The 3 membranes that cover and protect the brain and the spinal cord. The outer membrane, the dura mater, is tough and fibrous; it lines the inside of the skull and forms a loose sheath around the spinal cord. The middle membrane, the arachnoid mater, is elastic and web-like. The inner membrane, the pia mater, lies directly next to the brain. It is separated from the arachnoid mater by the subarachnoid space, which contains cerebrospinal fluid.


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