MEDULLATED NERVE-FIBRES. 21 



method is to treat the nerve with fuming nitric acid, after- 

 ward adding a solution of caustic potash. The fatty sub- 

 stance is thus discharged in small drops, the central band is 

 dissolved, and the empty sheath is seen, swollen and tinged 

 with yellow. These are the processes employed by Kolliker, 

 who demonstrates in this way the presence of nuclei. 1 



The medullary substance fills the tube and surrounds the 

 central band. This is called by various names ; as myeline, 

 white substance of Schwann, medullary sheath, nervous me- 

 dulla, etc. It does not exist either at the origin of the nerves 

 in the gray substance of the nerve-centres or at the periphe- 

 ral termination of the nerves, and is probably not an essential 

 conducting element. When the nerves are perfectly fresh, 

 this substance is transparent, homogeneous, and strongly 

 refracting, like oil ; but as the nerves become altered by des- 

 iccation, the action of water, acetic acid, and various other 

 reagents, it coagulates into an opaque, granular mass. The 

 consistence of this substance gives to the medullated fibres a 

 very peculiar appearance. The tubular membrane being 

 very thin and not elastic, the white substance, by very slight 

 pressure, is made to fill the tubes irregularly, giving them a 

 varicose appearance, which is entirely characteristic. In ex- 

 amining a preparation of the nervous tissue, large drops, 

 coagulated in irregular shapes, are seen scattered over the 

 field, and frequently fringing the divided ends of the tubes, 

 In the white substance of the encephalon and spinal cord, 

 where the tubular membrane is wanting, the varicose appear- 

 ance of the fibres is more remarkable than in any other situ- 

 ation. 



The axis-cylinder is, in all probability, the essential ana- 

 tomical element of the nerves. It exists in all the nerves 

 except in those termed gelatinous fibres, or fibres of Eemak, 

 which will be described hereafter. In the ordinary medul- 

 lated fibres, the axis-cylinder cannot be seen in the natural 

 condition of the tissue, because it refracts in the same man- 



1 KOLLIKER, op. tit., p. 318. 



