Three fibrous membranes that surround the brain and spinal cord to protect the central nervous system. The pia mater, a very thin membrane, adheres to the surface of the brain and spinal cord. The subarachnoid space, containing cerebrospinal fluid, separates the pia mater from a second membrane, the arachnoid. Around the brain, fine filaments connect these two membranes, which are believed to be impermeable to fluid. The third membrane, the dura mater, is strong, thick, and dense. It envelops the arachnoid, covers the inside of the skull, and surrounds and supports the large venous channels carrying blood from the brain. Several septa divide it and support different parts of the brain. In the spine, the dura mater and the arachnoid mater are separated by the subdural space; the arachnoid and pia mater are separated by the subarachnoid space. The extradural space (between the dura mater and the wall of the vertebral canal) is the site of epidural anesthesia (seeanesthesiology).