This is the name given to white or grayish threads which are attached by one extremity to the cerebro-spinal nervous centre, and at the other are distributed to the organs. The nerves are composed of very fine filaments, united at the point at which they spring from the nervous centre in bundles called the roots of the nerves; these roots unite and form the trunks which ramify and disappear, as it were, in the tissues of the body. A sheath of cellular tissue called neurilemma or perineurium, envelops the nerves, and penetrates between the fibres formed by the union of the nerve-tubes spoken of in treating of the tissues. The ramifications of the nerves unite and seem to be confounded at certain points where they form a very complicated net-work, which is termed a plexus; but this union is effected solely by the neurilemma. One nerve-fibre—properly speaking—never is confounded with another. It runs without interruption, and always distinct, through the most intricate net-work, from the nervous centre to the organ which it serves. From the analogy with the union of the blood-vessels, we speak of the anastomosis of the nerves, and we shall soon see that if the union of the vessels with each other is an essential condition of the circulation of the blood, the distinct and absolute independence of the nerves, even to their minutest ramifications, is no less necessary to the integrity of the nervous functions. We may therefore compare the union of the nerves by juxtaposition during their course to a bundle of electric wires which, though united, are always distinct; because of their isolating covering. The isolating covering of the nerves is the neurilemma, M. Sappey has recently described, under the name of the nervi nervorum, or nerves of the nerves, the filaments which run to the neurilemma, and which stand in precisely the same relation to the nerves that the nerves themselves do to the entire organism.
There are two orders of nerves: the one, under the influence of the will, causes motion in the organs; these are the nerves of animal life: the other presides over the functions of the viscera without our consciousness, and without any effort of will; these are the nerves of organic life. The first are cranial or spinal nerves, and spring directly from the nervous centres, they are white and generally of a resistant texture; the second, ganglionic or visceral nerves, although connected with the nervous centre, form a system apart, which is called the great sympathetic. These nerves are for the most part soft, and of a grayish colour.
These nerves are all disposed in twos, and form a series of pairs to the number of forty, of which nine pairs are cranial or cerebral, and thirty-one pairs are spinal.
The cranial nerves are classed as follows:—
Olfactory nerves, which are ramified in the organ of smell.
The common oculo-motor nerves, which are distributed to most of the muscles which move the eyeball.
Pathetic nerves, so called, because they give the power of motion to the great oblique muscle, the action of which, upon the eyeball, is one of the principal elements in the expression of the face.
The trigeminal or trifacial nerve forms on either side three nerves, the ophthalmic and the superior and inferior maxillary; they are distributed over the face, and to the organs which constitute it.
The external oculo-motor nerves. These go to the external straight muscle of each eyeball.
Is divided into three branches: 1. the glossopharyngeal, the nerve of taste, running to the tongue and pharynx, and furnishing branches to several muscles of the neck, to the tonsils, etc. 2. The pneumo-gastric nerve, which branches out to the cervical region, to the pharynx, the larynx, the lungs, and the stomach. 3. The spinal accessory of Willis, which sends branches to several muscles of the neck, to the pharynx, and to the larynx.
The great hypoglossal nerves, which give movement to the tongue.
The spinal nerves form eight cervical pairs, twelve dorsal, five lumbar, and six sacral. They all spring from the spinal cord in two bundles of roots, called the anterior and posterior roots, according to the portion of the cord from which they emerge. These roots are enveloped in a membranous sheath, and unite to form the trunk of the nerve at a point more or less distant from their origin, according to the region from which they proceed. The roots of the lumbar and sacral nerves form a bundle of independent cords in the inferior portion of the spinal canal, which, from their peculiar disposition, has been named the cauda equina or horse-tail.
On a level with the openings through which the nerves pass from the spinal canal, the posterior roots form a ganglion on each side to which the anterior roots unite themselves, and from which the nerve is distributed to the organism, by three classes of spinal branches, anterior, posteriory and gan-glionic; the latter unite with the great sympathetic.
The first four pairs of cervical nerves form by the contiguity of their branches the cervical plexus; the ramifications of which distribute themselves to the surface, and to the deep portions of the neck, to the outside of the head, to the shoulder, and to the upper portion of the back.
The last four pairs constitute the brachial plexus, which, after furnishing numerous branches to the shoulder and back, go to the arm as the brachio-cutaneous, musculo-cutaneous, median, radial, and ulnar nerves.
The twelve pairs of dorsal or intercostal nerves, as well as the five pairs of lumbar nerves, are ramified in the walls of the thorax and abdomen, and in the muscles of the back and loins. The lumbar plexus furnishes, among other principal branches, the crural nerve, which ramifies into the musculocutaneous of the leg, and the external and internal saphenous nerves, etc.
The six pairs of sacral nerves are distributed to the pelvis and to the lower limbs. The first four pairs with the last lumbar pair form the sacral plexus, the principal terminal branch of which is the sciatic nerve. This is the largest nerve in the body; it descends through the posterior portion of the thigh, to the muscles of which it furnishes several branches; and a little above the knee it divides into two trunks—the internal popliteal or tibial, and the external or peroneal, which distribute themselves by numerous ramifications to the muscles of the leg and foot.