Anatomy of the Mastoid Portion of the Facial Nerve

The chorda tympani arises on the average 5.3 mm proximal to the stylomastoid foramen (range, 1.2 mm distal to 10.9 mm proximal to the foramen). Only 6% originated less than 1 mm from the stylomastoid foramen. The mastoid segments of the fallopian canal of these temporal bones average 12.2 mm in length (range, 8.9 to 16.0 mm). The histograms of the diameters of myelinated fibers of the mastoid portion of the facial nerve proximal to the origin of the chorda tympani nerve in man usually are unimodal before 5 years of age and bimodal thereafter. Elimination of the contribution of the chorda tympani-nerve fibers to the fiber spectrum of the nerves from persons 10 years of age and older decreases the height of the small-fiber peak. Facial nerve size remains relatively constant from 7 to 69 years of age. The number of myelinated fibers per nerve decreases markedly after the fourth decade of life.