54 



BONES OF THE HEAD. 



of a pair of articular surfaces or condyles. It has the shape of an 

 inA^erted arch bent forwards upon itself, and consists of a middle 

 larger and horizontal part — the body, and of two rami or ascending- 

 branches. 



The hody is marked in the middle by a vertical ridge, indicating the 

 original division of the bone into two lateral parts, and thence named 

 the symphysis. The superior or alveolar border is hollowed out into 

 sockets for the teeth. The inferior border, thicker anteriorly than 

 beneath the ramus, is slightly everted in front, constituting the chin, or 

 menfum, a prominence peculiar to the human skull. On the outer 

 surface, on each side of the symphysis, below the incisor teeth, is a 

 shallow depression, the incisor fossa, which gives origin to the levator 

 menti muscle, and, more externally, the ladial or menial foramen, wliich 

 transmits the branches of the inferior dental nerve and artery. From 

 beneath the mental foramen an elevation, the external ohliqm line, 

 extends upwards and outwards to the anterior border of the ramus. 

 The deep surface is marked, on each side of the symphysis, along the 

 inferior margin, by an oval depression, indicating the anterior attach- 

 ment of the digastric muscles, and above them by two pairs of 



Fig. 47. Fig. 47. — The Ixferiok. SrAX- 



ILLAUY Bone, from the 



RIGHT SIDE AND ABOVE. (A. 

 T.) i 



1, the body ; 2, the sj-m- 

 physis ; 3, the ramus ; 4, the 

 angle, near it the oblique ridges 

 marked by the attachment of 

 the masseter muscle ; 5, the 

 coronoid process ; 6, the con- 

 dyle or articular head ; 7, placed 

 in the sigmoid notch, points to 

 the front of the neck ; 8, the 

 mental foramen ; 9, the external 

 oblique ridge ; 10, the inferior 

 dental foramen and mylo-hyoid 

 groove of the left side. This 

 figure represents a full set of 

 the teeth of the lower jaw in 



middle life. (See also, for the view of the inner surface of the lower jaw, figure 54, the 



vertical section of the skull. ) 



prominent tubercles, sjnncc men tales, placed closely together, giving 

 attachment, the upper pair to the genio-hyo glossi, and the lower to 

 the genio-hyoidei muscles. An oblique prominent line, the mijlo- 

 hyoulcan ridye, leading from beneath the spinas mentales, upwards and 

 outwards to the ramus, gives attachment to the mylo-hyoideus muscle. 

 Above this line is a smooth depression for the sublingual gland, and 

 beneath and external to it another for the submaxillary gland. 



The ramus is thinner than the body of the bono. Its posterior border 

 in meeting the line of the base forms the angle of the jaw. The 

 external surface is flat and uneven, and towards the angle stronger 

 ridges mark the place of attachment of the masseter muscle. The 

 internal surface presents at its middle the inferior dental foramen, leading 

 into the dental canal, which lodges the dental nerve and vessels. Passing 



