Myelinated or medullated nerve fibres are covered by two sheaths, outer neurilemma and inner medullary sheath. The medullary sheath is made of white shining lipid rich substance called myelin. Myelin or medullary sheath is secreted by Schwarm cells in peripheral nerve fibres and oligodendrocytes in central nervous system. The areas of myelinated nerve fibres devoid of myelin sheath are called nodes of Ranvier. Area of nerve fibre . between two adjacent nodes of Ranvier is called internode. Myelinated nerve fibres occur in white matter of central nervous system and in most peripheral nerves.