204 THE SKELETON. 



lower border of the ramus is thick, straight, and continuous with the body of the 

 bone. At its junction with the posterior border is the angle of the jaw, which is 

 either inverted or everted, and marked by rough, oblique ridges on each side, for 

 the attachment of the Masseter externally, and the Internal pterygoid internal Iv ; 

 the stylo-maxillary ligament is attached to the bone 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 muscle. 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 alveolar 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 forward 

 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. 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, and present externally a tubercle for 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. 



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

 sion, crossed bv the masseteric vessels and nerve. 



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

 partly from cartilage. The process of ossification commences early before, indeed, 

 any bone except the clavicle. Between the fifth and sixth week a centre of ossi- 

 fication appears in the membrane on the outer surface of Meckel's cartilage (see 

 page 118), from which the greater part of the bone is formed. A second centre 

 appears in the membrane on the inner surface of the tooth-sockets, from which the 

 inner wall of the sockets of the teeth is formed ; this terminates 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. And, finally, two supplemental patches 

 of cartilage appear at the condyle and at the angle, in which centres of ossification 

 for these parts appear. 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 



