Myelin sheath : A covering of axons (Kölliker, 1896)-and occasionally dendrites (His, 1889) or even cell bodies (Deiters, 1865)-above a particular diameter characteristic for each species that speeds up action potential conduction velocity and is formed by types of glia (Virchow, 1846): oligodendrocyes in the vertebrate central nervous system (Meckel, 1817), and Schwann cells in the vertebrate peripheral nervous system (Meckel, 1817); see Williams (1995, pp. 951-954), Toesca (1996). The myelin sheath was probably discovered by Remak (1837) who described a very thin-walled tube around his primitive band, the axon; see Van der Loos (1967, p. 19).