The nerves of the cerebro-spinal system and of the great sympathetic exhibit such different characters as regards their anatomy, that they may be examined separately.
Cerebra-spinal nerves.—In examining a cere bro-spinal nerve, we find it invested by a sheath of membrane, which has adherent to its inner surface thin layers of areolar tissue which pass, like so many partitions, between the threads or fibres of which the nerve is composed. This sheath is coinmonly called the neurilemma ; • it is analogous to the sheath which surrounds mus cles. Its office is chiefly mechanical, namely, that of binding the constituent fibrillm and fas cicles of the nerve together, so as to protect them and to support the delicate plexus of capillary bloodvessels from which they derive their nutriment.
The neurilemma is composed of fibres of the white fibrous kind. It exhibits to the naked eye the appearance of a fibrous membrane, white and almost silvery ; and its microscopic characters are those of the white fibrous ele ment, although not presenting much appearance of wavy fibres. The fibres are, for the most part, parallel to the axis of the nerve; but there are some which cross the nerve at right angles, or appear to pass spirally round it. The septa between the secondary bundles of the nerves seem to consist of a less perfect fibrous tissue, containing the remains of numerous cytoblasts. A yellow fibrous tissue of the finest kind exists here in very small quantity.
The bloodvessels are distributed upon the external investing sheath and upon the septa.. In some of the large nerves, the sciatic for example, these may be often seen minutely injected with blood. They are disposed some what similarly to those of muscles, running parallel to the fibres and fascicles of the nerve. The capillaries are among the sinallot in the body : they form oblong meshes of considerable length, completed at long intervals by vessels which cross the fibres of the nerve rnore or less in the traniverse direction. Henle assigns to them, when empty, a diameter not exceeding doth of an inch. These bloodvessels are generally derived frorn neighbouring arterial branches ; sometimes a special vessel accom panies a nervous trunk, and even perforates it, passing along its central axis, as is well known to descriptive anatomists in the sciatic and the optic nerves.
After the external part of the neurilemma has been dissected off; the nerve may be torn by needles and divided into secondary bundles and fibres. The ultimate fibre can then be readily distinguished by the aid of the microscope, from its being incapable of further subdivision by mechanical means ; and an accurate knowledge of its structure is of the utmost importance to the forniation of correct opinions respecting the actions of the nervous system.
The ultimate (or, as it is also called, the pri mitive) nervous fibre, is a tube composed of a fine transparent homogeneous membrane, in a great degree resembling the sareolemma of muscle. It is elastic, like that membrane, per fectly homogeneous, and, according to Schwann, in young nerves has the nuclei of cells connected with it. It may be called the tubular membrane of nerve (a, fig. 329). The contents of this tube consist in a soft, semifluid, whitish, pulpy sub stance, vvhich is readily pressed out of its cut extremity. In the nerve that is quite fresh, having been taken from an animal just dead, this pulpy matter is quite transparent and appa rently homogeneous. The tube meinbrane pre sents the appearance of a delicate line, resem bling, when perfectly fresh and unaltered by re-agents, the marg,in of an oil globule. When the nerve-tube has been treated w ith water, or has been allowed to remain a little time on a piece of glass, we observe within the tubular niembrane a double-edged layer of a whitish material of different refracting power from either that which occupies the centre of the nerve tube or the tubular membrane itself. The later after death the nerve is examined, the more dis tinct does this inner layer become. The addi tion of water, alcohol, and other re-agents always renders it more evident, and seems to destroy the apparent homogeneousness of the pulpy contents of the nerve tube. This layer within the tubular membrane is that which, according to Schwalm, gives to the nerve-tubes their white colour ; it is therefore called by him the white substance. Within this and occupying the centre of the tube is a transparent, somewhat flattened, band, which is extremely delicate, and in which it seems impossible to recognize any more definite structure.