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INTRODUCTION

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The spinal cord is vulnerable to injury arising in the structures that surround it. More than in any other part of the nervous system, pathologic lesions impinging on the spinal cord often originate in the membranes or vertebral column that surround it. The neurologic clinician must, therefore, be very familiar with these structures and their relationship to the spinal cord.

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INVESTING MEMBRANES

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Three membranes surround the spinal cord: The outermost is the dura mater (dura), the next is the arachnoid, and the innermost is the pia mater (pia) (Figs 6–1 and 6–2). The dura is also called the pachymeninx, and the arachnoid and pia are called the leptomeninges.

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FIGURE 6–1

Schematic illustration of the relationships between the spinal cord, spinal nerves, and vertebral column (lateral view), showing the termination of the dura (dura mater spinalis) and its continuation as the filum terminale externum. (Compare with Fig 5–4.)

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FIGURE 6–2

Drawing of a horizontal section through a vertebra and the spinal cord, meninges, and roots. Veins (not labeled) are shown in cross section. The vertebra and its contents are positioned as they customarily would be with CT and MR imaging procedures.

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Dura Mater

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The dura mater is a tough, fibrous sheath that extends from the foramen magnum to the level of the second sacral vertebra, where it ends as a blind sac (see Fig 6–1). The dura of the spinal cord is continuous with the cranial dura. The epidural, or extradural, space separates the dura from the bony vertebral column; it contains loose areolar tissue and a venous plexus. The subdural space is a narrow space between the dura and the underlying arachnoid.

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Arachnoid Mater

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The arachnoid is a thin, transparent sheath separated from the underlying pia by the subarachnoid space, which contains cerebrospinal fluid (CSF).

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Pia Mater

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The pia mater closely surrounds the spinal cord and sends septa into its substance. The pia also contributes to the formation of the filum terminale internum, a whitish fibrous filament that extends from the conus medullaris to the tip of the dural sac. The filum is surrounded by the cauda equina, and both are bathed in CSF. Its extradural continuation, the filum terminale externum, attaches at the tip of the dural sac and extends to the coccyx. The filum terminale stabilizes the cord and dura lengthwise.

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Dentate Ligament

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The dentate ligament is a long flange of whitish, mostly pial tissue that runs along both lateral margins of the spinal cord between the dorsal and ventral rootlets (see Fig ...

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