248 MUSCLES AND FASCIAE. 



placed on the side of the frasnum 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. 



Relations. .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 or Quadratics menti is a small quadrilateral 

 muscle, situated at the outer side of the preceding. It arises from the external 

 oblique line of the lower jaw, between the symphysis and mental foramen, and 

 passes obliquely upwards and inwards, to be inserted into the integument of the 

 lower lip, its fibres blending with the Orbicularis, and with those of its fellow of 

 the opposite side. It is continuous with the fibres of the Platysrna at its origin. 

 This muscle contains much yellow fat intermingled with its fibres. 



delations. 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 is triangular in shape, arising, by its broad base, 

 from the external oblique line of the lower jaw ; its fibres pass upwards, 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 and Eisorius at its 

 insertion. 



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 for- 

 wards ; at the same time it wrinkles the integument of the chin. The Depressor 

 labii inferioris draws the lower lip directly downwards and a little outwards. The. 

 Depressor anguli oris depresses the angle of the mouth, being the great antagonist 

 to the Levator anguli oris and Zygomaticus major : acting with these muscles, it 

 will draw the angle of the mouth directly backwards. 



8. INTERMAXILLARY EEGION. 

 Orbicularis Oris. Buccinator. Eisorius. 



Dissection. The dissection of these muscles may be consideraoly 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 is a sphincter muscle, elliptic in form, composed of con- 

 centric fibres, which surround the orifice of the mouth. It consists of two thick 

 semicircular planes of muscular fibre, which surround the oral aperture, and 

 interlace on either side with those of the Buccinator and other muscles inserted 

 into this part. On the free margin of the lips the muscular fibres are continued 

 uninterruptedly from one lip to the other, around the corner of the mouth, forming 

 a roundish fasciculus of fine pale fibres closely approximated. To the outer part 

 of each segment some special fibres are added, by which the lips are connected 

 directly with the maxillary bones and septum of the nose. The additional fibres 

 for the upper segment consist of four bands, two of which, the Accessorii Orbicularis 

 superioris, arise from the alveolar border of the superior maxilla, opposite the 

 incisor teeth, and, arching outwards on each side, are continuous at the angles of 

 the moiith with the other muscles inserted into this part. The two remaining 

 muscular slips, called the Naso-labialis, connect the upper lip to the septum of the 

 nose : as they descend from the septum, an interval is left between them, which 

 corresponds to that left by the divergence of the accessory portions of the Orbi- 

 cularis above described. It is this interval which forms the depression seen on 

 the surface of the skin beneath the septum of the nose. 



