THE INFERIOR MAXILLA. 



2H 



anterior, a posterior, and an inferior — appearing towards the end of the second foetal 

 month. They fuse in the course of the third. Sometimes, but very rarely in the 

 white races, the bone is divided by a fissure — as in some apes — into an upper and a 

 lower part. This is said to be relatively common (seven per cent. ) in the Japanese. 

 A division into three has been seen. The roughness for the masseter sometimes 

 gives a deceptive appearance of a separate piece to this portion. On the other hand, 

 an occasional slight horizontal cleft in the zygomatic process is probably a remnant 

 of a division. 



THE INFERIOR MAXILLA. 



The inferior maxilla,^ mandible, or lower jaw develops in two symmetrical 

 halves, which soon fuse. The bone, as a whole, consists of a central part — the body 

 — forming the chin and supporting the teeth, and two rami projecting upward from 

 the back on either side and articulating with the glenoid fossa of the temporal. 



The body is convex in front and concave behind. The line of junction of the 

 original halves is the syniphysis, marked by a slight line. There is a forward pro- 

 jection of the lower border of the chin which is a human characteristic. A short 



Fig. 241. 



Coronoid process 



External pterygoid 

 CONDYLE 



Temporal 



Sigmoid notch 

 Neck 



Masseter, 



ANGLE 



Incisor fossa 

 Levator nienti 



SYMPHYSIS 



Mental 

 tubercle 

 BODY """S^s^iEEg^^i-^ Depressor 



External oblique line | Mental foramen 



Platysma Depressor anguli oris 



Inferior maxillary bone, outer aspect. 



distance from the median line at the lower border is the mental hibercle"^ bounding 

 this projection laterally. The alveolar process, above the body, is of the same nature 

 as that of the upper jaw. A slight depression, the incisor fossa, is found below the 

 teeth of that name on the front of the bone. The mental foraine7i for the terminal 

 branches of the inferior dental nerve and artery is rather below the middle of the 

 bone under the second bicuspid, sometimes just before it. The external oblique 

 li7ie^ starting from the mental tubercle, passes below the mental foramen into the 

 front edge of the ramus. Sometimes it seems to spring from the lower border under 

 the molar teeth, and sometimes both these origins may be present at once. On the 

 lower border of the bone, rather to its inner side, there is a rough oval behind each 

 mental tubercle for the anterior belly of the digastric muscle. The inner side of the 

 body is in the main smooth. The superior and inferior genial ttibercles^ are two 

 pairs of small, sharp spines near the lower part of the inner side of the symphysis for 

 the genio-glossi and genio-hyoid muscles respectively. The internal obliqne line 

 begins at first very indistinctly near the genial tubercles, and is lost on the inner 

 side of the ramus. It is particularly prominent under the molars, and gives attach- 



' Mandibula. " Tubcrcnlum mentale. ^ I.inea obliqua. *Spinac mentales. ^ Linea mylohyoidea. 



