The outermost covering is the dura (Latin for “hard”) mater (Latin for “mother”), a tough, fibrous sheet composed of two layers. The outer periosteal layer is adherent to the skull, and the inner meningeal layer lies against the underlying arachnoid mater (eFig. 9.17). Although these two layers are closely adherent to one another, they do separate in some regions to form dural venous sinuses, which receive cerebral venous drainage (eFig. 9.18). The anatomy of the venous sinuses will be discussed in the “Cerebral Vasculature” section. Two potential spaces exist as the epidural (extradural) space, superficial to the periosteal layer, and subdural space, deep to the meningeal dural layer. These spaces can become filled with blood during vascular trauma (eTable 9.3). Within the cranial cavity, the meningeal layer of dura mater folds in on itself in several areas to form dural reflections, or septa. These reflections are known as the falx (Latin for “sickle”) cerebri between the cerebral hemispheres, as the tentorium cerebelli between the cerebral hemispheres and cerebellum, and as the falx cerebelli between the cerebellar hemispheres (eFig. 9.17). A smaller reflection, the diaphragm sellae, covers the pituitary fossa and underlying pituitary gland.