The membranes surrounding the BRAIN and SPINAL CORD. These membranes include the DURA MATER, a tough, fibrous membrane closely applied to the inside of the skull; the ARACHNOID MEMBRANE, a more delicate membrane, enveloping the brain but separated from its irregular surface by spaces containing fluid; and the pia mater, a delicate network of fibres containing blood vessels and uniting the arachnoid to the brain. The latter two are sometimes referred to as the pia-arachnoid.

These membranes bear the blood vessels which nourish the surface of the brain and the interior of the skull. Meningeal haemorrhage from these vessels forms one of the chief dangers arising from fracture of the skull.