128. Nerve-fibres are of two kinds--1°. The _dark-bordered_ or _medullary_ fibres, which have both sheath and myeline, as in the peripheral system; or only myeline, without the sheath, as in the central system. 2°. The _non-medullary_ fibres, which have the sheath, without appreciable myeline--such are the fibres of the olfactory, and the pale fibres of the sympathetic. Nerve-fibrils are neuroplasmic threads of extreme delicacy, visible only under high magnifying powers (700–800), which abound in the centres, where they form networks. The fibrils also form the terminations of the fibres. Many fibrils are supposed to be condensed in one axis cylinder. This is represented by Max Schultze in Figs. 17 and 20. 129. As may readily be imagined, the semi-liquid nature of the neuroplasm throws almost insuperable difficulties in the way of accurately determining whether the axis cylinder in the living nerve is fibrillated or not; whether, indeed, any of the aspects it presents in our preparations are normal. Authorities are not even agreed as to whether it is a pre-existent solid band of homogeneous substance, or a bundle of primitive fibrils, or a product of coagulation.[152] Rudanowsky’s observations on frozen nerves convinced him that the cylinder is a tubule with liquid contents.[153] My own investigations of the nerves of insects and molluscs incline me to the view of Dr. Schmidt of New Orleans, namely, that the cylinder axis consists of minute granules arranged in rows and united by a homogeneous interfibrillar substance, thus forming a bundle of granular fibrils enclosed in a delicate sheath[154]--in other words, a streak of neuroplasm which has a fibrillar disposition of its granules. We ought to expect great varieties in such streaks of neuroplasm; and it is quite conceivable that in the Rays and the Torpedo there are axis cylinders which are single fibrils, and others which are bundles, with finely granulated interfibrillar substance.[155] The fibres often present a varicose aspect, as represented in Fig. 21. It is, however, so rarely observed in the fresh tissue, that many writers regard it (as well as the double contour) as the product of preparation.[156] It is, indeed, always visible after the application of water. We need say no more at present respecting the structure of nerve-fibres, except to point out that we have here an organite not less complex than the cell. [Illustration: Fig. 21.--_Nerve-fibres from the white substance of the cerebrum._ _a_, _a_, _a_, the medullar contents pressed out of the tube as irregular drops. ] THE NEUROGLIA.