Skull and Hyoid 



255 



body of the hyoid, has its posterior fibres and " free edge " more vertical than hori- 

 zontal, whereas its front fibres are horizontal. The Genio-hyoids lie on these front 

 fibres, supporting the Genio-glossi, and the sublingual gland lies further out and comes 

 into contact with the bone. Thus we have the sublingual fossa outside the genial 

 tubercles and above the mylo hyoid ridge, and the mucous membrane, which covers 

 the alveolar part of the bone, and turns in to form the floor of the mouth, is separated 

 by the gland from the muscle here. 



But the submaxillary gland comes into relation with the bone in a different way. 

 Its " deep " part lies further back than the sublingual, up against the side of the tongue 

 (Hyo-glossus), but here the longer and more vertical fibres of the mylo-hyoid are 

 between it and the bone, so that the two are not in contact ; but this portion of the 

 gland is continuous round the free edge of the muscle with the " superficial part," which 

 is situated in the angle between the mylo-hyoid and the jaw (Fig. 203). Thus, 

 although the deep part of the gland does not touch the " buccal " part of the bone, the 

 superficial part comes into relation with the " cervical " portion, therefore below the 

 Mylo-hyoid ridge. This piece of the gland lies in the digastric triangle, behind the 



FIG. 202. Inner aspect of right half of mandible. 



anterior belly of the Digastric, so that the insertion of this muscle is further forward 

 than the submaxillary fossa, but not on the symphysis. 



Behind the position of the submaxillary fossa the gland extends back towards 

 the angle of the jaw : it is still under cover of the bone, but is separated from it here by 

 the lower part of the insertion of the Internal Pterygoid. 



On the outer surface (Fig. 204) the alveolus is covered by mucous membrane, which 

 is reflected from the jaw on to the cheeks and lip.. Thus the membrane-covered surface 

 extends to the origin of the Buccinator in the cheek region and is bounded in the region 

 of the lip by the origin of muscles running up to the Orbicularis oris : these are the 

 Depressor anguli oris below and behind the mental foramen, and the Depressor labii 

 inferioris in front of this. Further forward, but not shown in the figure, the lower 

 incisive slip of the Orbicularis arises from the incisive fossa. The muscle shown in the 

 figure as arising under the mucous membrane in this fossa has its fibres directed down 

 to be inserted into the skin of the chin ; therefore when it contracts it pulls up the skin 

 and so lifts the lip, and is therefore termed Levator menti or Lev. lab. inferioris. 



The remainder of the outer surface of the body is covered by the superficial tissues 

 of the face. The deep fascia of the neck is attached to the lower border, and the 

 Platysma, lying on this, gets some insertion into the bone from the chin outwards. In 



