Dura Mater: Layers and Folds

• The dura mater is described as having outer and inner layers, but this is somewhat misleading.
• The outer layer is essentially the periosteum, which covers all bones.
• This outer layer is continuous with the pericranium at the foramina of the skull and remains in contact with the bone.
• The inner layer is a dense, fibrous membrane and is considered the actual dura mater.
• In most areas, the inner and outer layers are fused and difficult to separate.
• Structures like the middle meningeal vessels appear to be extradural, located on the outer surface of the fused layers.
• The inner layer separates from the outer layer in certain areas to form the venous sinuses of the dura mater.
• Folds of the inner layer project into the cranial cavity.
• These folds include: the tentorium cerebelli, falx cerebri, falx cerebelli, and diaphragma sellae.
• The function of these folds is to minimize rotary displacement of the brain.
• At the foramen magnum, the inner layer separates to form the spinal dura mater.
• The inner layer also evaginates around the cranial and spinal nerve roots.

Tentorium Cerebelli

• A flange of the inner dural layer projects from the margins of the transverse and superior petrosal sinuses.
• It attaches to the posterior clinoid processes, the upper borders of the petrous temporal bones, and the internal occipital protuberance.
• The superior petrosal and transverse sinuses separate the upper and lower layers at their bony attachments.
• The free margin is U-shaped, located higher than the bony attachment, and forms the tentorial notch.
• The tentorial notch allows passage of the midbrain.

Falx Cerebri

• A sickle-shaped fold of the inner layer lies between the cerebral hemispheres
• Attaches anteriorly to the crista galli and foramen caecum
• The posterior margin is attached to the upper surface of the tentorium cerebelli.
• Its convex upper border attaches along the midline of the inner skull surface, from the foramen caecum to the internal occipital protuberance.
• The superior sagittal sinus is located between the two layers above the foramen caecum.
• The inferior sagittal sinus is contained within the free lower border, positioned just above the corpus callosum.

Falx Cerebelli

• A small elevation of the inner layer in the midline of the posterior cranial fossa.
• It extends from the internal occipital protuberance to the posterior margin of the foramen magnum.
• The occipital sinus is located between its layers.

Diaphragma Sellae

• A horizontal sheet of the inner layer forms the roof of the pituitary fossa.
• The dura of the floor of the fossa extends up the sides, positioned between the middle and posterior clinoid processes.
• It has a central perforation for the passage of the pituitary stalk, and extends laterally to form the roof of the cavernous sinus.

Blood Supply

• The inner layer has a minimal blood supply.
• The outer layer is richly supplied, along with the adjacent bone.
• In the supratentorial part: the middle meningeal artery, meningeal branches of the ophthalmic and anterior ethmoidal arteries, and meningeal branches of the internal carotid and the accessory meningeal artery supply blood.
• The accessory meningeal artery enters through the foramen ovale.
• In the posterior fossa, meningeal branches of the vertebral artery supply blood.
• All the arteries lie between the two dural layers.

Middle Meningeal Artery

• A branch of the maxillary artery, it arises in the infratemporal fossa.
• It passes through the foramen spinosum, accompanied by a sympathetic nerve plexus.
• Veins accompany it and create grooves on the inner skull surface.
• It divides into anterior and posterior branches on the greater wing of the sphenoid.
• The frontal (anterior) branch ascends towards the pterion, curves back, and ascends towards the vertex over the precentral gyrus.
• The parietal (posterior) branch courses horizontally backward, grooving the squamous part of the temporal bone and ramifying over the posterior skull.
• Supplies the bones of the vault of the skull.
• Scalping does not cause necrosis of the underlying bones due to the bone's primary blood supply being the middle meningeal artery

Middle Meningeal Veins

• Sinuses in the dura mater that accompany the branches of the middle meningeal artery.
• They lie between the artery and the bone, grooving the latter.
• Some veins converge to exit the skull through the foramen spinosum and foramen ovale to join the pterygoid plexus.
• Most middle meningeal veins join the sphenoparietal sinus.

Surface Markings

• The foramen spinosum is located just above the articular eminence at the posterior root of the zygomatic arch.
• Surgically approach the frontal branch through a burr hole at the pterion, 3 cm above the midpoint of the zygomatic arch.
• The posterior branch runs parallel with the upper border of the zygomatic arch and the supramastoid crest, usually vertically above the mastoid process at the level of the upper margin of the orbit.

Nerve Supply

• The ophthalmic division of the trigeminal nerve supplies most of the supratentorial part of the dura mater.
• Tentorial nerves supply the falx, dura of the vault, and upper surface of the tentorium cerebelli.
• The anterior ethmoidal and maxillary nerves provide twigs to the anterior cranial fossa.
• The middle meningeal nerve (a branch of the maxillary division of the trigeminal nerve) supplies the anterior portion of the middle fossa.
• The meningeal branch of the mandibular nerve (nervus spinosus) supplies the posterior part of the middle fossa.
• The ninth and tenth cranial nerves supply the posterior fossa.
• The upper three cervical nerves supply the dura around the foramen magnum suggesting the spinal dura migrated upwards to surround the brain.

Explore the dura mater, the outermost layer of the meninges. It features inner and outer layers, with the outer layer acting as the periosteum. Folds of the inner dural layer, such as the tentorium cerebelli and falx cerebri, minimize brain displacement.