called the cervical or brachial enlargement, extends from opposite the third cervical to the first dorsal vertebra, and from it arise the nerves which supply the upper limbs; the lower, called the crural or lumbar enlargement, is opposite the last dorsal vertebra, and supplies with nerves the lower limbs. The cord is almost completely divided into right and left lateral halves by two fissures, named respectively anterior and posterior median fissures, which do not quite reach the centre of the cord, for at the bottom of the anterior fissure are the transverse fibres of the anterior white commissure, and at the bottom of the posterior fissure the fibres of the posterior grey commissure. By these commissures the two halves of the cord are united together. The fibres of the posterior commissure surround a canal, called the central canal, which extends along the whole length of the cord, and even passes into the upper end of the filum terminale. This canal is lined by a ciliated columnar endothelium, and expands superiorly into the cavity of the fourth ventricle. Each lateral half of the cord is subdivided into three columns by two depressions, which mark the points of emergence of the roots of the spinal nerves. The anterior nerve roots pass through the antero-lateral depression or fissure, and between it and the antero-median fissure is the anterior column of the cord. The posterior nerve roots pass through the postero-lateral fissure, and between it and the postero-median fissure is the posterior column, whilst between the anterior and posterior nerve roots lies the lateral column. In the cervical region, the part of the posterior column which lies next the postero-median fissure is marked off by a fissure into a small internal or postero-median column. The subdivision of each lateral half of the cord into the columns, and the arrangement of its nervous tissues, are well seen in transverse sections through its substance. The cord is composed of white and grey matter. The white matter is external,. FIG. 64.-Transverse section through the spinal cord. and forms the columns of the cord. The grey matter is surrounded by the white, and has in each lateral half of the cord a crescentic shape. The horns of the crescent are directed towards the fissures of emergence of the nerve roots; the anterior horn is rounded; the posterior long and narrow. The proportion of grey matter to the white varies in different parts of the cord. At the commencement of the filum terminale there is scarcely any white matter; but the white matter increases in amount from below upwards, so that its absolute quantity is greatest in the cervical part of the cord. The grey crescents are thicker in the upper and lower enlargements than in the intermediate part. AF, antero-median, and PF, postero-median fissures; PC, posterior, LC, lateral, and AC, anterior columns: AR, anterior, and PR, posterior nerve roots; C, central canal of cord, with its columnar endothelial lining. The pia mater is shown investing the cord, sending processes into the anterior and posterior fissures, as well as delicate prolongations into the columns. The crescentic arrangement of the grey matter is shown by the darker shaded portion. The cord contains both nerve fibres and nerve cells. The external, columnar, white part of the cord consists of nerve fibres, with a supporting reticular framework of connective tissue and blood-vessels derived from the pia mater. Wellformed stellate connective tissue corpuscles lie in this supporting framework. The nerve fibres of the various columns extend longitudinally, and lie parallel to each other, so that in transverse sections through the columns the fibres are transversely divided. The individual fibres vary much in diameter, but in all the axial cylinder and medullary sheath can be distinctly seen. Wherever the nerve roots enter into the cord, the fibres of these roots pass transversely or obliquely in their course inwards to the grey matter. Horizontal fibres are also found in the white anterior commissure, and a similar appearance can be seen in the posterior commissure. Horizontal fibres have also been traced from the lateral columns into the adjacent part of the grey matter. The grey crescentic portion of the cord contains connective tissue, blood-vessels, nerve fibres, and nerve cells. The nerve fibres in the grey matter are numerous; and whilst some possess a medullary sheath, others consist only of the axial cylinder; they divide and subdivide, and, as Gerlach has shown, form a narrow-meshed network of extremely minute fibres. The nerve cells are multipolar, and are chiefly collected in the anterior and posterior horns of each crescent. The cells of the anterior cornu are large, distinct, and stellate, and form a well-defined group of nerve cells. Those of the posterior cornu are smaller in size, more elongated in shape, but with stellate branched processes. They are not so distinct as in the anterior horn, owing to the connective tissue with its corpuscles being so abundant. This tissue is best marked at the tip of the posterior horn, where it forms the substantia gelatinosa of Rolando. Lockhart Clarke has described an intermedio-lateral group of nerve cells situated at the outer side of the grey matter, about midway between the anterior and posterior horns, in the upper part of the cervical portion of the cord, and in the thoracic part between the brachial and crural enlargements. The course of the fibres in the cord and their relations to the nerve cells should now be considered. There can be no doubt that of the longitudinal fibres some ascend from below upwards, and conduct either excito-motory impulses to the regions of the spinal cord itself, or sen sory impulses to the brain. Other longitudinal fibres again descend from the brain and higher regions of the cord to the lower, and conduct motor and vaso-motor impulses from above downwards. The horizontal and oblique fibres of an anterior or motor nerve root enter the grey matter of the anterior cornu, and seem to have the following arrangement: some become directly continuous with the axial cylindrical processes of the nerve cells; others pass into the anterior commissure; others extend as far as the grey matter of the posterior horn. The nerve cells of the anterior cornu give origin, therefore, directly to nerve fibres by their unbranched processes. Gerlach's observations show that the branched of these cells become continuous with processes the network of extremely minute fibres already described in the grey matter; from this network medullated fibres appear to arise which leave the grey matter; some enter the lateral column, and ascend as the fibres of this structure; others pass as fibres of the anterior commissure to the opposite side of the cord, and ascend as the anterior column of that side. The anterior and lateral columns, therefore, are constantly receiving accessions of fibres from the enclosed grey matter. The fibres of a posterior or sensory nerve root on entering the cord subdivide into two bundles; one does not enter the grey matter, but applies itself to the posterior column, of which it forms some of the vertical fibres. These fibres may ascend to the brain, or they may at some higher point in the cord enter the grey matter of the posterior horn. The other bundle of posterior root fibres at once enters the posterior horn of grey matter. The connections and ultimate arrangement of these fibres in the grey matter have not been satisfactorily made out. Gerlach states that, as they frequently subdivide on entering the grey matter, it Spinal erves. is possible they may form the fine nerve fibre plexus of the The anterior nerve root joins the posterior immediately outside the ganglion, and by their 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. 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 The structure of the spinal cord shows it to be both a its general resemblance in arrangement to the hairs of a nerve centre and a conductor of nervous impulses. The horse's tail, has been named cauda equina. 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 coluinns 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 thirtyone 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 spine 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 vertebræ. Twelve dorsal or thoracic nerves pass out on each side in relation to the dorsal vertebræ: 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 roots, 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 impulses from the centre to the periphery. The ganglion is situated on the posterior root, as a rule, in the intervertebral 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 PD. AD 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 vertebræ, 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 arrangement. Each nerve, called from its position an intercostal nerve, runs outwards, immediately below the lower border of a rib, FIG. 65.-Diagram of the arrangement of a and gives origin to three series of branches, named communicating, muscular, and cutaneous. By the Communicating branch each intercostal nerve is connected with an adjacent 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. AC pair of thoracic spinal nerves. SC, spinal cord; AR, anterior nerve root; PR, posterior root, with its ganglion; PD, posterior primary division; AD, anterior primary division, or intercostal nerve; SG, sympathetic ganglion, with the communicating branches between it and the anterior division; M, muscles, with the motor branches entering them; LC, lateral cutaneous, and AC, anterior cutaneous 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 Nervous nerves. In the regions of the neck, loins, and pelvis, the anterior plexuses. divisions of the spinal nerves do not pass simply outwards to their distribution. In each region adjacent nerves interlace with each other, and form what is technically called a nervous plexus. When a branch arises from a thoracic nerve, it contains fibres derived from that nerve only; but when a branch arises from a plexus, it may contain fibres, not of one only, but of two or more of the nerves which, by their interlacement, form the plexus. Hence the parts which are supplied by these branches are brought into connection with a greater number of nerves, and consequently with a greater extent of the spinal cord or nerve centre, than are the parts which receive branches from a single nerve only. These plexuses are especially found in connection with the nerves which supply the extremities, where, owing to the complexity of the muscular movements, the co-ordination of these movements through the nervous system is rendered necessary. the upper arm; hence they are called intercosto-humeral | colli, rhomboid, and subclavius muscles; the supra and The anterior divisions of the eight cervical nerves are The Brachial plexus (Pl. XVII. 1, 2, 3, 4) is formed The Lumbar plexus, of large size, is situated at the back of the abdominal cavity in the region of the loins, and is formed by the four upper lumbar nerves, which form a series of loop-like interlacements in front of the transverse processes of the lumbar vertebræ. It gives origin to communicating, muscular, cutaneous, and mixed branches. The Communicating branches join the four upper lumbar ganglia of the sympathetic system. The Muscular branches supply the quadratus lumborum muscle, and give branches to the psoas. The Cutaneous branches are named-a, Ilio-hypogastric, which gives an iliac branch to the skin of the buttock, and a hypogastric branch to the skin of the abdomen above the pubic symphysis; b, |