308 THE MUSCLES AND FASCIAE. 



and assists the Levator labii superioris in producing the naso-labial ridge. The 

 Zygomaticus major draws the angle of the mouth backward and upward, as in 

 laughing ; whilst the Zygomaticus minor, being inserted into the outer part of the 

 upper lip and not into the angle of the mouth, draws it backward, upward, and 

 outward, and thus gives to the face an expression of sadness. 



7. Inferior Maxillary Region (Fig. 195). 



Levator labii inferioris (Levator menti). 

 Depressor labii inferioris (Quadratus menti). 

 Depressor anguli oris (Triangularis menti). 



Dissection. The muscles in this region may be dissected by making a vertical incision 

 through the integument from the margin of the lower lip to the chin : a second incision should 

 then be carried along the margin of the lower jaw as far as the angle, and the integument care- 

 fully removed in the direction shown in Fig. 194. 



The Levator labii inferioris (Levator menti) is to be dissected by everting the 

 lower lip and raising the mucous membrane. It is a small conical fasciculus placed 

 on the side of the frgenum of the lower lip. It arises from the incisive fossa, 

 external to the symphysis of the lower jaw; its fibres descend to be inserted into 

 the integument of the chin. 



Relation. On its inner surface, with the mucous membrane ; in the median 

 line, it is blended with the muscle of the opposite side ; and on its outer side, with 

 the Depressor labii inferioris. 



The Depressor labii inferioris (Quadratus menti) is a small quadrilateral 

 muscle. It arises from the external oblique line of the lower jaw, between the 

 symphysis and mental foramen, and passes obliquely upward and inward, to be 

 inserted into the integument of the lower lip, its fibres blending with the Orbicularis 

 oris and with those of its fellow of the opposite side. It is continuous with the 

 fibres of the Platysma at its origin. This muscle contains much yellow fat inter- 

 mingled with its fibres. 



Relations. By its superficial surface, with part of the Depressor anguli oris 

 and with the integument, to which it is closely connected ; by its deep surface, 

 with the mental vessels and nerves, the mucous membrane of the lower lip, the 

 labial glands, and the Levator menti, with which it is intimately united. 



The Depressor anguli oris (Triangularis menti) is triangular in shape, arising, 

 by its broad base, from the external oblique line of the lower jaw, from whence its 

 fibres pass upward, to be inserted, by a narrow fasciculus, into the angle of the 

 mouth. It is continuous with the Platysma at its origin and with the Orbicu- 

 laris oris and Risorius at its insertion, and some of its fibres are directly continuous 

 with those of the Levator anguli oris. 



Relations. By its superficial surface, with the integument ; by its deep surface, 

 with the Depressor labii inferioris and Buccinator. 



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



Actions. The Levator labii inferioris raises the lower lip and protrudes it 

 forward, and at the same time wrinkles the integument of the chin, expressing 

 doubt or disdain. The Depressor labii inferioris draws the lower lip directly 

 downward and a little outward, as in the expression of irony. The Depressor 

 anguli oris depresses the angle of the mouth, being the antagonist to the Levator 

 anguli oris and Zygomaticus major; acting with these muscles, it will draw the 

 angle of the mouth directly backward. 



8. Intermaxillary Region. 



Orbicularis oris. Buccinator. Risorius. 



Dissection. The dissection of these muscles may be considerably facilitated by filling the 

 cavity of the mouth with tow, so as to distend the cheeks and lips ; the mouth should then be 

 closed by a few stitches and the integument carefully removed from the surface. 



The Orbicularis oris (Fig. 195) is not a sphincter muscle, like the Orbicularis 



