SYSTEM.] minute structures connected with tlie peripheral extrem ities of the nerves. These end-organs are situated in the skin and other organs of sense, in the glands, blood-vessels, and muscles. The nerves establish communications and con duct nervous impulses, either between different nerve centres, or between nerve centres and peripheral end-organs, so as to associate together in their action parts of the nervous system often widely separated from each other. Nerves, therefore, are internuncial structures. When a nerve con nects two nerve centres together it is intercentral. When a nerve connects a nerve centre with a peripheral end- organ, and conducts impulses from the centre to the end-organ, it is a centro-peripheral or centrifugal nerve. When a nerve connects a peripheral end-organ with a centre, and conducts impulses from the end-organ to the centre, it is a periphero-central or centripetal nerve. Owing to the different directions in which impulses are conducted by nerves, the varying nature of their end- organs, and the functional differentiation of the nerve centres, or portions of the nerve centres in which their central extremities terminate, nerves vary so in their functions, that a classification of the nerves, based upon their functional properties, has been proposed. Of the centro-peripheral nerves, those which end in the muscles are motor nerves ; those which end in the muscular coat of the blood-vessels are vaso-motor nerves; whilst some physiologists have named nerves which they believe to terminate in connection with the secreting cells of a gland, secretory nerves; and others, which they believe to terminate in the tissues and to be concerned in the regulation of their nutrition, trophic nerves. It should be stated, however, that it is not yet absolutely determined that the secreting cells of glands and the cell elements of the tissues have special nerves terminating in connection with them for the purpose of exercising a direct influence over secretion and nutrition. Should these special nerves be non-existent, then the secretory and nutritive functions would be influenced solely by the vaso-motor nerves, which regulate the size of the blood-vessels and the amount of blood which flows through a part in a given time. Of the periphero-central nerves, those which arise in the end-organs in the skin, terminate in a nerve centre, and excite in it the sense of touch, are nerves of common sensation; those which arise in the end-organs in the eye, ear, nose, and tongue, and excite in their appropriate nerve centres the sensations of sight, sound, smell, and taste, are nerves of special sense ; whilst nerves which conduct impulses from peripheral end-organs to a nerve centre, and, instead of exciting in the latter a sensation, have the impiilses reflected to motor nerves, are reflex or excito-motory nerves. The nerve centres, nerves, and peripheral end-organs are arranged in two groups or systems a Cerebro-spinal and a Sympathetic. The Cerebro-spinal nervous system con sists of the brain and spinal cord, the nerves which arise from or terminate in these large centres, the small ganglia connected with these nerves, and the end-organs at their peripheral terminations. The Sympathetic nervous system consists of the sympathetic ganglia, with their nerves and end-organs. NERVOUS TISSUE. The several parts of the nervous system are not uniform in colour, some being white, others grey. The nerves, at least those of the cerebro- spinal system, are invariably white, and white masses, variable in size, are met with in the brain and spinal cord ; they constitute the white matter of the nervous system. In the nerve centres, both of the cerebro-spinal and sym pathetic systems, grey matter is found, sometimes in con siderable quantities. This grey colour is so characteristic, 859 that it may be regarded as marking the position of a nerve centre. The nervous system possesses a characteristic form of tissue the nervous tissue which in part consists of fibres (Nerve Fibres), and in part of cells (Nerve Cells). The nerve cells are found in the grey matter that is, in the nerve centres and sometimes also in the peripheral end-organs. The nerve fibres constitute the nerves, enter into the nerve centres, and pass into the peripheral end-organs ; they form the white matter. But in addition to the characteristic nervous tissue, the nervous system also contains a con siderable quantity of connective tissue, numerous blood vessels, and some lymph vessels. Nerve Fibres. Nerve fibres are of two kinds : a, the white, medullated, or dark-bordered fibres, which are the characteristic fibres of the cerebro-spinal nervous system, though they do also sparingly occur in the sympathetic system ; 5, the pale, non-medullated, or gelatinous nerve fibres, which are the characteristic fibres of the sympa thetic nervous system. Medidlated Nerve Fibres. To examine the structure of these fibres, a portion of a cerebro-spinal nerve may be selected. In the first place, it will be seen to be invested by a sheath of connective tissue, the perineurium, which gives off processes that pass into the nerve, and subdivide it into fasciculi or funiculi. Each fasciculus is in its turn composed of nerve fibres, which are separated from each other by bundles of delicate connective tissue, prolonged from the perineurium, in which the nutrient blood vessels of the nerve ramify. The size of a nerve is in relation to the number and size of its fasciculi, and the size of a fasciculus is in relation to the number of its fibres. The fibres and the fasciculi lie parallel to each other in the same nerve ; but as nerves branch at intervals, the more external of the fasciculi diverge from the main stem to form the branches. In the white matter of the brain and spinal cord the nerve fibres are not arranged in such definite fasciculi as in a distributor^ nerve, and the con nective tissue between the fibres is the soft, delicate form called neuroglia. A medullated nerve fibre is an elongated cylinder, which, when examined in the body of a living animal, or im mediately after removal from the living body, consists apparently of a soft, homogeneous, or glassy-looking sub stance enclosed within a limiting membrane. When ex amined some time after death, or after the addition of re agents, such as water, spirit, ether, collodion, acetic acid, &c., it loses its homogeneous aspect, and the following struc tures can be distinguished in it : A (Fig. 5 6), a delicate trans parent investing membrane, the so-called tubular or primitive membrane, or neuri- lemma ; C, a delicate thread, extending along the axis of the fibre, the axial cylinder or central band of Eemak ; B, a substance which lies be tween the primitive mem brane and the axial cylin der, the white substance of Schwann, or the medullary sheath. Within the external outline of the fibre, formed by the investing membrane, is a second line, not quite parallel to the first, and the presence of these two lines gives to the fibre a characteristic double-contoured appearance. The investing membrane is a perfectly pellucid, homogeneous structure, with nuclei arranged at intervals in it. 3 FIG. 56. 1. Medullated nerve fibres, showing the double contour. 2. .A similar fibre in which A is the j>ri- mitive membrane, B the medullary sheath, C the axial cylinder, pro truding beyond the broken end oi the fibre. 3. Transverse section through tlie medullated fibres of a nerve, showing the axial cylinder in e:-.ch fibre. Bet ween the fibres is the interfibrous connective tissue.
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