The facial is the motor nerve of the face. Having traversed the temporal bone, it emerges at the stylomastoid foramen, runs forwards, and forms the pes anserinus in the parotid gland, from which radiating branches are given off. A unilateral facial paralysis may result from an affection of (a) centre in the brain, when the side of the face affected is opposite to that of the lesion and the paralysis is incomplete, the mimetic play of the features being retained ;

(b) the nerve in the temporal bone, where the paralysis is on the same side and is complete once the nerve is destroyed ;

(c) an affection of the peripheral portion of the nerve from exposure to cold : here the paralysis comes on suddenly, and is generally complete, and often very intractable. In a complete paralysis of one side the eyelid cannot be closed; the eyeball appears prominent; tears may overflow on to the cheek (epiphora) from drooping of the lower eyelid and corneal irritation, and the brow droops; the natural furrows of the face disappear ; the nostril does not expand on inspiration, and hence sense of smell is impaired; the patient cannot whistle; he frequently complains that when taking fluid the material runs out of the corner of his mouth; and when he smiles, the muscles/ of the unaffected side draw the affected portions toward them, and thus cause distortion. Speech, also, is generally thick. In incomplete facial paralysis, on the other hand, while the patient may be unable to close the eye voluntarily, he generally does so bilaterally, as in blinking, and when he smiles, the affected side of the face responds slightly and is not mask-like. The sensory nerve-supply is from the trigeminal, of which the ophthalmic division supplies the brow, nose, canthi, and upper eyelid ; the superior maxillary division supplies the lower eyelid, cheek, side of nose, and upper lip ; the inferior maxillary division supplies the skin of the temporal region, that covering the masseter muscle, lower lip, and chin, and the mucous membrane of the cheek and lower lip.

The supra-orbital nerve comes out through the supra-orbital foramen, at the junction of the middle and inner thirds of the upper orbital margin. A line drawn from this to the interval between the bicusps of both jaws (Holden's line) passes through the infra-orbital foramen, which lies about inch below the lower orbital margin, and through the mental foramen, which transmits the terminal branches of the third division, and which lies midway between the alveolus and lower border of the jaw.

Facial neuralgia may be caused by affections of the Gas-serian ganglion, which give rise to a severe trigeminal type ; by tumours, such as sarcomas, springing from the base of the skull, or osteomas projecting into one of the nerve canals ; or by peripheral irritation, such as caries of the teeth or inflammatory conditions surrounding the exit of one of the foramina.

Sometimes the neuralgia gives rise to spasms (tic douloureux), and is frequently associated, when severe, with flushing and swelling of the affected parts and watering of the eye and nose, and, as these parts are then extremely sensitive, the patient's condition is very miserable. In severe trigeminal cases it is generally necessary to excise the Gasserian ganglion, and this operation, if successful, is followed by complete relief. Sometimes, also, Meckel's ganglion is removed, or peripheral portions of the nerves excised, as a rule with only temporary success, unless the cause be peripheral. Meckel's ganglion lies in the spheno-maxillary fossa, and is generally reached by following up the infra-orbital nerve and artery (Carnochan's operation). The ganglion, a small reddish body about 1/5 inch in diameter, lies in front of the foramen rotundum, and the Vidian canal of the sphenoid (which transmits the Vidian nerve to the ganglion) below the second division of the fifth nerve. To its outer side lie the terminations of the internal maxillary artery, the external pterygoid muscle, and the pterygo-maxillary fissure (which communicates with the fossa of that name), and to its inner side the vertical plate of the palate and the spheno-palatine foramen (which communicates with the nasal fossa and transmits the artery of that name) and the nasal branches of the ganglion. In addition to orbital and nasal branches, it gives off three palatine nerves-anterior, posterior, and external-which supply the palate, posterior arch of the fauces, and tonsil. As the branches of the fifth overlap one another in distribution, excision of a division is followed by very limited anaesthesia.

The face is developed from five processes : an upper central, or fronto-nasal; two upper lateral, or maxillary ; and two lower lateral, or mandibular. The fronto-nasal process grows down and forms the nose, central portion of the upper lip, and pre-maxilla, which carries the upper central incisor teeth. The maxillary processes grow inwards and join the fronto-nasal process, forming the upper jaw, cheek, and palate, while the mandibular processes grow inwards, fuse centrally, and form the lower jaw. A hare-lip is produced when the fronto-nasal and maxillary processes do not fuse properly at their lower extremities. A coloboma facialis is produced by a want of union between the fronto-nasal process and the superior maxillary process throughout their length. It presents below as an ordinary hare-lip, then passes up around the ala of the nose to the orbital cavity, forming a coloboma of the lower eyelid. Macrostoma, or transverse facial cleft, is a partial persistence of the cleft between the maxillary and mandibular processes, and extends upwards and outwards from the angle of the mouth. Microstoma is produced by excessive union of the processes.