Meninges
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.
Pia 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.
