THE INFERIOR MAXILLARY BONE. 99 



the attachment of the Masseter externally, and the Internal pterygoid internally ; 

 the stylo-maxillary ligament is attached to the angle between these muscles. The 

 anterior border is thin above, thicker below, and continuous with the external 

 oblique line. The posterior border is thick, smooth, rounded, and covered by the 

 parotid gland. The upper border of the ramus is thin, and presents two processes, 

 separated by a deep concavity, the sigmoid notch. Of these processes, the anterior 

 is the coronoid, the posterior the condyloid. 



The Coronoid Process is a thin, flattened, triangular eminence of bone, which 

 varies in shape and size in different subjects, and serves chiefly for the attachment 

 of the Temporal muscle. Its external surface is smooth, and affords attachment 

 to the Temporal and Masseter muscles. Its internal surface gives attachment to 

 the Temporal muscle and presents the commencement of a longitudinal ridge, 

 which is continued to the posterior part of the alveolar process. On the outer 

 side of this ridge is a deep groove, continued below on the outer side of the alve- 

 olar process ; this ridge and part of the groove afford attachment, above, to the 

 Temporal; below, to the Buccinator muscle. 



The Condyloid Process, shorter but thicker than the coronoid, consists of two 

 portions; the condyle, and the constricted portion which supports the condyle, 

 the neck. The condyle is of an oblong form, its long axis being transverse, and 

 set obliquely on the neck in such a manner that its outer end is a little more for- 

 ward and a little higher than its inner. It is convex from before backward and 

 from side to side, the articular surface extending farther on the posterior than on 

 the anterior aspect. At its outer extremity is a small tubercle for the attach- 

 ment of the external lateral ligament of the temporo-mandibular joint. The 

 neck of the condyle is flattened from before backward, and strengthened by ridges 

 which descend from the fore part and sides of the condyle. Its lateral margins 

 are narrow, the external one giving attachment to part of the external lateral 

 ligament. Its posterior surface is convex ; its anterior is hollowed out on its 

 inner side by a depression (the pterygoid fossa), for the attachment of the 

 External pterygoid muscle. 



The Sigmoid Notch, separating the two processes, is a deep semilunar depres- 

 sion, crossed by the masseteric vessels and nerve. 



Development. The lower jaw is developed principally from membrane, but 

 partly from cartilage. The process of ossification commences early earlier than 

 in any other bone except the clavicle. The greater part of the bone is formed from 

 a centre of ossification (dentary), which appears between the fifth and sixth week 

 in the membrane on the outer surface of Meckel's cartilage. A second centre 

 (splenial) appears in the membrane on the inner surface of the cartilage, and 

 from this centre the inner wall of the sockets of the teeth is formed ; this termi- 

 nates above in the lingula. The anterior extremity of Meckel's cartilage becomes 

 ossified, forming the body of the bone on each side of the symphysis. Two supple- 

 mental patches of cartilage appear at the condyle and at the angle, in each of 

 which a centre of ossification for these parts appears ; the coronoid process is also 

 ossified from a separate centre. 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 symphysis, and proceeding backward : Levator menti, Depressor labii infe- 

 rioris, Depressor anguli oris, Platysma myoides, Buccinator, Masseter ; a portion 

 of the Orbicularis oris (Accessorii orbicularis inferioris) is also attached to this 

 surface. To its internal surface, commencing at the same point : Genio-hyo- 

 glossus, Genio-hyoideus, Mylo-hyoideus, Digastric, Superior constrictor, Temporal, 

 Internal pterygoid, External pterygoid. 



CHANGES PRODUCED IN THE LOWER JAW BY AGE. 



The changes which the lower jaw 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 



