Three membranes that surround the central nervous system and line its bony coverings. They consist of the dura or dura mater, which lines and adheres to the brain case and the vertebral canal; the arachnoid, or arachnoid mater, a gossamer membrane just below; and the pia or pia mater, which adheres very tightly to the underlying structures of the central nervous system and the take-off of the nerve roots. Spinal fluid circulates between the pia and the arachnoid, in the subarachnoid space.

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