THE MANDIBLE, OR LOWER JAW 119 



week. Accessory nuclei develop to form the condyle and the coronoid process, in the front part 

 of both alveolar walls and along the front of the lower border of the bone. 



These accessory nuclei possess no separate ossific centres, but ossification extends into them 

 from the adjacent membrane bone and they undergo absorption. The inner alveolar border, 

 usually described as arising from a separate ossific centre (spenial centre), is formed in the human 

 mandible by an ingrowth from the main mass of the bone. At birth the bone consists of two 

 halves, united by a fibrous symphysis, in which ossification takes place during the first year. 



Articulation. With the glenoid fossae of the two temporal bones. 



Attachment of Muscles. To fifteen, pairs to its external surface, commencing at the sym- 

 physis, and proceeding backward: Levator menti, Depressor labii inferioris, Depressor anguli 

 oris, Platysma, Buccinator, Masseter; a portion of the Orbicularis oris is also attached to this 

 surface. To its internal surface, commencing at the same point: Geniohyoglossus, Geniohyoid, 

 Mylohyoid, Digastric, Superior constrictor, Temporal, Internal pterygoid, External pterygoid. 



Meckel's cartilage 



FIG. 88. Scheme showing ossification of the mandible, inner side (Low). The membrane bone is colored 

 red. The greater part of Meckel's cartilage is colored blue. The upturned, stippled portion near the symphysis 

 represents the part of Meckel's cartilage, which is surrounded and invaded by the membrane bone. The accessory 

 nuclei of cartilage in the condyle, coronoid process, alveolar border, and body are indicated by stippled areas. 



I-'K;. 89. Scheme showing ossification of mandible from the outer side (Low). Membrane bone colored red. 



Accessory nuclei of cartilage stippled. 



CHANGES PRODUCED IN THE MANDIBLE BY AGE. 



The changes which the mandible undergoes after birth relate (1) to the alterations effected 

 in the body of the bone by the first and second dentitions, the loss of the teeth in the aged, and 

 the subsequent absorption of the alveoli; (2) to the size and situation of the dental canal; and 

 (3) to the angle at which the ramus joins with the body. 



At birth (Fig. 90) the bone consists of lateral halves, united by fibrous tissue. The body is 

 a mere shell of bone, containing the sockets of the two incisor, the canine, and the two tem- 

 porary molar teeth, imperfectly partitioned from one another. The dental canal is of large size, 

 and runs near the lower border of the bone, the mental foramen opening beneath the socket of 

 the first molar. The angle is obtuse (17.5 degrees), and the condyloid portion nearly in the 

 same horizontal line with the body; the neck of the condyle is short, and bent backward. The 

 coronoid process is of comparatively large size, and situated at right angles with the rest of 

 the bone. 



SIDE VIEW OF THE MANDIBLE AT DIFFERENT PERIODS OF LIFE. 



After birth (Fig. 91) the two segments of the bone become joined at the symphysis, from 

 below upward, in the first year; but a trace of separation may be visible in the beginning of the 

 second year near the alveolar margin. The body becomes elongated in its whole length, but 

 more especially behind the mental foramen, to provide space for the three additional teeth 

 developed in this part. The depth of the body becomes greater, owing to increased growth of 

 the alveolar part, to afford room for the fangs of the teeth, and by thickening of the subdental 

 portion, which enables the jaw to withstand the powerful action of the masticatory muscles; but 

 the alveolar portion is the deeper of the two, and, consequently, the chief part of the body lies 

 above the oblique line. The dental canal after the second dentition is situated just above the 

 level of the mylohyoid ridge, and the mental foramen occupies the position usual to it in the 

 adult. The angle becomes less obtuse, owing to the separation of the jaws by the teeth. (About 

 the fourth year it is 140 degrees.) 



