Artist illustration of cranial meninges. Dura mater, the most superficial menix, also functions as periosteum for bones of the cranial cavity. Arachnoid is applied to the deep surface of dura mater. Arachnoid trabeculae connect to pia mater. Pia mater, the deepest menix, is attached to a glial limiting membrane formed by surface astrocytes of the brain. The subarachnoid space between arachnoid and pia mater is lined by flattened fibrocytes and contains cerebrospinal fluid.