Meninges

Three layers wrap the central nervous system. The dura mater is the tough outermost membrane; over the brain it has two layers, an outer periosteal layer against the skull and an inner meningeal layer, which separate in places to form the dural venous sinuses and fold inward as the falx cerebri and tentorium cerebelli. The arachnoid mater is a delicate avascular middle layer. The pia mater is a thin vascular membrane that adheres directly to the surface of the brain and spinal cord, following every gyrus and sulcus.

The spaces between and around these layers are clinically important. The subarachnoid space, between arachnoid and pia, holds cerebrospinal fluid and the major cerebral vessels; CSF returns to venous blood through arachnoid granulations projecting into the superior sagittal sinus. The epidural space above the dura contains the middle meningeal artery over the skull, and the subdural space is a potential space crossed by bridging veins.

Which vessel tears determines the pattern of bleeding. Epidural hematoma typically follows temporal bone fracture with middle meningeal artery injury, producing a rapidly expanding lens-shaped collection that does not cross suture lines, often after a lucid interval. Subdural hematoma comes from torn bridging veins, is more common in the elderly and in alcohol use disorder, and forms a crescent-shaped collection that crosses sutures but not the midline. Subarachnoid hemorrhage, classically from a ruptured berry aneurysm, presents as a sudden severe headache. Infection or inflammation of these layers is meningitis, which causes headache, fever, and nuchal rigidity and is sampled by lumbar puncture below the level of the spinal cord.

USMLE Step 1 tests meningeal anatomy through hemorrhage patterns, CSF flow, and headache differentials. CCMA and NPTE-PTA candidates cover the meninges as part of central nervous system structure, including recognition of meningitis signs and the protective role of CSF.