THE MANDIBLE, OR LOWER JAW 117 



ridge, the external oblique line (linea obliqua). The ridge is at first nearly hori- 

 zontal, but afterward inclines upward and backward, and is continuous with the 

 anterior border of the ramus; it affords attachment to the Depressor labii inferioris 

 and Depressor anguli oris; below it the Platysma is attached. 



The internal surface (Fig. 87) 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 two prominent tubercles, one above and one below, the genial 

 tubercles (spinae mentales), which afford attachment, the upper pair to the Genio- 

 hyoglossi, the lower pair to the Geniohyoidei, 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 (fovea sublingualis), for the lodgement of the sublingual gland; 

 and beneath the fossa a rough depression, the digastric fossa (fossa digastrica), 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, or mylohyoid 

 ridge (linea mylohy&idea), 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 Mylohyoid muscle; the Superior constrictor of the pharynx with 

 the pterygomaxillary 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 mucous membrane of the mouth; the portion below presents an 

 oblong depression, the submaxillary fossa (fovea submaxillaris), wider behind than 

 in front, for the lodgement of the submaxillary gland. The external oblique line 

 and the internal or mylohyoidean line divide the body of the bone into a superior 

 or alveolar and an inferior or basilar portion. 



Borders. The superior or alveolar portion of the body (pars alveolaris) has 

 above a narrow border which is wider and the margins of which are thicker 

 behind than in front. It is hollowed -into numerous cavities (alveoli dentales), for 

 the reception of the teeth ; these cavities are sixteen in number, and vary in depth 

 and size according to the teeth which they contain. To the outer side of the alveo- 

 lar border the Buccinator muscle is attached as far forward as the first molar tooth. 

 The inferior or basilar portion (basis mandibulae) is rounded, longer than the superior 

 and thicker in front than behind; it presents a shallow groove, just where the body 

 joins the ramus, over which the facial artery turns. 



The Perpendicular Portions or Rami (rami mandibulae). The perpendicular 

 portions or rami are of a quadrilateral form. Each presents for examination two 

 surfaces, four borders, and two processes. 



Surfaces. The external surface is flat, marked with ridges, and gives attachment 

 throughout nearly the whole of its extent to the Masseter muscle. 



The internal surface presents about its centre an oblique foramen (foramen man- 

 dibulare), the beginning of the inferior dental canal (canalis mandibulae}, which 

 transmits the inferior dental vessels and nerve. The margin of this opening 

 is irnMEular; it presents in front a : ' ' ' << ;.y a sharp spine, 



the lingr-ii. internal lateral 



1 io a ! :>. .ding to a groove, 



the id|UivQidean groov ]iiely downward 



to t utivoid vessels and 



nen of the Internal 



ptei -a rd and forward 



in t t in the body; it is 



her $ small openings. 



