The spinal cord is surrounded by three membranes (dura mater, arachnoid mater and pia mater; Fig. 8.34), which are continuous through the foramen magnum with the cranial meninges.
The dura mater surrounding the spinal cord forms a sheath that corresponds to the inner (meningeal) layer of the cranial dura (p. 374). The spinal dura is separated from the periosteum of the vertebral canal by the extradural (epidural) space (Figs 8.16 & 8.32), which contains fat, vessels and loose connective tissue. Anaesthetic agents are instilled into this space to produce epidural anaesthesia. Spinal dura is attached to the margins of the foramen magnum and via fibrous slips to the posterior longitudinal ligament within the vertebral canal. Inferiorly the dura covers the filum terminale (Fig. 8.35). At the level of the second sacral vertebra the dura attaches to the filum terminale and these continue onto the back of the coccyx to fuse with the periosteum.
Each spinal nerve root is surrounded by a sleeve of dura mater which extends through the intervertebral foramen before fusing with the epineurium of the spinal nerve.
The arachnoid mater is a delicate membrane that surrounds the spinal cord and the nerves within the vertebral canal. Above it, is continuous with the cranial arachnoid through the foramen magnum and below it ends at the level of the second sacral vertebra. The arachnoid is connected to the pia mater by numerous delicate strands that cross the subarachnoid space between the two meninges. This space is filled with cerebrospinal fluid and also contains blood vessels, which supply the spinal cord. The cerebrospinal fluid is produced in the ventricular system of the brain and circulates in the subarachnoid space around the spinal cord and brain (p. 376). Cerebrospinal fluid is usually sampled by inserting a needle between the arches of the third and fourth or fourth and fifth lumbar vertebrae. In this procedure, which is called a spinal tap or lumbar puncture, the needle is carefully orientated to pass between the spinous processes and enter the subarachnoid space below the termination of the spinal cord so that there is no danger of damage to the spinal cord. In spinal anaesthesia, anaesthetic agents may be injected into the subarachnoid space via a lumbar puncture technique. In a midline puncture, the needle would traverse skin, superficial tissues, supraspinous and interspinous ligaments, the epidural space, the dura and arachnoid mater.
The pia mater is a highly vascular layer that closely invests the spinal cord and nerves. On each side, the pia mater is attached to the dura by the ligamentum denticulatum between the anterior and posterior spinal roots. The pia surrounds the termination of the spinal cord (conus medullaris) and continues as the filum terminale as far as the posterior surface of the coccyx.