A cell that forms the myelin sheath of nerve fibres (axons) of the peripheral nervous system. Each cell is responsible for a given stretch (called an internode) along any particular axon and typically serves several neighbouring axons simultaneously; adjacent internodes are separated by small gaps (nodes of Ranvier) where the axon is bare. During its development the cell wraps itself around the axon, so the sheath consists of concentric layers of Schwann cell membrane. These cells are named after the German physiologist Theodor Schwann (1810–82).