The meninges are a series of membranes that envelop and protect the central nervous system. They are made up of three layers: * The dura mater, a thick membrane that coats the inside of the skull; * The arachnoid mater, a spider web like, transparent, loose membrane that envelops the brain. * The pia mater, a thin, flexible membrane that surrounds the brain and spinal cord. Some diseases that affect the meninges are meningitis and meningioma.