A soft white substance constituting the medullary sheaths of nerve fibers, and composed mainly of cholesterin, lecithin, cerebrin, protein, and some fat.

One of a group of phosphorized principles occurring in nerve tissue, both in the brain and nerve fibers.

The whitish, fatty substance forming the segmented, multilayered wrappings or "sheaths" around certain long nerve fibers or axons. Myelin sheaths electrically insulate axons, serving to speed the transmission of nerve signals (action potentials).

The fat-like substance which surrounds the axon of nerve fibers and forms an insulating material.