Part 57

Nerve-supply.—Facial nerve.

5. =Incisivus superior= (Depressor labii superioris).—This lies under the mucous membrane of the upper lip.

Origin.—The alveolar border of the premaxilla from the second incisor to the first cheek tooth.

Insertion.—The upper lip.

Action.—To depress the upper lip.

6. =Incisivus inferior.=—This is arranged in the lower lip like the preceding muscle in the upper one.

Origin.—The alveolar border of the mandible from the second incisor to a point near the first cheek tooth.

Insertion.—The skin of the lower lip and the prominence of the chin.

Action.—To raise the lower lip.

7. =Mentalis= (Levator menti).—This is situated in the prominence of the chin. Its fibers arise from each side of the body of the mandible and are inserted into the skin of the chin. It is mingled with fat and strands of connective tissue, in which the roots of the tactile hairs are embedded. It raises and corrugates the skin to which it is attached.

8. =Depressor labii inferioris.=—This muscle lies on the outer surface of the ramus of the mandible, along the ventral border of the buccinator.

Origin.—The alveolar border of the mandible near the coronoid process and the maxillary tuberosity, in common with the buccinator.

Insertion.—The lower lip.

Action.—To depress and retract the lower lip.

Structure.—The tendon of origin and the belly are fused with the buccinator as far forward as the first cheek tooth. From this point forward the belly is distinct and rounded, terminating in a tendon which spreads out in the lower lip, blending with the orbicularis and the muscle of the opposite side.

Relations.—Superficially, the skin, masseter, facial vessels, and parotid duct; deeply, the mandible and inferior labial artery.

Blood-supply.—Facial artery.

Nerve-supply.—Facial nerve.

9. =Buccinator.=—This muscle lies in the lateral wall of the mouth, extending from the angle of the mouth to the maxillary tuberosity.

Origin.—The external surface of the maxilla above the interdental space and the molar teeth; the alveolar border of the mandible at the interdental space and also posteriorly where it turns upward to the coronoid process; the pterygo-mandibular ligament.

Insertion.—The angle of the mouth, blending with the orbicularis oris.

Action.—To flatten the cheeks, thus pressing the food between the teeth; also to retract the angle of the mouth.

Structure.—Two layers may be recognized. The =superficial layer= (Pars buccalis) extends from the angle of the mouth to the masseter. It is incompletely pennate, having a longitudinal raphé on which most of the muscle-fibers converge. The upper fibers are directed chiefly downward and backward, the lower ones upward and backward. The =deep layer= (Pars molaris) consists mainly of longitudinal fibers. It blends in part with the superficial layer of the orbicularis; it has a small tendinous attachment to the coronoid process behind, and is united below with the depressor labii inferioris.

Relations.—Superficially, the skin and fascia, the zygomaticus, levator nasolabialis, lateral dilator of the nostril, the superior buccal glands, the parotid duct, the facial vessels, and branches of the facial nerve; deeply, the mucous membrane of the mouth and the inferior buccal glands.

Blood-supply.—Facial and buccinator arteries.

Nerve-supply.—Facial nerve.

MUSCLES OF THE NOSTRILS

1. =Levator nasolabialis.=—This has been described (p. 214).