The main trunk of the facial nerve emerges from the skull base at the stylomastoid foramen. It lies medial, deep, and slightly anterior to the middle of the mastoid process at the lower end of the tympanomastoid fissure. After giving off the posterior auricular and branches to the posterior digastric and stylomastoid muscles, it passes obliquely inferiorly and laterally into the substance of the parotid gland. The length of the facial nerve trunk that is visible to the surgeon is about 1,3 cm. It divides into the temporofacial and cervicofacial divisions at a point inferior to the lowest part of the bony external auditory meatus (Fig. 10-1). The average distance from the lowest point on the external bony auditory meatus to the bifurcation of the facial nerve is 2,3 cm (S.D. 0,28 cm) (1). Posterior to the parotid gland, the nerve trunk is at least 2 cm deep to the surface of the skin. The two divisions proceeds forward in the substance of the parotid gland and divide into their terminal branches (Fig. 10-2).
The marginal mandibular branch courses obliquely downward and anteriorly. It frequently arises from the main trunk well behind the posterior border of the mandible and crosses the posterior border in the lower one third of the ramus. This positioning leaves a void between the buccal branches and the marginal mandibular branch or branches through which the mandible can be approached safely (Fig. 10-3).
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