Which layer of meninges forms the tough and fibrous outer sac?

The tough and fibrous outer sac of the meninges is formed by the dura mater. This layer provides protection to the brain and spinal cord, and it is the thickest of the three meninges. The dura mater also has two layers: the periOSteal layer, which is attached to the skull, and the meningeal layer, which covers the brain and spinal cord.