This electron micrograph is a cross section through a portion of an axon and its associated myelin wrap from the cochlear nerve of the cat. This high magnification view showcases the regular periodicity with which the myelin wraps the axon. Each dense line represents where the internal surfaces of the apposed Schwann cell membranes are in contact. The faint intraperiod lines are where the outer surfaces of the two membranes are in contact. This micrograph, by Enrico Mugnaini is Figure 15 from Chapter 1 (The Cell Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D., and accompanies a lower magnification micrograph shown in Fig. 14 of this chapter. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.