PALE OR NON-MEDULLATED FIBRES. 315 



extreme variability of appearance and size of mesh (fig. 363) even in the course 

 of the same nerve-fibre, lends much probability to the view that it is produced 

 artificially by the precipitation by the reagent employed, of some material entering 

 into the constitution of the myelin of the medullary sheath. Gerlach has in fact 

 shown that the character of the reticulum varies with the strength of the alcohol 

 employed to acb upon the nerve. 



It will be seen from the above that there is still much diversity of opinion with regard to 

 the minute structure of the medullary sheath of the nerves. As to its chemical composition, 

 the white substance or myelin consists chiefly of lecithin and neurin together with cholesterin 

 and one or two other substances in less amount. When escaped from the nerve-fibres it forms 

 .drops either rounded or irregular in shape, which always show the double contour which is so 

 .characteristic of the medullated nerve-fibres, the appearance being due to the peculiar manner 

 in which myelin refracts the light. In contact with water it combines with that fluid, and as 

 a result of the imbibition the myelin-drops undergo a considerable increase in bulk, accom- 

 panied by remarkable changes of form, often growing out into tube-like filaments for a con- 

 siderable distance into the fluid. In this behaviour myelin is not peculiar but resembles 

 .certain other substance of a fatty and resinous nature. 



It is generally believed that medullated nerve- fibres occur only in vertebrates, but Retzius 

 has described the nerve-fibres in Palasmon (a crustacean) as also possessing a myelin-sheath, 

 with nodes and internodal nuclei. 



Sheath of Schwann, primitive sheath, or nenrolemma. The sheath of 

 Schwann forms the outermost covering of the white nerve-fibres. It has the appear- 

 ance of a delicate homogeneous membrane with nuclei disposed at intervals along 

 its inner surface. As already mentioned, these nuclei bear a definite relation to the 

 segments of the nerve-fibre, for they lie about midway between the nodes, only one 

 nucleus being found in each internode (fig. 359). The nuclei are oval and some- 

 times flattened, they usually lie in a depression of the medullary sheath, and at each 

 <end of the nucleus, especially in young nerves, there is a small amount of granular 

 protoplasm which may spread for a short distance between the primitive and the 

 medullary sheath. The primitive sheath usually dips in at the nodes and is there only 

 separated from the axis-cylinder by the annular band of Eanvier before mentioned. 



So long as the primitive sheath is accurately filled by the contained medullary 

 substance its outline can seldom be distinguished, but sometimes, when the white 

 substance separates at various points from the inside of the tube, the contour of the 

 fibre becomes indented and irregular, and then the membrane in question may, in 

 favourable circumstances, be discerned as an extremely faint line, running outside 

 the deeply shaded border formed by the white substance. 



In the white fibres of the brain and spinal cord the nucleated sheath is absent, 

 and these are only invested by a medullary sheath. In consequence of the absence 

 .of the comparatively tough primitive sheath the fibres from these situations cannot 

 be isolated for any distance without rupture, and it is found that for the same reason 

 the medullary sheath readily breaks away from the axis-cylinder, so that this is thus 

 left bare. It is usually stated that the nodes of Eanvier do not exist on these 

 white fibres of the brain and spinal cord ; but various observers have described 

 appearances which seem to indicate that the nodes may be present in these fibres also. 



Pale or noii-medullatecl fibres ; fibres of Bemak. These occur princi- 

 pally in branches of the sympathetic nerve, but they are found also in greater or less 

 .amount in the cerebro-spinal nerves. They are transparent, faintly striated fibres of 

 varying size, which exhibit nuclei at frequent intervals. The nuclei are applied to 

 the surface of the fibre, and, according to the generally received account, belong to a 

 delicate homogeneous sheath, similar to the primitive sheath of the medullated fibre. 

 It must be admitted, however, with Ranvier that it is difficult or impossible to 

 exhibit the sheath, and if this is the case, the nuclei must be regarded as embedded 

 in the peripheral layer of the fibre itself, and as belonging to this. Many of these 



VOL. I. Y 



