This electron micrograph shows a cross section through an axon, its myelin wrap, and the associated myelin-producing Schwann cell from the the cochlear nerve of the cat. The external mesaxon is the location where the wrapping edges of the Schwann cell meet, as the cell extends membrane that encircles the axon. This image, by Enrico Mugnaini, is Figure 14 from Chapter 1 (The Cell Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.