A unique, sheathlike structure that encloses major nerve trunks, somewhat like insulation around a wire. It is composed of approximately 80% lipid, the balance being made up of proteins, polysaccharides, salts, and water. The lipid fraction is composed of sphingolipids and glycerophosphates, which in turn contain long-chain fatty acids. It has a low concentration of polyunsaturated lipids and high concentration of long-chain sphingolipids. Its composition is essentially constant in different species of animals and also between adults and infants. The breakdown of the lipid structure of myelin is a characteristic of multiple sclerosis.