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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The Quadratus labii superioris is a broad sheet, the origin of which extends from the side of the nose to the zygomatic bone. Its medial fibers form the angular head, which arises by a pointed extremity from the upper part of the frontal process of the maxilla and passing obliquely downward and lateralward divides into two slips. One of these is inserted into the greater alar cartilage and skin of the nose; the other is prolonged into the lateral part of the upper lip, blending with the infraorbital head and with the Orbicularis oris. The intermediate portion or infraorbital head arises from the lower margin of the orbit immediately above the infraorbital foramen, some of its fibers being attached to the maxilla, others to the zygomatic bone. Its fibers converge, to be inserted into the muscular substance of the upper lip between the angular head and the Caninus. The lateral fibers, forming the zygomatic head, arise from the malar surface of the zygomatic bone immediately behind the zygomaticomaxillary suture and pass downward and medialward to the upper lip.
  The Caninus (Levator anguli oris) arises from the canine fossa, immediately below the infraorbital foramen; its fibers are inserted into the angle of the mouth, intermingling with those of the Zygomaticus, Triangularis, and Orbicularis oris.
  The Zygomaticus (Zygomaticus major) arises from the zygomatic bone, in front of the zygomaticotemporal suture, and descending obliquely with a medial inclination, is inserted into the angle of the mouth, where it blends with the fibers of the Caninus, Orbicularis oris, and Triangularis.

Nerves.—This group of muscles is supplied by the facial nerve.

Actions.—The Quadratus labii superioris is the proper elevator of the upper lip, carrying it at the same time a little forward. Its angular head acts as a dilator of the naris; the infraorbital and zygomatic heads assist in forming the nasolabial furrow, which passes from the side of the nose to the upper lip and gives to the face an expression of sadness. When the whole muscle is in action it gives to the countenance an expression of contempt and disdain. The Quadratus labii superioris raises the angle of the mouth and assists the Caninus in producing the nasolabial furrow. The Zygomaticus draws the angle of the mouth backward and upward, as in laughing.
  The Mentalis (Levator menti) is a small conical fasciculus, situated at the side of the frenulum of the lower lip. It arises from the incisive fossa of the mandible, and descends to be inserted into the integument of the chin.
  The Quadratus labii inferioris (Depressor labii inferioris; Quadratus menti) is a small quadrilateral muscle. It arises from the oblique line of the mandible, between the symphysis and the mental foramen, and passes upward and medialward, to be inserted into the integument of the lower lip, its fibers blending with the Orbicularis oris, and with those of its fellow of the opposite side. At its origin it is continuous with the fibers of the Platysma. Much yellow fat is intermingled with the fibers of this muscle.
  The Triangularis (Depressor anguli oris) arises from the oblique line of the mandible, whence its fibers converge, to be inserted, by a narrow fasciculus, into the angle of the mouth. At its origin it is continuous with the Platysma, and at its insertion with the Orbicularis oris and Risorius; some of its fibers are directly continuous with those of the Caninus, and others are occasionally found crossing from the muscle of one side to that of the other; these latter fibers constitute the Transversus menti.

Nerves.—This group of muscles is supplied by the facial nerve.

Actions.—The Mentalis raises and protrudes the lower lip, and at the same time wrinkles the skin of the chin, expressing doubt or disdain. The Quadratus labii inferioris draws the lower lip directly downward and a little lateralward, as in the expression of irony. The Triangularis depresses the angle of the mouth, being the antagonist of the Caninus and Zygomaticus; acting with the Caninus, it will draw the angle of the mouth medialward. The Platysma which retracts and depresses the angle of the mouth belongs with this group.

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