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Encyclopedia Britannica - Main :: SOU-STE |
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SPINAL CORD , in anatomy, that part of the central nervous system in man which lies in the spinal canal formed by the vertebrae, and reaches from the foramen magnum to the lower margin of the first lumbar vertebra. It is about 18 in. long , and only occupies the upper two-thirds of the spinal canal. The . cord isR4c1aNCiut l-.'.~ LIO. Denreov. protected by the same three Q o wTU. membranes which surround OsTER,OR s _ uRF~~~ P' MATER the brain. Outside is the (From Gray's Anatomy, Descriptive and dura mater, which differs Surgical.) from that of the brain in FIG. I.Transverse Section of the not forming a periosteum Spinal Cord and its Membranes. to the bones, in sending no processes inward, and in having no blood sinuses enclosed within its walls. In other words the spinal dura mater is the continuation of only the inner or cerebral layer of the dura mater of the skull. Inside the dura mater is the arachnoid, which is delicate and transparent, while between the two lies the sub-dural space, which reaches down to the second or third sacral vertebra. The pia mater is the innermost covering, and is closely applied to the surface of the cord into the substance of which it sends processes. Between it and the arachnoid is the sub-arachnoid space, which is much larger than the sub-dural and contains the cerebro-spinal fluid. Across this space, on each side of the cord, run a series of processes of the pia mater arranged like the teeth of a saw; by their apices they are attached to the dura mater, while their bases are continuous with the pia mater surrounding the cord.. These ligaments, each consisting of twenty-one teeth, are the ligamenta denticulata, and by them the spinal cord is moored in the middle of the cerebro-spinal fluid.The spinal cord itself is a cylinder slightly flattened from before backward. In the cervical region it is enlarged where the nerves forming the brachial plexus come off, while opposite the lower thoracic vertebrae the lumbar enlargement marks the region whence the lumbo-sacral nerves are derived. (See fig. 2.) Opposite the second lumbar vertebra the cylindrical cord becomes pointed and forms the conus medullaris, from the apex of which a glistening membranous thread runs down among the nerves which form the cauda equina, and, after blending with the termination of the dural sheath, is attached to the back of the coccyx.In a transverse section of the cord two median fissures are seen; the antero-median (see fig. 3, A) is wide, and reaches about a third of the way along the antero-posterior diameter of the cord; it is lined by the pia mater, which, at its orifice, is thickened to form aglistening hand, known as the linea splendens; in front of this lies the single anterior spinal artery. The postero-median fissure (fig. 3, P.) is much deeper and narrower, and has no reflection of the pia mater into it. Where the posterior nerve roots emerge (fig. 3, P.R.) is a depression which is called the posterolateral fissure, while between this and the postero-median a slight groove is seen in the cervical region, the paramedian fissure (fig. 3, P.M ; see also fig. 2). On looking at fig. 3 it will be seen that the anterior nerve roots (A.R.) do not emerge from a definite fissure. The spinal cord, like the brain, consists of grey and white matter, but, as there is here no representative of the cortical grey matter of the brain, the white matter entirely surrounds the grey. In section the grey matter has the form of an H, the cross
DVn out by the naked eye (see fig. 4). The anterior limbs of the H form the anterior cornua, while the posterior; which in the greater part of-the cord are longer and thinner, are the pos- terior cornea. At the tips of these is a lighter-coloured cap (fig. 3, S.G.) which is known as the substantia gelatinosa Rolandi. On each side of the H is a slighter projection, the lateral cornu, which is best marked in the thoracic region (see fig. 4). On referring to fig. 4 it will be seen that the grey matter has different and characteristic appearances in different regions of the cord, and it will be noticed that in the cervical and lumbar enlargements, where the nerve to the limbs comes off, the anterior horns are broadened. O.A P p (From D. J. Cunningham, in Cunning- 'ham's Text-Book of Anatomy.) CV( shows the level of the 1st cervical vertebra; CVv of the 5th cervical vertebra; DVTI of the 2nd dorsal vertebra; DVx of the loth dorsal vertebra; DVxii of the 12th dorsal vertebra; L\'n of the 2nd lumbar vertebra. The posterior vesicular or Clarke
which the sensory fibres of the sympathetic system (visceral afferents) run. The white matter, as has been shown, surrounds the grey and passes across the middle line to form the while commissure, which lies in front of the grey. It is composed of neuroglia and medullated nerve fibres, which are arranged in definite :tracts, although in a section of a healthy cord these tracts cannot be distinguished even with the microscope. They FIG. 4.Sections of Spinal Cord, have been and are still being twice scale of nature. gradually mapped out by 1. Cervical enlargement. pathologists, physiologists 2. Thoracic region. and embryologists. 3. Lumbar enlargement. On tracing a sensory nerve 4. Sacral region. into the cord (fig. 3; P.R.) through the posterior nerve root it will be seen to lie quite close to the mesial side of the posterior horn of grey matter, where most of it runs upward. The next root higher up takes the same position and pushes the former one toward the middle line, so that the lower nerve fibres occupy an area close to the postero-median fissure known as the tract
tract
Clarke
oval
A few fibres of each sensory nerve ascend in a small area known as Lissauer's tract (fig. 3, L.T.) on the outer side of the posterior nerve roots, and eventually enter the substantia gelatinosa. To the outer side of Lissauer's tract and lying close to the lateral surface of the cord is the direct cerebellar tract (fig. 3, D.C.T.), the fibres of which ascend from the cells of Clarke's column to the cerebellum. As Clarke's column is only well developed in the thoracic region this tract obviously cannot go much lower. In front of the last and also close to the lateral surface of the cord is another ascending tract, the tract of Gowers (fig. 3, T.G.), or, as it is sometimes called, the lateral sensory fasciculus. It probably begins in the cells of the posterior horn,,and runs up to join the fillet and also to reach the cerebellum through the superior cerebellar peduncle. The crossed pyramidal tract (fig. 3, C.P.T.) lies internal to the direct cerebellar tract, between it and the posterior cornu. It is the great
touch with the motor cells in the anterior cornu of the opposite side. This tract extends right down to the fourth sacral nerve.In front of the crossed pyramidal tract is the lateral basis bundle (fig. 3, L.B.B), which probably consists of association fibres linking up different segments of the cord. The anterior basis bundle (fig. 3, A.B.B.) lies in front and on the mesial side of the anterior cornu, and through it pass the anterior nerve roots. Like the lateral bundle it consists chiefly of association fibres, but it is continued up into the medulla as the posterior longitudinal bundle to the optic nuclei. The direct pyramidal tract (fig. 3, D.P.T.) is a small bundle of the motor fibres from the Rolandic area, which, instead of crossing to the other side at the decussation of the pyramids in the medulla, runs down by the side of the antero-median fissure. Its fibres, however, keep on gradually crossing to the opposite side through the anterior white commissure of the cord, and by the time the mid-thoracic region is reached it has usually disappeared. The roots of the spinal nerves in the upper part of the canal rise from the cord nearly opposite the points at which they emerge between the vertebrae, but the farther one passes down the higher the origin of each root becomes above its point of emergence. Consequently the lumbar and sacral nerves run a long way down from the lumbar enlargement to their spinal foramina and are enclosed in the dural and arachnoid sheaths to form a mass like a horse's tail, which is therefore known as the cauda equina. The relation between the origin of each nerve and the spinous processes of the vertebrae has been worked out by R. W. Reid (Tourn. Anat. and Phys., xxi:i. 341).Embryology.The early development of the neural tube from the ectoderm is outlined in the article on the BRAIN. When the neural groove becomes a tube it is oval
D PT &R. in the Spinal Cord. A. Antero-median fissure. P. Postero-median fissure. A.R. Anterior nerve roots. P. R. Posterior nerve roots. P.M. Paramedian fissure. S.G. Substantia gelatinosa. G.T. Tract of Goll. B.T. Tract of Burdach. C.T. Comma tract. O.A. Oval area. L.T. Lissauer's tract. D.C.T. Direct cerebellar tract. T.G. Gowers' tract. C,P.T. Crossed pyramidal tract. L.B.B. Lateral basis bundle. A.B.B. Anterior basis bundle. D.P.T. Direct pyramidal tract. Histologically the grey matter is made up of neuroglia, medullated and non-medullated nerve fibres, and nerve cells (for details see End of Article: SPINAL CORD If you wish, you can link directly to this article.
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