THE NERVOUS SYSTEM. 



71 



(2.) The medullary sheath or white substance of Sclnvann is the part 

 to which the peculiar white aspect of the cerebro-spinal nerves is princi- 

 pally due. It is a thick, fatty, semi-fluid substance, as we have seen, pos- 

 sessing a double contour. It is said to be made up of a fine reticulum 

 (Stilling, Klein), in the meshes of which is embedded the bright fatty 

 material. 



According to McCarthy, the medullary sheath is composed of small 

 rods radiating from the axis-cylinder to the sheath of Schwann. Some- 

 times the whole space is occupied by these rods, whilst at other times the 

 rods appear shortened, and compressed laterally into bundles embedded in 

 some homogeneous substance. 



(3.) The axis-cylinder consists of a large number of primitive fibrillm. 

 This is well shown in the cornea, where the axis-cylinders of nerves break 

 up into minute fibrils which go to form terminal networks (see Cornea), 

 and also in the spinal cord, where these fibrillae form a large part of the 

 grey matter. From various considerations such as its 

 invariable presence and unbroken continuity in all 

 nerves, though the primitive sheath or the medullary 

 sheath may be absent, there can be little doubt that 

 the axis-cylinder is the conductor of nerve-force^ the 

 other parts of the nerve having the subsidiary function 

 of support and possibly of insulation. 



At regular intervals in most medullated nerves, the 

 nucleated sheath of Schwann possesses annular con- 

 strictions (nodes of Ranvier). At these points (Figs. 

 304, 305), the continuity of the medullary white sub- 

 stance is interrupted, and the primitive sheath comes 

 into immediate contact with the axis-cylinder. 



Size. 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 -g-^ to -g-^gr of an inch in 

 diameter. As they approach the brain or spinal cord, 

 and generally also in the tissues in which they are dis- 

 tributed, they gradually become smaller. In the grey 

 or vesicular substance of the brain or spinal cord, they 

 generally do not measure more than from YMTS * 

 TTO~O ^ an inch. 



(B.) Non-Medullated Fibres. The fibres of the second kind (Fig. 

 306), which constitute the whole of the branches of the olfactory and 

 (uulitory nerves, the principal part of the trunk and branches of the sym- 

 pathetic nerves, and are mingled in various proportions in the cerebro- 

 spinal nerves, differ from the preceding, chiefly in their fineness, being 



FIG. 305. A node of 

 Ranvier in a medul- 

 1 a t e d n erve-fibre, 

 viewed from above. 

 The medullary sheath 

 is interrupted, and the 

 primiti ve sheath 

 thickened. Copied 

 from Axel Key and 

 Retzius. X750. (Klein 

 and Noble Smith.) 



