The facial nerve is directly and indirectly involved in numerous pathological conditions affecting the temporal bone, ranging from infection to neoplasia. In each instance, a solid understanding of its complex anatomy is crucial to the physician's ability to both diagnose and treat disorders of the facial nerve.
The facial nerve (Figure 69–1) begins its development near the end of the first month of gestation, when the acousticofacial primordium, giving rise to both the facial and acoustic nerves, develops adjacent to the primordial inner ear, the otic placode. The geniculate ganglion, which arises from the second branchial arch, develops early in the second month of the gestation. Adjacent to the developing geniculate ganglion, the acousticofacial primordium differentiates into a caudal and a rostral trunk. The caudal trunk progresses into the mesenchyme of the second branchial arch, becoming the main trunk of the facial nerve. The rostral branch becomes associated with the first arch, eventually developing into the chorda tympani nerve, providing taste to the anterior two-thirds of the tongue. This development partially explains the close association of the chorda tympani with the facial nerve.
A schematic illustration demonstrating the embryology of the facial nerve. (A) The location of the primitive facial nerve in the developing embryo is shown in relation to other important nerves in the head and neck. (B) The location of the second branchial arch giving rise to the main trunk of the facial nerve is shown in relation to the other branchial arches. (C) Other derivatives of the second branchial arch are shown and help explain the complex innervation pattern of the facial nerve. (Reproduced with permission from Langman J & Sadler TW, Langman's Medical Embryology, 5th ed. Williams and Wilkins: Baltimore, MD; 1985.)
Both the geniculate ganglion and the nervus intermedius, arising from the second branchial arch, form independently of the motor division of the seventh nerve. During the sixth week of gestation, the motor division of the facial nerve establishes its position in the middle ear between the membranous labyrinth (an otic placode structure) and the developing stapes (a second arch structure). The nerve then passes into the mesenchyme of the second arch. During this time, the chorda tympani nerve becomes associated with the trigeminal nerve, which will carry the chorda tympani on its way to the tongue via the lingual nerve. The greater superficial petrosal nerve, which carries preganglionic parasympathetic fibers toward the pterygopalatine ganglion, also develops during this time period.
Most of the anatomic relationships of the facial nerve are established by the end of the second gestational month. Although the fallopian canal, the bony canal that transmits the facial nerve through the temporal bone, begins its development in the fifth gestational month, it ...