The lipid-rich sheath surrounding axons in both the CENTRAL NERVOUS SYSTEMS and peripheral nervous system. The myelin sheath is an electrical insulator and allows faster and more energetically efficient conduction of impulses. The sheath is formed by the cell membranes of glial cells (schwann cells in the peripheral and oligodendroglia in the central nervous system). Deterioration of the sheath in demyelinating diseases is a serious clinical problem.