Spinal Cord

Meninges. The cord is surrounded by dura, arachnoid, and pia mater, which are continuous with the corresponding layers of the cerebral meninges at the foramen magnum. The spinal dura mater, unlike the cerebral, consists only of a meningeal layer that is not adherent to the vertebrae; it is separated from the boundaries of the vertebral canal by an epidural space containing fatty areolar tissue and many veins. The spinal and cranial subarachnoid spaces are continuous and contain cerebrospinal fluid. The pia mater closely invests the cord; on each side, it sends out a series of 22 triangular processes, the denticulate ligaments, which are attached to the dura mater and thus anchor the cord (see Plate 2-2). The spinal cord is considerably smaller than the vertebral canal; the meninges, the cerebrospinal fluid and the epidural fatty tissue and veins combine to cushion it against jarring contacts with its bony and ligamentous surroundings.

Spinal Nerves. There are 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) of symmetrically arranged spinal nerves, attached to the cord in linear series by anterior and posterior nerve rootlets, or filaments, which coalesce to form the nerve roots. Each posterior spinal nerve root possesses an oval enlargement, the spinal (sensory) ganglion.

In early embryonic life, the cord is as long as the vertebral canal, but as development proceeds, it lags behind the growth of the vertebral column. Consequently, the cord segments move upward in relation to the vertebrae, and the nerve roots, originally horizontal, assume an increasingly oblique direction from above downward as they proceed to their foramina of exit. In the adult, except in the upper cervical region, the cord segments lie at varying distances above the corresponding vertebrae. For clinical purposes, it is customary to localize them in relation to the vertebral spinous processes. In the lower cervical region, the vertebral spines are one lower in number than the corresponding cord segments; in the upper thoracic region, two lower in number; and in the lower thoracic region, three lower in number. For example, the fourth thoracic spinous process is approximately level with the sixth thoracic cord segment. The lumbar, sacral, and coccygeal segments of the cord are crowded together and occupy the space approximately opposite the ninth thoracic to the first lumbar vertebrae. These alterations of the cord segments relative to the vertebral segments explain why the cervical enlargement (C4 to T2) lies approximately opposite the corresponding vertebrae, whereas the lumbar enlargement (L3 to S3) lies opposite the last three thoracic vertebrae. The nerve roots attached to the lower part of the cord descend to their points of exit as the cauda equina, named for their resemblance to the tail of a horse.

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