Dalley, 10th Edition

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Chapter 2 ■ Back

134

Spinal cord Gray matter White matter

Vertebral bodies:

Posterior ramus Anterior ramus Posterior root

Spinal cord

T10

Nerves:

Posterior rootlets

T10

Dura mater/ dural sac

Dura mater Arachnoid mater Pia mater

Anterior root Spinal ganglion Spinal nerve Subarachnoid space

T11

Spinal meninges

Arachnoid mater

T11

T12

Subarachnoid space

T12

Dural root sheath

Epidural fat

Conus medullaris

Anterolateral view

Cauda equina in lumbar cistern

The spinal dura forms the spinal dural sac , a long tubu lar sheath within the vertebral canal (Figs. 2.45B, 2.46, and 2.47). This sac adheres to the margin of the foramen mag num of the cranium, where it is continuous with the cranial dura mater. The sac is anchored inferiorly to the coccyx by the lum terminale externum ( coccygeal ligament ). The spinal dural sac is evaginated by each pair of posterior and anterior roots as they extend laterally toward their exit from the verte bral canal (Fig. 2.50). Thus, tapering lateral extensions of the spinal dura surround each pair of posterior and anterior nerve roots as dural root sheaths , or sleeves (Figs. 2.47, 2.48, and 2.51). Distal to the spinal ganglia, these sheaths blend with the epineurium (outer connective tissue covering of spinal nerves) that adheres to the periosteum lining the IV foramina. Innervation of Dura Mater. Nerve bers are distributed to the spinal dura by the ( recurrent ) meningeal nerves (see Figs. 2.30 and 2.31). The function of these afferent and sym pathetic bers is unclear, although it is known that the afferent bers supply pain receptors that are involved in the referred pain characteristic of spinal disorders and become irritated when there is in ammation of the meninges ( meningitis ). SPINAL ARACHNOID MATER The spinal arachnoid mater is a delicate, avascular mem brane composed of brous and elastic tissue that lines the spinal dural sac and its dural root sheaths. It encloses the CSF- lled subarachnoid space containing the spinal cord, spinal nerve roots, and spinal ganglia (Figs. 2.47, 2.48, 2.49, and 2.50). The spinal arachnoid is not attached to the spinal dura but is held against its inner surface by the pressure of the CSF. In a lumbar spinal puncture, the needle traverses the spinal dura and arachnoid simultaneously. Their apposition is the dura–arachnoid interface (Fig. 2.49), often errone ously referred to as the “subdural space.” No actual space occurs naturally at this site; it is, rather, a weak cell layer FIGURE 2.48. Spinal cord, spinal nerves, and spinal meninges. Three membranes (the spinal meninges) cover the spinal cord: dura mater, arachnoid mater, and pia mater. As the spinal nerve roots extend toward an IV foramen, they are surrounded by a dural root sheath (sleeve) that is continuous distally with the epineurium of the spinal nerve.

Filum terminale internum

Sacrum

Spinal (sensory) ganglia in dural sleeves

Filum terminale externum

Coccyx

Posterolateral view

FIGURE 2.47. Cauda equina and lum terminale in lumbar cistern. Note the dural sleeves extending from the dural sac, enclosing the spinal sensory ganglia.

SPINAL DURA MATER The spinal dura mater (or simply, spinal dura), composed mainly of tough brous tissue with some elastic bers, is the outermost covering membrane of the spinal cord (Figs. 2.47, 2.48, and 2.49). The spinal dura is separated from the peri osteum-covered bone and the ligaments that form the walls of the vertebral canal by the epidural space . This space is occupied by the internal vertebral venous plexus embed ded in a fatty matrix ( epidural fat ). The epidural space runs the length of the vertebral canal, terminating superiorly at the foramen magnum and laterally at the IV foramina, as the spinal dura adheres to the periosteum surrounding each opening, and inferiorly, as the sacral hiatus is sealed by the sacrococcygeal ligament.

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