Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/920

Rh 8(56 ANATOMY [NERVOUS called the cervical or Irachial 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 re spectively 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 com missures 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 terminate. 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 sub division of each lateral half of the cord into the columns, and the arrangement of its nervous tissues, are well seen in trans verse sections through its sub stance. The cord is composed of white and grey matter. The white matter is external, FIG. 64. Transverse section through the spiral cord. A fin-ma +Vio r&amp;gt;n- AF, antcro-mediun, and PK, postero-median fissures; PC, posterior, LC, lateral, and AC, anterior columns; lumilS Of the COrd. AIi - anterior, and I ll, posterior nerve roots; C, central canal of cord, with its columnar cndothelial 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 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 com mencement of the filum terminate 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. 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. Well- formed stellate connective tissue corpuscles lie in this sup porting 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 com missure, 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 con nective 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 an terior cornu are large, distinct, and stellate, and form a well-defined group of nerve cells. Those of the pos terior 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 Kolando. 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 arrange ment : some become directly continuous with the axial cylin drical processes of the nerve cells; others pass into the an terior 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 un- branched processes. Gerlach s observations show that the branched processes of these cells become continuous with 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 ulti mate 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, if