76 



THE BONES. 



labial face, and a circumference. The anterior face is smooth and concave, is lined 

 by the buccal mucous membrane, and supports the free extremity of the tongue. 

 The posterior face is convex, more extensive than the preceding, and continuous 

 with the external face of the branches. It presents : 1. On the median line, a 

 slight crest or small groove— traces of its being originally separated into two 

 pieces. 2. On the sides and above, the mental foramen — the inferior orifice of the 

 maxillo-dental canal. On a level with this foramen, the bone very markedly 

 contracts to form the neck. The circumference describes a parabolic curve, the 

 concavity being uppermost, and joins, by its extremities, the anterior border 

 of each branch. It is excavated in its middle part by the six alveoli for the 

 lodgment of the inferior incisors, and behind these— in male animals only— there 

 is an additional alveolus for the tusk. The portion included on each side 



Fig. 44. 



INFERIOR MAXILLA. 



1, Mental foramen; 1', superior orifice of the maxillo-dental canal ; 2, surface of implantation for 

 the masseter muscle ; 3, myloid ridge ; 4, coronoid process ; 5, condyle. 



between the last incisor and first molar, forms a more or less sharp ridge, which 

 constitutes the inferior interdental space or bar {diastema). 



Structure and development. — Formed, like all the flat bones, by two compact 

 plates separated by spongy tissue, the inferior maxilla is developed from two 

 centres of ossification, which correspond to each branch, and which coalesce some 

 time after birth. 



But in the human foetus, there can oe seen five pieces developed around the 

 dental canal — the coronary, articular, angular, opercular, and premaxillary — 

 which proves that the maxilla of Mammalia is formed on the same type as that 

 of oviparous Vertebrata (Lavocat). 



DiFFEKENTIAL CHARACTERS IN THE INFERIOR MaXILLA OP OTHER ANIMALS. 



A. Ox, Sheep, Goat.— In these animals, the part of the posterior border of the inferior 

 maxilla below the molars is convex, and cannot rest on a horizontal plane by all its points at 

 the same time. The condyle is convex in its small diameter, and slightly concave laterally The 

 coronoid process is bent backwards and outwards. The body does not show any alveolus for 

 the tusk, because this tooth is not present in these animals ; but it is hollowed by eight alveoli 

 for the incisor teeth. The two branches of the bone are never consolidated, but remain 

 movable on each other during life. 



