheaths, such as are found in the central 

 organs. 



The central or axis-fibre of the nerve-tubes ("primitive band," 

 Remak, cylinder axis, Purkinje, fig. 137, 2, 3, 4, 5. 139, 1), is 

 a cylindrical or slightly flattened filament, which, in entire and 



unaltered nerve-fibres, is as 

 little recognizable as the 

 sheath, being surrounded by 

 the pulp, and possessing the 

 same refractive power, whilst 

 it comes readily into view 

 when the fibre is torn or 

 treated with various reagents; 

 and it may thus be recognised as 

 a constant structure, sometimes 

 in the interior of the fibre, 

 and sometimes isolated. Un- 

 der natural circumstances it 

 is pale, most generally homogeneous, more rarely, finely 

 granular or striated, bordered by straight or occasionally by 

 irregular, pale contour lines, and it is, generally, everywhere 

 of uniform thickness : it is distinguished from the medullary 

 sheath, especially by the circumstance, that although soft and 

 flexible, it is still, nut fluid and viscous, but elastic and solid, 

 something like coagulated albumen, with which it also appears 

 to agree in most of its chemical characters. This axis-cylinder 

 exists in all nerve-fibres without exception, even in the finest, 

 and invariably presents the same characters, except only, that it 



Fig. 139. Nerve-fibres, x 350 diam. 1, from the Frog, boiled with alcohol and 

 acetic acid ; a, sheath ; b, axis-cylinder ; c, crystals (fat ?) : 2, isolated sheath of a 

 Frog's nerve, boiled with soda : 3, from the floor of the fourth ventricle in Man, after 

 treatment with soda; a, sheath; b, medulla flowing out in drops, the axis-cylinder 

 is wanting (having been drawn out in the preparation), and the pale streak is 

 medulla: 4, from the root of the n. abducens of Man, treated with soda; a, sheath ; 

 b, medulla, axis-cylinder not visible. 



