THE MUSCLES OF THE MOUTH 383 



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 

 medial ward 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 inclina- 

 tion, 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 Quad- 

 ratus 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 medial- 

 ward, 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. 



