Thin membranous sheath around a nerve fiber. The outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system. A covering or wrapper of a nerve. Also called neurilemma, this thin membrane enwraps the myelin layers of some myelinated nerve fibers, especially peripheral ones, or the extensions of certain unmyelinated nerve fibers.