meninges (mĭnĭnˈjēz) [key], three membranous layers of connective tissue that envelop the brain and spinal cord (see nervous system). The outermost layer, or dura mater, is extremely tough and is fused with the membranous lining of the skull. In the brain it forms a vertical sheet that separates the cerebral hemispheres and a horizontal sheet that lies between the cerebrum and the cerebellum. The thin arachnoid membrane lies below and in close contact with the dura mater. The innermost layer, or pia mater, is in direct contact with the brain and spinal cord and contains the blood vessels that supply them. The pia mater and arachnoid membrane are separated by the subarachnoid space containing the cerebrospinal fluid, which carries nutrients, absorbs the impact of shocks, and acts as a barrier to disease organisms. Thus, the meninges provide a fluid-filled jacket for the protection of neural tissues and allow for the flexing and twisting of the vertebral column about the spinal cord.