Dura mater

The most superficial of the 3 meninges that surround the brain and spinal cord.  The dura mater has two layers, and outer periosteal or endosteal layer and an inner meningeal layer.  The periosteal layer is fused to the inside of the cranial cavity.  In most places the two layers are fused, but they do divide in certain regions to form the dural sinuses and folds.