The meninges is the system of membranes which envelops the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater.
The arachnoid mater is the midddle and is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater. It is separated from the pia mater by the subarachnoid space.
It is, attached to the inside of the dura, surrounds the brain and spinal cord. Cerebrospinal fluid flows under this membrane in the subarachnoid space.
The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or leptomeninges..