Subarachnoid space

The Subarachnoid Cavity (subarachnoid space) is the interval between the arachnoid and pia mater.

It is occupied by a spongy tissue consisting of trabeculæ of delicate connective tissue, and intercommunicating channels in which the subarachnoid fluid is contained.

This cavity is small on the surface of the hemispheres of the brain; on the summit of each gyrus the pia mater and the arachnoid are in close contact; but in the sulci between the gyri, triangular spaces are left, in which the subarachnoid trabecular tissue is found, for the pia mater dips into the sulci, whereas the arachnoid bridges across them from gyrus to gyrus.

At certain parts of the base of the brain, the arachnoid is separated from the pia mater by wide intervals, which communicate freely with each other and are named subarachnoid cisternæ; in these the subarachnoid tissue is less abundant.