Deep to the arachnoid mater is the only normally occurring fluid-filled space associated with the meninges, the subarachnoid space (Fig. 8.35). It occurs because the arachnoid mater clings to the inner surface of the dura mater and does not follow the contour of the brain, while the pia mater, being against the surface of the brain, closely follows the grooves and fissures on the surface of the brain. The narrow subarachnoid space is therefore created between these two membranes (Fig. 8.35).