374 THE MUSCLES AND FASCIA 



7. The Mandibular Region (Fig. 289). 



Levator menti. Depressor labii inferioris. 



Depressor anguli oris. 



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 mandible as far as the angle, and the integument carefully 

 removed in the direction shown in Fig. 287. 



The Levator menti (m. mentalis) is a small conical fasciculus placed on the 

 side of the frenum of the lower lip. It arises from the mandibular incisive fossa, 

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

 the integument of the chin. 



The Depressor labii inferioris, or Quadratus menti (m. quadratus labii 

 inferioris) (Fig. 295), is a small quadrilateral muscle. It arises from the external 

 oblique line of the mandible, 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 fat intermingled with its fibres. 



The Depressor anguli oris (m. triangularis) (Fig. 289) is triangular in shape, 

 arising, by its broad base, from the external oblique line of the mandible, 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 Orbicularis oris and Risorius at its insertion, and some of its fibres are directly 

 continuous with those of the Levator anguli oris. 1 



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



Actions. The Levator menti 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 the Levator anguli oris, it will draw 

 the angle of the mouth directly inward. 



8. The Buccal 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 (Figs. 289 and 295) is not a simple sphincter muscle, like 

 the Orbicularis palpebrarum, but consists of numerous strata of muscle fibres, 

 having different directions, which surround the orifice of the mouth. These 

 fibres are partially derived from the other facial muscles which 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 the Buccinator fibres namely, those near 

 the middle of the muscle decussate at the angle of the mouth, those arising 

 from the maxilla passing to the lower lip, and those from the mandible to the 



i Muscle fibres connecting the two muscles below the chin are occasionally met with; they constitute the 

 Musculus transversus menti of His and Waldeyer. 



