M. quadratus labii superioris ([Fig. 63], p and q; [Fig. 64], f and g).—This is a complex of muscle-fibres attached chiefly to the fibrous pad on which the whiskers rest. In it two parts can be distinguished more or less completely.
(1) M. levator labii superioris alæque nasi ([Fig. 63], p; [Fig. 64], g) on the sides of the nose. This arises (a) as a continuation of the frontal portion of the epicranius, and (b) in a small bundle ([Fig. 63], p) from near the dorsal end of the frontal process of the maxillary bone, in common with the next. The two slips thus formed unite craniad and are inserted partly into the integument on the outer side of the wing of the nose, partly into the skin of the upper lip, lateroventrad of the nose, and partly into the pad on which the whiskers are located, in connection with the next.
(2) The angular head (caput angulare), or levator labii superioris proprius ([Fig. 63], q; [Fig. 64], f) is much larger than the last. It takes origin from a small tubercle at the cranial border of the orbit, close to the origin of the orbicularis oculi. The fibres diverge from their origin to their insertion among the whiskers and in the fibrous pad on which the whiskers rest.
Action.—Erects the whiskers and raises the upper lip.
M. caninus or levator anguli oris ([Fig. 64], f′).
Origin in a depression on the lateral surface of the maxillary bone, just craniad of the last premolar tooth. The fibres pass craniad, diverging, toward the whiskers, and have their insertion into the pad of tissue on which the whiskers rest.
Action.—Retracts the whiskers and raises the upper lip.
M. buccinator.—A thin muscle against the mucous membrane of the upper lip, partly united with the orbicularis oris and lying beneath M. caninus.
Origin on the outer surface of the superior maxillary bone in the depression between the infraorbital foramen and the first premolar tooth. The fibres pass ventrad, diverging, and mingle with those of the deep part of the orbicularis oris.
Relations.—Outer surface with the caninus and with the orbicularis oris. Inner surface with the mucous membrane of the upper lip.