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SPINAL CORD

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Originally appearing in Volume V25, Page 669 of the 1911 Encyclopedia Britannica.
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SPINAL See also:CORD , in See also:anatomy, that See also:part of the central See also:nervous See also:system in See also:man which lies in the spinal See also:canal formed by the vertebrae, and reaches from the foramen magnum to the See also:lower margin of the first lumbar vertebra. It is about 18 in. See also:long, and only occupies the upper two-thirds of the spinal canal. The . cord is R4c1aNCiut l-.'.~ LIO. Denreov. protected by the same three Q o wTU. membranes which surround OsTER,OR s _ uRF~~~ P°' MATER the See also:brain. Outside is the (From See also:Gray's Anatomy, Descriptive and dura mater, which differs Surgical.) from that of the brain in FIG. I.—Transverse See also:Section of the not forming a periosteum Spinal Cord and its Membranes. to the bones, in sending no processes inward, and in having no See also: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 See also: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 See also: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 See also:

side of the cord, run a See also:series of processes of the pia mater arranged like the See also: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 See also:middle of the cerebro-spinal fluid. The spinal cord itself is a See also: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 See also:apex of which a glistening membranous See also:thread runs down among the nerves which See also: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 See also:diameter of the cord; it is lined by the pia mater, which, at its orifice, is thickened to form a glistening See also: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 See also: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 See also:

grey and See also:white See also: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 See also:cross See also:bar forming the grey commissure. In the middle of this the central canal can just be made DVn out by the naked See also: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 See also:projection, the lateral See also: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. See also:

Cunningham, in Cunning- 'See also:ham's See also:Text-See also: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 See also:Clarke's See also:column is also largely confined to the thoracic region, and lies in the mesial part of the posterior cornu. It is the See also:place to which the sensory See also:fibres of the sympathetic system (visceral afferents) run. The white matter, as has been shown, surrounds the grey and passes across the middle See also: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 See also:microscope. They FIG. 4.—Sections of Spinal Cord, have been and are still being twice See also: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 See also:

root it will be seen to See also:lie quite See also:close to the mesial side of the posterior See also: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 See also:area close to the postero-median fissure known as the See also:tract of Goll (fig, 3, G.T.), while the higher lie more externally in the tract of Burdach (B.T.).

The greater part of each nerve sooner or later enters the grey matter and comes into close relation with the cells of Clarke's column, but some fibres run right pp to the See also:

nucleus gracilis andcuneatus in the medulla (see BRAIN), while a few turn down and form a descending tract, which, in the upper part of the cord, is situated in the inner part of the tract of Burdach and is known as the See also:comma tract (fig. 3, C.T.), but lower down gradually shifts quite close to the postero-median fissure and forms the See also:oval area of Flechsig (fig. 3, O.A,.): It will be obvious that both these tracts could not be seen in the same section, and that fig. 3 is only a diagrammatic outline of their position. A few fibres of each sensory nerve ascend in a small area known as Lissauer's tract (fig. 3, L.T.) on the See also: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 See also: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 See also: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 See also:fillet and also to reach the cerebellum through the See also:superior cerebellar peduncle.

The crossed pyramidal tract (fig. 3, C.P.T.) lies See also:

internal to the direct cerebellar tract, between it and the posterior cornu. It is the See also:great motor tract, by which the fibres coming from the Rolandic area of the cerebral cortex are brought into See also:touch with the motor cells in the anterior cornu of the opposite side. This tract extends right down to the See also: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 See also: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 See also: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 See also:time the See also: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 See also:

mass like a See also: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. See also:Reid (Tourn. Anat. and Phys., xxi:i. 341). See also:Embryology.—The See also:early development of the neural See also:tube from the ectoderm is outlined in the See also:article on the BRAIN. When the neural groove becomes a tube it is oval in section with a very large laterally 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.

End of Article: SPINAL CORD

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