SYSTEM.] ANATOMY 867 is possible they may form the fine nerve fibre plexus of the grey substance; but a direct continuity between them and the axial -cylinder processes of the cells of the posterior horn does not seem to have been observed. From the plexus, formed by the much subdivided processes of these cells, fibres arise, which, forming the fibres of the pos terior commissure, pass both in front of and behind^ the central canal to the opposite side, where they ascend towards the brain, " partly in the vertical fasciculi of the posterior cornua and partly in the posterior columns." The structure of the spinal cord shows it to be both a nerve centre and a conductor of nervous impulses. The nerve cells in its grey matter give rise either directly, or through the delicate plexus formed by their branching processes, to nerve fibres, which may either pass out of the cord as the anterior and posterior roots of the spinal nerves, or may ascend to the brain as the columns of the cord. Hence the cord is anatomically continuous, on the one hand, through the nerves which arise from it, with the peripheral end-organs in the skin, and muscular system in which those nerves terminate; and, on the other hand, it is continuous with the brain. It serves, therefore, to conduct the impulses of touch-sensation from the skin upwards to the brain, and the motor impulses from the brain downwards to the muscles. But further, the cord is the great nerve centre concerned in reflex excito-motory actions. It must, also, be remembered that the two halves of the cord are anatomically continuous with each other through the nerve fibres of the commissures, so that it acts as a single organ, and not as two organs. Experiments have shown that sensory impulses are conducted upwards through the cord, not by that half from which the nerves arise that have been excited, but by the opposite half of the cord, which is obviously due to the crossing of the fibres of the posterior commissure. Motor impressions are, however, conducted downwards by that half of the cord from which the nerves arise that pass to supply the muscles to be moved. The spinal cord is well supplied with blood by numerous arteries, which terminate in a diffused capillary network. The capillaries are much more numerous in the grey matter of the cord than in the white columns. ORIGIN, ARRANGEMENT, AND DISTRIBUTION OF THE SPINAL NERVES. The spinal cord gives origin to thirty- one pairs of SPINAL nerves, which pass out of the spinal canal through the intervertebral foramina. These nerves are arranged in groups, according to the region of the spino through the foramina in which they proceed. There are eight pairs of cervical nerves; the first or sub-occipital emerges between the occipital bone and the atlas, the eighth between the seventh cervical and first dorsal vertebrae. Twelve dorsal or thoracic nerves pass out on each side in relation to the dorsal vertebrae : five pairs of lumbar nerves in the region of the loins; five pairs of sacral nerves through the sacral foramina; and one pair of coccygeal nerves through the lowest openings in the spinal canal. Each spinal nerve arises by two rooU, an anterior and a posterior, from the side of the cord. These roots are distinguished from each other both anatomically and physiologically. The posterior root has a swelling or ganglion on it, whilst no ganglion exists on the anterior root. The posterior root consists of sensory nerve fibres, i.e., of fibres which conduct impulses from the periphery into the nerve centre; whilst the anterior root is composed of motor nerve fibres, i.e., of fibres which conduct im pulses from the centre to the periphery. The ganglion is situated on the posterior root, as a rule, in the interverte bral foramen; but the lower sacral nerves have the ganglia on their posterior roots in the spinal canal. These ganglia contain bipolar nerve cells, and the nerve fibres, as they pass through each ganglion, are apparently connected with the poles of the cells. The roots of the spinal nerves vary in direction and length. Those of the cervical nerves are short, and run almost horizontally outwards to their respective intervertebral foramina; those of the dorsal are longer and more oblique; whilst the roots of the lumbar and sacral nerves, owing to the cord ending much above the foramina through which the nerves proceed, are very long, and form a leash of nerves in the lower part of the spinal canal, which surrounds the filum terminale, and, from its general resemblance in arrangement to the hairs of a horse s tail, has been named cauda equina. The anterior nerve root joins the posterior immediately outside the ganglion, and by then: junction a spinal nerve is formed. This nerve contains a mixture of both motor and sensory fibres, and is compound therefore in function. Almost immediately after its formation the nerve separates into two divisions, an anterior and a posterior, and each division, like the nerve itself, contains both motor and sensory fibres. The Posterior Primary Divisions of the spinal nerves, smaller than the anterior, are distributed both to the muscles and skin on the back of the axial part of the body. Their general arrangement is as follows : each division, with some three or four exceptions, subdivides into an. internal and an external branch. In the back of the neck and the back of the upper part of the chest, the external branches of these nerves supply the deep muscles; the internal branches pierce the muscles close to the spines of the vertebrae, and end in the skin; the internal branch of the second nerve, called great occipital, and that of the third cervical, pass to the skin over the occipital bone. In the back of the lower part of the chest and of the loins, the internal branches supply the deep muscles, the external branches pass to the skin, those of some of the lumbar nerves extending as far as the skin of the buttock. The Anterior Primary Divisions are not so uniform either in arrangement or distribution as are the pos terior. They supply the front and sides of the axial part of the neck and trunk, and the extremities. The anterior divisions of the twelve thoracic nerves have the most simple arrange ment. Each nerve, called from its position an inter costal nerve, runs out wards, immediately below the lower border of a rib. Fl - GS. Diagram of the arrangement of a i . . , ,-, pair of thoracic spinal nerves. SC, spinal and gives Origin to three cord; AR, anterior nerve root; I ll, pos-
- e r "!* with its ganglion; PD P? s-
tenor primary division ; AD, anterior primary division, or intercostal nerve; SG, sympathetic ganglion, with the com- municating branches between it and the anterior division; M, muscles, with the motor branches entering them ; LC, lateral cutaneous, and AC, anterior ^uneous nerves. series of branches, named Communicating, muscular, and riitanprm<? "Re- fhp us - D J 1 Gommunicatinq branch i , , J . each intercostal nerve IS connected with an ad- jacent ganglion on the thoracic portion of the sympathetic system. By the Muscular or motor branches these nerves supply the intercostal muscles, the levatores costarum, and the triangularis sterni, whilst the lower intercostal nerves run forwards and downwards into the wall of the abdomen, and supply the two oblique, the transverse, rectus, and pyramidalis muscles. The skin of the sides of the thorax and abdomen receives its nervous supply from the Lateral Cutaneous branches, whilst the skin on the front of the trunk is supplied by the Anterior Cutaneous terminations of these nerves. The lateral cutaneous branches of the second and third intercostal nerves are comparatively large in size,
and assist in the supply of the skin of the inner side of
