NERVE-FIBKES. 



CXXXVll 



The axis cylinder, axis-bandy or axial fibre is situated in, or near, the 

 miklle of the nerve-tube, where it may occasionally be seen, on a careful 

 inspection, as a greyish stripe or baud, bounded on each side by a very 

 faint even outline, having no share in the sinuosities of the white substance 

 (fig. LXXIV, c). 



The axis is of a more tenacious consistence than the white substance, and may accord- 

 ingly be sometimes seen projecting beyond it at the end of a broken nerve-tube, either 

 quite denuded, or covered only by the tubular membrane, the intervening white sub- 

 stance having escaped. Although the name of axis-cylinder would seem to imply that 

 it has actually a cylindrical figure, yet this is by no means certain; and whether 

 naturally cylindrical or not, it certainly very generally appears more or less flattened 

 when subjected to examination. To all outward appearance, usually, it is solid and 

 homogeneous, but sometimes it is striated longitudinally, and towards its termination 

 at the peripheral extremity of the nerve, it very commonly divides into finer filaments. 

 The axis-cylinder consists of a solid albuminoid substance, whereas the medullary 

 sheath or white substance consists mainly of fat and a certain proportion of albu- 

 minous matter, combined with it as a colloid into an oleo-albuminous liquid. Accord- 

 ingly, whilst water, especially when cold, rapidly produces congelation of the white 

 substance, ether, on the other hand, causes it speedily to disappear as if by solution, 

 and globules of oil then make their appearance both within and without the tube, the 

 remaining contents becoming granular from precipitation of albumen. 



The existence of an axial fibre is probably universal in nerve-fibres, though it is not 

 generally visible without preparation. To bring it into view, a solution of carmine or 

 aniline-red (magenta) may be used, which stains it red first colouring the denuded 

 and projecting ends, but finally also the parts still surrounded with the medullary 

 sheath. Glacial acetic acid, chromic acid, iodine, alcohol, chloroform, collodium, and 

 other reagents, are also employed with greater or less advantage. 



Many of the tubular nerve-fibres, when Fig- LXX V. 



subjected to the microscope, appear di- 

 lated or swollen out at short distances 

 along their length, and contracted in the 

 intervals between the dilated parts. Such 

 fibres have been named varicose (fig. 

 LXXV). They occur principally in the 

 brain and spinal cord, and in the intra- 

 cranial part of the olfactory, in the optic, 

 and acoustic nerves ; they are occasion- 

 ally met with also in the other nerves, 

 especially in young animals. These fibres, 

 however, are naturally cylindrical like 

 the rest, and continue so while they re- 

 main undisturbed in their place ; and the 

 varicose character is occasioned by pres- 

 sure or traction during the manipulation, 

 which causes the soft matter contained 

 in the nerve-tube to accumulate at cer- 

 tain points, whilst it is drawn out and 

 attenuated at others. Most probably 

 the change takes place before the white 

 substance has coagulated. The fibres in 

 which it is most apt to occur are usually 

 of small size, ranging from ^ ^th to 

 j-g^- -tli of an inch in diameter ; and 

 when a very small fibre is thus affected, 

 the varicosities appear like a string of (from' Valentin). 



Fig. 



LXXV. FIBRES FROM THE ROOT 

 OF A SPINAL NEKVE. 



At a, where they join the spinal 

 cord, they are varicose ; lower down 

 at b, they are uniform and larger 



