Next Lesson - Cranial Nerves I-VI
Core
The meninges are a series of connective tissue membranes which encapsulate the brain and spinal cord. They act as a supportive framework for the brain and create a space to hold the cerebrospinal fluid (CSF).
The meninges consist of three membranes:
- Dura Mater - the outermost thick, fibrous layer.
- Arachnoid Mater - a thinner intermediate layer.
- Pia Mater - a very thin and delicate layer, which is closely adhered to the grey matter of the brain.
Diagram - Layers of the meninges
Creative commons source by SVG by Mysid, original by SEER Development Team [1], Jmarchn [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Between the meningeal layers are two spaces:
- Subdural Space - a potential space between the meningeal dura and arachnoid mater. The subdural space is normally empty.
- Subarachnoid Space - the space between the arachnoid and pia mater. This space is filled with CSF and contains the cerebral vessels.
The dura mater has two layers:
- Periosteal Dura - the outer layer of dura, adhered to the inner table (the inner surface) of the skull bone. It is also called the endosteal layer.
- Meningeal Dura - the inner layer of dura, adjacent to the arachnoid mater.
In most parts, these two layers of dura are closely adhered to each other. However, there are some places where these layers separate, giving rise to dural venous sinuses. The meningeal layer also folds inwards to form dural folds.
In some places, the meningeal dura folds inwards on itself (this is called a reflection) to form dural folds. These are tough fibrous sheets which divide the cranial cavity into rigid compartments.
Important dural folds to note are:
- Falx Cerebri - runs within the longitudinal fissure of the brain (the groove between the two cerebral hemispheres) to separate the cranial cavity in the sagittal plane. It attaches anteriorly to the crista galli (a bony ridge on the ethmoid bone at the front of the skull base) and posteriorly to the superior surface of the tentorium cerebelli.
- Tentorium Cerebelli - separates the occipital lobes and cerebellum. It runs in the transverse plane. It forms a tent shape because the falx cerebri is attached along its upper surface.
Diagram - Anatomical prosection (a dissection specimen prepared for teaching) showing the falx cerebri and tentorium cerebelli
Creative commons source by Rabjot Rai, Joe Iwanaga, Gaffar Shokouhi, Rod J. Oskouian, R. Shane Tubbs [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
In some places, the periosteal and meningeal dura separate to form spaces which are filled with venous blood. These are called dural venous sinuses. These sinuses receive blood from the cerebral veins and eventually drain into the internal jugular vein.
Diagram - Schematic in the transverse plane showing the dural venous sinuses
Creative commons source by Wieslaw L. Nowinski, Thant S. L. Thaung [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The dural venous sinuses receive blood from:
- Bridging Veins - cerebral veins which cross the subdural space to drain blood from the brain into the sinuses.
- Emissary Veins - these veins connect the scalp veins (and other veins outside the skull) to the venous sinuses, passing through holes in the skull. They can carry infection from outside the skull into the sinuses.
For more information, check out our article on Blood Supply of the Head and Neck.
Head trauma can shear (tear) blood vessels, resulting in bleeding in and around the meninges. These bleeds are named by where the blood collects: extradural (outside the dura, between the dura and the skull), subdural (between the dura and the arachnoid mater) and subarachnoid (between the arachnoid and pia mater) haemorrhages.
Bleeding can also occur within the brain itself. This is an intracerebral haemorrhage, otherwise known as a haemorrhagic stroke.
An extradural haemorrhage arises from one of the meningeal arteries, particularly the middle meningeal artery. Blood accumulates between the periosteal dura and inner table (the inner surface) of the skull bone. This bleed does not usually cross the suture lines (the joins between the skull bones), because the dura is firmly attached there. Its CT appearance is a biconvex (lens-shaped, or lentiform) area of hyperdensity (it looks whiter than the brain on CT).
Image - Axial head CT showing an extradural haemorrhage (indicated by the red arrow)
Creative commons source by James Heilman, MD [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
A subdural haemorrhage arises from the bridging veins. Blood accumulates within the subdural space. The risk of subdural haemorrhage increases in the elderly due to cerebral atrophy (shrinkage of the brain). This stretches the bridging veins, which increases their tension and makes them more fragile. Subdural bleeds can be acute or chronic. On CT these bleeds are crescent-shaped. Acute bleeds appear hyperdense (brighter than the brain), whereas chronic bleeds appear hypodense (darker than the brain).
Image - Axial head CT showing an acute subdural haematoma (indicated by the red arrow)
Creative commons source by James Heilman, MD [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
A subarachnoid haemorrhage most often arises from a ruptured aneurysm (a bulge in the wall) of one of the cerebral arteries. These haemorrhages are sudden and can be life-threatening. For more information, check out our article on Subarachnoid Haemorrhage.
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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