The spinal cord is enclosed in three fibrous layers called the meninges. From superficial to deep, they include: the dura mater, arachnoid mater, and pia mater. The dura mater forms a dural sheath, a tough collagenous membrane around the cord. The arachnoid mater consists of a simple squamous epithelium adhering to the inside of the dura. The subarachnoid space is filled with CSF. The pia mater is a delicate, transparent membrane that closely lines the spinal cord.