THE INFERIOR MAXILLA. 



213 



the appearance of being set rather on the front of the neck,^ which is merely a 

 constriction below the head ; the articular surface, however, extends at least as far 

 down behind as in front. There is a depression for a part of the insertion of the 

 external pterygoid on the front of the neck internal to the sigmoid notch, "^ which 

 is the deep depression separating the coronoid process from the condyle. The 

 dental canaP sweeps downward and forward with a slight curve, and then runs 



Fig. 243. 



I liner surface 



Dental canal 



Fig. 244. 



Alveolar process 



Coronoid process 



Condyle 



Section through body of lower 

 jaw, anterior surface. 



Symphysis Dental canal 



Right inferior maxilla at about birth, inner aspect. 



Fig. 245. 



Condyle 



Coronoid 

 process 



horizontally nearer the lower than the upper border of the body of the jaw.* It 

 lies at first against the inner wall, but soon is nearer the outer. This relation 

 then varies, but towards the anterior end of its course it is against the inner wall. 

 It divides under the second bicuspid into the viental canal, some five millimetres 

 long, running to the mental foramen, and into the incisive canal, much smaller, for 

 the vessels and nerves of the front teeth, which, 

 after dividing, is lost in the cancellated tissue under 

 the lateral incisor. 



Structure.. — The jaw is of very tough bone, 

 especially at the symphysis, where it is almost solid. 

 On section the body shows very thick walls below, 

 before, and behind. The alveolar processes, on the 

 contrary, are made of very light plates that are ab- 

 sorbed rapidly after the loss of the teeth. 



Development and Growth. — The two halves 

 of the inferior maxilla are formed separately, each 

 from six centres. They are at first connected by 

 ligament. Even before birth the union seems very 

 close, but they become cobssified only in the course 

 of the first year. The centres appear from the sixth 

 to the eighth week of foetal life in the membrane of 

 Meckel's cartilage, except as otherwise mentioned. 

 They fuse during the third month. The centres 



are : ( i ) the dentary, which is a line of ossific deposit forming the lower border 

 and the front of the alveolar process ; (2) one in the distal end of Meckel's carti- 

 lage, for the region of the symphysis ; (3) one for the coronoid ; (4) one appearing 

 in cartilage, not that of Meckel, for the condyle and top of the ramus ; (5) one for 

 the angle ; (6) the splenial, for the inner alveolar plate, extending back to include 

 the lingula. This one appears some three weeks later than the others. Still 

 another minute one is said to help to form the mental foramen (Rambaud et 

 Renault). All these, except that for the condyle, which unites at fifteen, fuse 

 shortly after their appearance. The mandible, being at first nothing but a hollow 

 bar to hold tooth-sacs, is very shallow. The ramus is small, the head bent back- 

 ward and the angle very large. At birth it is about 140°. With the loss of teeth, 

 from whatever cause, the alveolar process atrophies. In old age the bone is very 

 small and of light structure, and the angle enlarges consideralDly, so as to mimic 

 the infantile form. 



^Fawcett : Journal of Anatomy and Physiology-, vol. xxix., 1895. 



^ Collum mandibulae. - Incisura maodibulae. ^ Canalis mandibulae. 



Alveolar 

 process 



Right half of lower jaw at about birth, 

 from above. 



