NERVES

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NERVES. The nerves of the animal frame are composed of bundles of white parallel medullary threads. Every bundle is surrounded with a soft sheath full of bloodvessels, and whose finest branches terminate in the substance of the nerves. These' nerves are spread through the whole animal frame, and variously connected witn each other. Only the epidermis, the hair and nails are destitute of them. They are of various size, according as they are composed of more or fewer bundles of medullary threads. In the course of the nerves there are a number of knots; these are called ganglions ; they are commonly of an oblong shape, and of a grayish color, somewhat inclining to red, which is perhaps owing to their being extremely vascular. In particular parts of the body, the nerves come in contact with each other, and the bundles composing them are mutually interwoven to such a degree that they cannot be disjoined without violence. These communications are called plexuses, and are found particularly in the abdomen, behind the stomach, and in the region of the pit of the stomach, near the liver, mesentery, heart, &c. The final terminations of the nerves are various, particularly those which run to the organs of sense. In the auricular organ, for instance, the nerves terminate in a soft mass like pap, surrounded with moisture ; the optic nerve terminates in a medullary skin ; the nerves of taste terminate in little papilla; those of feeling in the points of the fingers, and the surface of the skin in general; those belonging to the muscles are lost in the texture of the same, so that theii terminations cannot be accurately ascertained. All the nerves are embraced under the general head of the nervous system. This is most intimately connected with the brain and the spinal marrow, which may be regarded as a prolongation of it. The brain is the centre, from which or to which proceed all impressions communicated to the nerves. The substance of the nerves is the same medullary matter which constitutes the brain, resembling the white of an egg, and appearing, to the unassisted eye, as if composed of little balls. The central termination of all the nerves is in the brain and spinal marrow, where they branch out into the skin or the interior of the organs. The various isolated, and, in part, heterogeneous structures of which the body consists, which are mechanically joined by the cellular tissue, the membranes and the ligaments, are united into one harmonious whole by means of the nerves. The vascular system connects them only so far as it, furnishes the supply of blood required for their support and their operations ; but it is properly the nervous system which imparts to all their life, governs their operations, and establishes their sympathy and mutual action. This is effected by means of that portion of the nervous system which is diffused through the abdomen, forming many nets and plexuses, and constituting what is called the vegetative, or reproductive, or organic nervous system, because the growth and support of the body are effected by it Another part of the nervous system af fords the means of consciousness and voluntary action. This is the brain or cerebral system (see these two articles), which excites the nerves that put in action the muscles of voluntary motion, and those which supply sensibility to the organs of sense, and convey to the brain the impressions thence received. The nerves which communicate with the organs of sense, run in pairsthe first pair (olfactory nerve) to the nose, where it is spread over the surface of the nostrils, and forms the power of smell; the second (optic nerve) to the eyes; this is round, thick and penetrates from behind the ball or globe of the eye (through a round plate of the firm coat of the ball, containing many little apertures), and is spread out on the inner and concave surface of the globe into a thin coat called the retina, on which the images of external objects are formed ; the eighth pair (auditory nerves) are spread over the interior of the ear, and are sensible to the vibrations of the air. From the numerous ramifications of the ninth pair come the nerves of the tongue, which give rise to the sense of taste. The general sense of feeling is situated particularly in the skin; and peculiarly in the points of the fingers. This sense is produced by a variety of nerves diffused over the skin, and those parts which are most sensitive are supplied with the greatest quantity of nerves, which form entire series of contiguous nervous papillae; for instance, at the lips, the points of the fingers, &c. Thus the action of the nerves is reciprocal from without inwards, and from within outwardsthe first, because the impressions on the organs of sense are communicated by the nerves to the brain, and there form perceptions and feelings; the second, because the voluntary motions are produced by communications from the brain to the nerves, while the reproductive part of the nervous system quietly supports the whole machine, and, in a sound state of the body, is recognised only by the operation of the appetites, and by a general feeling of ease throughout the system, but, in a diseased state, gives rise to general uneasiness and pain. The power of the nervous system has no fixed point, but is variable, even in the same subject. In sleep, the activity of the cerebral system is impaired, that of the reproductive system heightened; therefore, in quiet sleep, the operations of the senses and the voluntary motions cease, while the activity of the organs of respiration and circulation, of digestion, secretion and nourishment continue?. From what has been said, it appears that the whole action of the body depends upon the nervous system. (See Nervous Diseases.)