Ii6 



A MANUAL OF ANATOMY 



of which the bone is originally composed. This ridge bifurcates at 

 the lower third, and its two divisions, diverging, pass to the lower 

 border, where each terminates in the mental tubercle. Between 

 these diverging divisions there is a triangular elevated surface, 

 called the mental protuberance or chin. On either side of the sym- 

 physis is the incisor fossa, which gives origin to the levator menti 

 and a deep slip of the orbicularis oris. A little external to this 

 fossa is the mental foramen, which opens outwards from the inferior 

 dental canal, and transmits the mental nerve and vessels. This 



Sigmoid Notch 



Left Coronoid Process 

 Temporal Muscle j 



External Pterygoi 

 Left Condyle 



Impression for Temporal 

 Muscle 



Incisor Fossa 



Levator Menti 

 Depressor Labii 

 Inferioris 



Mental Protuberance 



Mental Tubercle 



Posterior 

 — Border of Rami 

 -Masseter 



Mental Foramen 



Base 

 Platysma Myoides 

 External Oblique Line and 

 Depressor Anguli Oris 



Fig. 72. — The Inferior Maxillary Bone (External View). 



foramen is in line with the septum between the two bicuspid alveoli, 

 and in the adult it is midway between the superior and inferior 

 borders. Below it is the external oblique line, which extends from 

 the mental tubercle to the lower extremity of the anterior border 

 of the ramus. This line gives origin to the depressor anguli oris. 

 The lower part of the external surface, from near the symphysis to 

 about the level of the mental foramen, gives origin to the depressor 

 labii inferioris. 



The internal surface presents a slight median groove over about 

 its upper two-thirds, which coincides with the symphysis. Lower 

 down there are four small projections, called collectively the genial 

 , spines, which are arranged in pairs on either side of the middle line. 

 The upper spine gives origin, at either side, to the genio-hyo- 

 glossus, and the lower to the genio-hyoid. Close to the lower border, 

 at either side of the symphysis, is the oval digastric impression, 



