part of part of surrounds surrounds composed primarily of Schwann cell axon peripheral nervous system cell layer sheath of Schwann http://upload.wikimedia.org/wikipedia/commons/3/34/Gray631.png FMA def is 'Neural tissue which consists of Schwann cells' which does not fit, but we still use 'neural tissue' here. WP def specifically states Schwann cells, which would restrict it to PNS UMLS:C0027807 neurilemma Schwann's membrane MESH:D009441 NCIT:C33544 endoneural membrane UBERON:0001031 sheath of Schwann BTO:0003048 GAID:730 neurolemma Wikipedia:Neurilemma 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. The neurolemma is underlain by the basal lamina (referred to as the medullary sheath in the included illustrations. In CNS, axons are myelinated by oligodendrocytes, thus lack neurolemma. FMA:62976