A nerve stem, whether met with embedded in the tissues of an organ or lying free, is composed of one or more bundles of nerve fibres united together by connective tissue. The accompanying figure (Fig. 301)' shows the general arrangement of this connective tissue in a stem composed of a single bundle of nerve fibres. There is an external layer of connective tissue, the perineurium (a), binding the whole bundle together. But inside the bundle there is connective tissue binding the individual nerve fibres together and forming the endoneurium (b), the nuclei of which are prominently seen in the figure. In a nerve stem made up of several bundles these also are bound together by connective tissue, the epineurium or neurilemma. The nerves within the skull and the nerve-roots inside the spinal canal are, in general, less furnished with connective tissue than the peripheral nerves, and in particular the perineurium is less consistent and continuous.
The nerve fibres of such peripheral nerves are, for the most part, medullated and when examined in the fresh state they present an opaque appearance and a double outline as in Fig. 302, A. When examined in the fresh state it is only this appearance that is visible, but by proper methods of preparation the constituent structures of the fibre can be shown as indicated in Fig. 302, B and C, and in Fig. 301. These are the axis cylinder (3, Fig. 302), the medullary sheath or white substance of Schwann (2), and the primitive sheath (1). The axis cylinder is the conducting part of the fibre, and runs continuously from end to end. The medullary sheath is composed of a fatty substance (myeline) and is prone to undergo a kind of coagulation which gives rise to the double contour. This substance is semi-fluid, and when the nerve fibre is broken up, either during life or after death, it is apt to flow out, and so we may have free drops of myeline which have a strongly refracting outline (4, Fig. 303). The primitive sheath is a transparent membranous tube which covers the fibre and keeps the medullary sheath together. When transverse sections of a nerve which has been hardened and stained are examined these various constituents appear, as in Fig. 301. The axis cylinder is a coloured point in the middle of each fibre. The medullary sheath around this is transparent and colourless. The primitive sheath forms a coloured ring around the fibre.
If a medullated nerve fibre be examined after preparation with osmic acid, it will be seen that, as Ranvier has shown, the medullary sheath is not continuous, but is interrupted at intervals, the axis cylinder and primitive sheath being alone present throughout. These nodes divide the nerve fibre into sections, and each section receives a further individuality from the fact that about its middle an oval nucleus is present inside the primitive sheath, between it and the medullary sheath.
Non-medullated or pale nerves have no medullary •sheath, and consist essentially of axis cylinders each covered with a primitive sheath in which nuclei occur at intervals. As the white appearance of ordinary nerves depends on the medullary sheath, non-medullated nerves are grey in colour. Most nerves at their peripheral terminations lose the medullary sheath and become pale, but some are so throughout, chiefly the olfactory nerve and the whole nerves of the sympathetic system.
Fig. 301. - Transverse section of a nerve consisting of a single bundle; from a specimen stained and mounted by Clarke's method, a, perineurium; b, endoneurium. Inside the perineurium is the lymphatic space between it and the nerve bundle. The nerve fibres are represented by rings with a central dot - the axis cylinder, x 120. (Klein).
Fig. 302. - Medullated nerve fibres. A, the natural appearance; li and C, diagrams showing the constituent structures as in text. (Quain).