326 MUSCLES AND FASCIA. 



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 (Quadratus 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 Platysma 

 at its origin. This muscle contains much yellow fat intermingled 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 is triangular in shape, arising, by its broad base, 

 from the external oblique line of the lower jaw, from whence 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, 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 

 forwards, and at the same time wrinkles the integument of the chin. The De- 

 pressor Labii Inferioris draws the lower lip directly downwards and a little 

 outwards. 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 backwards. 



8. INTEEMAXILLARY EEGION. , *. 

 Orbicularis Oris. Buccinator. Eisorius. 



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 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 interlace on either side with those 

 of the Buccinator and other muscles inserted into the lips. 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 maxil- 

 lary bones and septum of the nose. The additional fibres for the upper segment 

 consists of four bands, two of which (accessorii orbicularis superiores) 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 mouth 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 corre- 

 sponds to that left by the divergence of the accessory portions of the Orbicu- 

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

 the surface of the skin beneath the septum of the nose. The additional fibres 

 for the lower segment (accessorii orbicularis inferiores) arise from the inferior 

 maxilla, externally to the Levator Labii Inferioris, and arch outwards to the 

 angles of the mouth, to join the Buccinator and the other muscles attached to 

 this part. 



