Myelin is an electrically-insulating ielectric phospholipid layer that surrounds only the axons of many neurons. It is an outgrowth glial cell: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes supply it to those of the central nervous system. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates. Myelin was discovered in 1878 by Louis-Antoine Ranvier.