An electrically insulating fatty layer that surrounds the axons of many neurons. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while oligodendrocytes supply it to those of the central nervous system. [ GOC:cjm GOC:jl http://www.neurolex.org/wiki/sao593830697 http://en.wikipedia.org/wiki/Myelin ]