The trigeminal nerve [V] divides into three major divisions— the ophthalmic [V1], maxillary [V2], and mandibular [V3]—before leaving the middle cranial fossa (Fig. 8.64). Each of these divisions passes out of the cranial cavity to innervate a part of the face, so most of the skin covering the face is innervated solely by branches of the trigeminal nerve [V]. The exception is a small area covering the angle and lower border of the ramus of the mandible and parts of the ear, where the facial [VII], vagus [X], and cervical nerves contribute to the innervation.
The ophthalmic nerve [V1] exits the skull through the superior orbital fissure and enters the orbit. Its branches (Fig. 8.64) that innervate the face include: the supra-orbital and supratrochlear nerves, which leave the orbit superiorly and innervate the upper eyelid, forehead, and scalp; the infratrochlear nerve, which exits the orbit in the medial angle to innervate the medial half of the upper eyelid, the skin in the area of the medial angle, and the side of the nose; the lacrimal nerve, which exits the orbit in the lateral angle to innervate the lateral half of the upper eyelid and the skin in the area of the lateral angle; and the external nasal nerve, which supplies the anterior part of the nose (Fig. 8.65).
The maxillary nerve [V2] exits the skull through the foramen rotundum. Branches (Fig. 8.64) that innervate the face include: a small zygomaticotemporal branch, which exits the zygomatic bone and supplies a small area of the anterior temple above the zygomatic arch; a small zygomaticofacial branch, which exits the zygomatic bone and supplies a small area of skin over the zygomatic bone; and the large infra-orbital nerve, which exits the maxilla through the infra-orbital foramen and immediately divides into multiple branches to supply the lower eyelid, cheek, side of the nose, and upper lip (Fig. 8.65).
The mandibular nerve [V3] exits the skull through the foramen ovale. Branches (Fig. 8.65) innervating the face include: the auriculotemporal nerve, which enters the face just posterior to the temporomandibular joint, passes through the parotid gland, and ascends just anterior to the ear to supply the external acoustic meatus, the surface of the tympanic membrane (eardrum), and a large area of the temple; the buccal nerve, which is on the surface of the buccinator muscle supplying the cheek; and the mental nerve, which exits the mandible through the mental foramen and immediately divides into multiple branches to supply the skin and mucous membrane of the lower lip and skin of the chin (Fig. 8.65).
The muscles of the face, as well as those associated with the external ear and the scalp, are derived from the second pharyngeal arch. The cranial nerve associated with this arch is the facial nerve [VII] and therefore branches of the facial nerve [VII] innervate all these muscles.
The facial nerve [VII] exits the posterior cranial fossa through the internal acoustic meatus. It passes through the temporal bone, giving off several branches, and emerges from the base of the skull through the stylomastoid foramen (Fig. 8.66). At this point it gives off the posterior auricular nerve. This branch passes upward, behind the ear, to supply the occipital belly of the occipitofrontalis muscle of the scalp and the posterior auricular muscle of the ear.