Meninges of the Brain

• The dura mater lines the interior of the cranium, while the pia mater covers the brain's surface.
• The arachnoid mater lies between the dura and pia mater, connecting to the pia via fine filaments, hence its spider-like name.
• The subarachnoid space, located between the arachnoid and pia mater, contains cerebrospinal fluid.
• The meninges consist of the dura mater, arachnoid mater, and pia mater.

General Arrangement

• The brain is suspended in cerebrospinal fluid, which reduces its effective weight from 1500 g to 50 g.
• Cerebrospinal fluid is held in a thin bag of arachnoid mater.
• The arachnoid mater is supported by the dura mater, a tough sheet of fibrous tissue, because it cannot handle hydrostatic pressure on its own.
• In the cranial cavity, the dura mater is largely fused with the periosteum lining the skull bones.
• The vascular pia mater closely covers the brain's surface, fitting snugly over its convexity against the arachnoid mater.
• Along the sulci and across the base of the brain, the pia mater is out of contact with arachnoid mater and "floats" freely in the cerebrospinal fluid.

Pia Mater

• The pia mater invests the brain and spinal cord like periosteum invests bone and contains blood vessels.
• No structure intervenes between the pia mater and the underlying nervous tissue.
• It invests the surface of the central nervous system to the depths of the deepest fissures and sulci.
• The pia mater is made of vascular fibrous tissue and can be stripped away from the brain surface.
• It extends over the cranial nerves and spinal nerve roots to fuse with their epineurium.
• The pia mater is invaginated into the brain by entering cerebral arteries, which lie loose in sheaths of pia and are surrounded by a perivascular space containing cerebrospinal fluid.
• The pia mater of the spinal cord projects laterally to form the denticulate ligament.

Arachnoid Mater and Subarachnoid Space

• The arachnoid mater is an impermeable, delicate membrane supported by the inner layer of the dura mater.
• A thin film of tissue fluid (lymph) lies in the subdural space between the dura and arachnoid mater, except at the aqueduct of the cochlea.
• Vessels and nerves pierce the dura and arachnoid mater at the same place and do not run between the two membranes.
• The subarachnoid space is beneath the arachnoid mater.
• In some areas, the pia and arachnoid fuse, obliterating the subarachnoid space, giving rise to the term "pia-arachnoid".
• Delicate strands connect the pia and arachnoid, and in the spinal part of the space, these strands form a thin posterior midline lamina, creating an incomplete posterior median septum.
• The subarachnoid space serves as a pathway for the circulation and absorption of cerebrospinal fluid after its escape from the fourth ventricle.
• Cranial nerves, spinal nerve roots, arteries, and veins of the brain and spinal cord pass through the subarachnoid space.
• The space extends to the termination of the spinal arachnoid and dura at the level of the S2 vertebra.
• Arachnoid villi are herniae of the arachnoid mater through the dura mater into the venous sinuses, allowing cerebrospinal fluid to 'ooze' back into the blood.
• Arachnoid villi are most numerous in the superior sagittal sinus and its laterally projecting blood lakes.
• In children, the villi are discrete, but with age, they aggregate into visible clumps called arachnoid granulations (Pacchionian bodies).
• Arachnoid granulations leave indentations on the inner table of the cranial vault alongside the superior sagittal sinus.
• Subarachnoid cisterns are larger spaces between the base of the brain and the base of the skull due to incongruities in the contours of bone and brain.
• The cerebellomedullary cistern (cisterna magna) is the largest, located between the cerebellum and the medulla, receiving cerebrospinal fluid from the foramen of Magendie in the fourth ventricle.
• The lateral part of the cisterna magna contains the vertebral artery and its posterior inferior cerebellar branch on each side.
• The cisterna magna can be tapped in the midline through the posterior atlanto-occipital membrane and spinal dura.
• The pontine cistern lies between the clivus and the pons and medulla, receiving cerebrospinal fluid from the foramina of Luschka of the fourth ventricle.
• It contains the basilar artery, its pontine and labyrinthine branches, and the fifth to twelfth cranial nerves.
• The interpeduncular cistern lies between the dorsum sellae and the cerebral peduncles, roofed by the floor of the third ventricle.
• The floor of the interpeduncular cistern is formed by the arachnoid membrane in contact with the dura mater, between the temporal lobes.
• It contains the terminal branches of the basilar artery (including the posterior part of the circle of Willis), the pituitary stalk, and the third and fourth cranial nerves.
• The chiasmatic cistern lies above the optic chiasma, beneath the rostrum of the corpus callosum.
• It contains the anterior communicating artery and the intracranial part of the optic nerves.

The meninges—dura mater, arachnoid mater, and pia mater—protect the brain. Cerebrospinal fluid cushions the brain, reducing its weight within the skull. The dura mater lines the cranium, the arachnoid mater connects to the pia, and the pia mater covers the brain's surface.