Deep to the dura mater is the arachnoid (from the Greek word arachne meaning “spider’s web”) mater. The outer layer of the arachnoid mater is composed of several layers of flattened cells that lie adjacent to the meningeal layer of the dura mater (eFig. 9.19). Strands of connective tissue extend from this outer layer to form arachnoid trabeculae, which connect internally to the pia mater (eFig. 9.19). The pia mater forms a thin, veil-like layer that closely follows the gyri and sulci on the surface of the brain. The pia and arachnoid matter are separated by a subarachnoid space that contains CSF and the major blood vessels supplying the brain.