INFERIOR MAXILLARY AND INTERMAXILLARY REGIONS. 401 



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 Avith 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, Avith 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 narroAv fasciculus, into the angle of the 

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

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

 Avith 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 Avrinkles the integument of the chin, expressing 

 doubt or disdain. The Depressor labii inferioris draAvs the loAver lip directlv 

 doAvmvard and a little outAvard, 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 draAv the 

 angle of the mouth directly backward. 



8. Intermaxillary Region. 

 Orbicularis oris. J 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 remoA'ed from the surface. 



The Orbicularis oris (Fig. 269) is not a sphincter muscle, like the Orbicularis 

 palpebrarum, but consists of numerous strata of muscular fibres, having different 

 directions. Avhich surround the orifice of the mouth. These fibres are partially 

 derived from the other facial muscles Avhich are inserted into the lips, and are 

 partly fibres proper to the lips themselves. Of the former, a considerable number 

 are derived from the Buccinator and form the deeper stratum .of the Orbicularis. 

 Some of them namely, those near the middle of the muscle decussate at the angle 

 of the mouth, those arising from the upper jaw passing to the lower lip, and those 

 from the loAver jaAV to the upper lip. Other fibres of the muscle, situated at its 

 upper and loAver part, pass across the lips from side to side without interruption. 

 Superficial to this stratum is a second, formed by the LeA'ator and Depressor 

 anguli oris, Avhich cross each other at the angle of the mouth, those from the 

 Depressor passing to the upper lip, and those from the LeA'ator to the lower lip, 

 along Avhich they run to be inserted into the skin near the median line. In 

 addition to these there are fibres from the other muscles inserted into the lips the 

 Levator labii superioris, the Levator labii superioris alaeque nasi, the Zygornatici, 

 and the Depressor labii inferioris : these intermingle Avith the transverse fibres 

 above described, and haA'e principally an oblique direction. The proper fibres of 



