The thickest and outermost of the three meninges surrounding the brain and spinal cord. It consists of two closely adherent layers, the outer of which is identical with the periosteum of the skull. The inner dura extends downward between the cerebral hemispheres to form the falx cerebri and forward between the cerebrum and cerebellum to form the tentorium. A thin film of fluid (not cerebrospinal fluid) separates the inner dura from the arachnoid.