202 



THE SKELETON. 



above downward. In the .median line is a vertical ridge, the symphysis, which 

 extends from the upper to the lower border of the bone, and indicates the point of 

 junction of the two pieces of which the bone is composed at an early period of life. 

 The lower part of the ridge terminates in a prominent triangular eminence, the 

 mental process. This eminence is rounded below, and often presents a median 

 depression separating two processes, the mental tubercles. It forms the chin, a 

 feature peculiar to the human skull. On either side of the symphysis, just below the 

 cavities for the incisor teeth, is a depression, the incisive fossa, for the attachment 

 of the Levator menti (or Levator labii inferioris) ; more externally is attached a 

 portion of the Orbicularis oris (Accessorii Orbicularis inferioris), and, still more 

 externally, a foramen, the mental foramen, for the passage of the mental vessels 

 and nerve. This foramen is placed just below the interval between the two 

 bicuspid teeth. Running outward from the base of the mental process on 

 each side is a ridge, the external oblique line. The ridge is at first nearly 

 horizontal, but afterward inclines upward and backward, and is continuous 

 with the anterior border of the ramus : it affords attachment to the Depressor 



Coronoid process. 



Condyle. 



Angle. 



FIG. 166. Inferior maxillary bone. Outer surface. Side view. 



labii inferioris and Depressor anguli oris ; below it the Platysma myoides is 

 attached. 



The internal surface (Fig. 167) is concave from side to side, convex from above 

 downward. In the middle line is an indistinct linear depression, corresponding 

 to the symphysis externally ; on either side of this depression, just below its centre, 

 are four prominent tubercles, placed in pairs, tAvo above and two below ; they are 

 called the genial tubercles or mental spines, and afford attachment, the upper to 

 the Genio-hyo-glossi, the lower to the Genio-hyoidei muscles. Sometimes the 

 tubercles on each side are blended into one ; at others they all unite into an irregular 

 eminence ; or, again, nothing but an irregularity may be seen on the surface of the 

 bone at this part. On either side of the genial tubercles is an oval depression, the 

 sublingual fossa, for lodging the sublingual gland; and beneath the fossa a rough 

 depression on each side which gives attachment to the anterior belly of the 

 Digastric muscle. At the back part of the sublingual fossa the internal oblique 

 line (mylo-Tiyoidean) commences ; it is at first faintly marked, but becomes more 

 distinct as it passes upward and outward, and is especially prominent opposite 

 the last two molar teeth ; it affords attachment throughout its whole extent to the 

 Mylo-hyoid muscle ; the Superior constrictor of the pharynx with the pterygo- 

 maxillary ligament being attached above its posterior extremity, near the alveolar 

 margin. The portion of the bone above this ridge is smooth, and covered by the 



