Membranes enclosing brain.
Three membranes enclosing the brain and spinal cord. The thickest is the outermost layer, the dura mater; the middle layer is the arachnoid mater and the innermost layer is the pia mater. The cerebrospinal fluid circulates between the two innermost layers.

Any of the three membranes that envelop the brain and spinal cord
A membrane (one of 3) that envelops the brain and spinal cord; SYN. meninges.