THE MANDIBLE 95 



which closes the anterior extremity of the inferior orbital (spheno-maxillary) 

 fissure. 



All the four angles of the zygomatic bone have distinguishing featui-es. The superior, 

 forming the fronto-sphenoidal process, is the most prominent, and is serrated for articulation 

 with the zygomatic process of the frontal; the anterior or infra-orbital process, sharp and 

 pointed, articulates with the maxilla and occasionally forms the superior boundary of the infra- 

 orbital foramen; the posterior or temporal process is blunt and serrated mainly on its medial 

 aspect for articulation with the zygomatic process of the temporal; the inferior angle, blunt 

 and rounded, is known as the malar tubercle. 



Of the four borders, the orbital is the longest and extends from the fronto-sphenoidal to the 

 infra-orbital process. It is thick, rounded, and forms more than one-third of the circumference 

 of the orbit; the temporal border, extending from the fronto-sphenoidal to the temporal process, 

 is sinuously curved and gives attachment to the temporal fascia. Near the frontal angle is 

 usually seen a slight elevation, the processus marginalis, to which a strong shp of the fascia is 

 attached; the masseteric border, thick and rough, completes the lower edge of the zygomatic 

 arch and gives origin to the anterior fibres of the niasseter; the maxillary border, rough and con- 

 cave, is connected by suture with the maxilla, and near the margin of the orbit gives origin to 

 the infra-orbital head of the quadratus Inbii svperioris. 



Blood-supply. — The arteries of the zygomatic are derived from the ^infra-orbital, lacrimal, 

 transverse facial, and deep temporal arteries. 



Articulations. — With the maxilla, frontal, temporal, and sphenoid. 



Ossification.^The zygomatic is ossified in membrane from three centres which appear in 

 the eighth week of intra-uterine life. The three pieces, which have received the names of pre- 

 nialar, posimalar, and hypomalar, miite about the fifth month. Occasionally the primary nuclei 

 fail to coalesce, and the bone is then represented in the adult by two or three portions sepa- 

 rated by sutures. In those cases in which the premalar and postmalar unite and the hypo- 

 malar remains distinct, the suture is horizontal; if the independent portion is the premalar, 

 then the suture is vertical. The bipartite zygomatic has been observed in skulls obtained from 

 at least a dozen different races of mankind, but because of the greater frequency in which it 

 occm-s in the crania of the Japanese (seven per cent.), the name of o.« Japonicum has been 

 given to it. 



THE MANDIBLE 



The mandible [mandibula] or lower jaw-bone (figs. 120, 121) is the largest and 

 strongest bone of the face. It supports the mandibular teeth, and by means of a 

 pair of condyles, moves on the skull at the mandibular fossae of the temporal 

 bones. It consists of a horizontal portion — the body — strongly curved, so as to 

 somewhat resemble in shape a horseshoe, from the ends of which two branches 

 or rami ascend almost at right angles. 



The body is marked in the middle line in front by a faint groove which in- 

 dicates the symphysis or place of union of the two originally separate halves 

 of the bone. This ends below in the elevation of the chin known as the mental 

 protuberance, the lowest part of which is slightly depressed in the centre and 

 raised on each side to form the mental tubercle. Each half of the mandible 

 presents two surfaces and two borders. On the lateral surface, at the side of 

 the symphysis, and below the incisor teeth, is a shallow depression, the incisor 

 fossa, from which the mentalis and the incisivus lahii inferioris muscle arise; and 

 more laterally, opposite the second bicuspid tooth, and midway between the 

 upper and lower margins, is the mental foramen, which transmits the mental 

 nerve and vessels. Below the foramen is the oblique line, extending backward 

 and upward from the mental tubercle to the anterior border of the ramus; it 

 divides the body into an upper or alveolar part and a lower or basilar part, and 

 affords attachment to the quadratus lahii inferions and the triangularis oris. 



The medial surface presents at the back of the sj^mphysis four small pro- 

 jections, called the mental spine (genial tubercles). These are usually arranged 

 in two pairs, one above the other; the upper comprising a pair of prominent 

 spines, gives origin to the genio-glossi, and the lower, represented in some bones 

 by a median ridge or only a slight roughness, gives origin to the genio-hyoid muscles. 

 At the side of the symphysis near the inferior margin is an oval depression, the 

 digastric fossa, for the insertion of the digastric muscle. Commencing below 

 the mental spine, and extending upward and backward to the ramus, is the 

 mylo-hyoid line, which becomes more prominent as it approaches ,the alveolar 

 border; it gives attachment along its whole length to the mylo-hyoid muscle, 

 at its posterior fifth to the superior constrictor of the pharynx, and at the pos- 

 terior extremity to the pterygo-mandibular raphe. Above this line at the side 

 of the symphysis is a smooth depression [fovea sublingualis] for the sublingual 

 gland, and below it, farther back, is another for the submaxillary gland. 



