The pia mater is the delicate, innermost, mesh-like layer of the three meninges which surround the brain and spinal cord. The pia mater closely envelops the entire surface of the brain, running down into the fissures of the cortex. It joins with the ependyma which lines the ventricles to form choroid plexuses that produce cerebrospinal fluid. In the spinal cord, the pia mater attaches to the dura mater by the denticular ligaments through the arachnoid membrane.