MYELIN

white, insulating sheath on the axon of many nerve fibres. It is composed of fatty materials, protein, and water. The myelin sheath is deposited by Schwann cells in layers surrounding the nerve fibres of the central and peripheral nervous systems of many animals. Nerve fibres containing myelin sheaths are white. Nonmyelinated nerve fibres form gray nerves, which are common in invertebrates and in the sympathetic nervous system of vertebrates.