The meninges are the three layers of connective tissue that wrap around the spine and brain. These layers are the: dura mater, arachnoid mater and pia mater.

Dura mater

The dura mater (“tough mother”) is the outermost layer of the meninges, made of dense connective tissue. While only one layer surrounds the spinal cord, it is divided in the cranium into an outer endosteal layer which is continuous with the pericranium (deepest layer of the scalp) through sutures and foramina, and an inner meningeal layer. These layers are only separated at the dural venous sinuses.

While mostly lining the skull interior beside the endosteal layer, the meningeal dura reflects on itself at certain places to form four dural folds/septa. These are the:

• falx cerebri: between the cerebral hemispheres
• tentorium cerebelli: between the cerebrum and cerebellum
• falx cerebelli: between the cerebellum
• diaphragma sellae: covers the pituitary gland in the sella turica

Arachnoid mater

The web-like middle layer, the arachnoid mater, travels with the meningeal dura. It is thicker in the cranium than the spine and connected to the pia by numerous fibrous filaments (arachnoid trabeculae), forming a cerebrospinal fluid (CSF) filled subarachnoid space. Since the arachnoid mater does not line cranial or spinal grooves (besides the longitudinal fissure), this space includes the cranial sulci, with subarachnoid cisterns being areas of particularly large CSF accumulations.

CLINICAL CORNER

Lumbar punctures occur at L3/L4. They can be diagnostic and collect CSF from the lumbar cistern, part of the subarachnoid space between L2 (where the spinal cord ends) and S2, or be therapeutic (e.g. spinal anaesthesia).

Pia mater

The pia mater is a thin vascular layer, thicker in the spine than cranium. It invests in the ventral median fissure of the spinal cord as well as the many grooves and sulci of the brain. Denticulate ligaments are collagenous thickenings of the pia that attach to the dura and suspend the spinal cord in CSF.

Dural spaces

Between the dura and vertebral periosteum, the spinal epidural space contains loose connective tissue, venous plexuses and lymphatics. The cranial epidural/extradural space between the endosteal layer and skull is only noticeable pathologically. The subdural space is a potential space between dura and arachnoid while the subarachnoid space is CSF filled between the arachnoid and pia.

CLINICAL CORNER

There are many types of intracranial haemorrhage including:
• Epidural: when meningeal arteries rupture, forming a crescent-shaped haematoma lifting periosteum from the bone.
• Subdural: when emissary veins (connecting extracranial veins with the dural venous sinuses) tear, often occurring after a minor head injury and in the elderly, forming a convex-shaped haematoma.
• Subarachnoid and cerebral: which can be spontaneous (e.g. aneurysm, haemorrhagic stroke) or result from head injury.