214 



FACE. 



along this surface of the bone to terminate at 

 the anterior border of the ramus of the jaw; 

 it gives attachment to muscles and separates 

 the external surface of the bone into two parts, 

 viz. an anterior superior, which presents, ex- 

 ternal to the symphysis, 1. a depression (the 

 J'HSSU mentalis) for the attachment of a muscle; 

 2. to the outer side of this the mental foramen, 

 which is directed obliquely upwards and out- 

 wards; it is the lower orifice of the inferior 

 dental canal, which conveys nerves and vessels 

 to the teeth of the lower jaw; 3. a number of 

 ridges and grooves near the alveolar border of 

 the jaw, which correspond to the sockets of the 

 teeth and to the septa which divide them : 

 this part of the bone is covered by the gums. 

 The surface below and behind the oblique line 

 is smooth, or only faintly marked with irre- 

 gular lines for the attachment of the platysma 

 myoides. 



The internal surface of the body of the 

 lower jaw is concave, and presents in the 

 median line, at the symphysis, a vertical crest 

 (crista mentalis interna), which is not so 

 distinct as the corresponding ridge on the outer 

 surface of the bone : at its lower extremity is 

 a tubercle having four summits (the genial 

 processes, yimo, chin, spina interna,) which 

 give attachment to two pairs of muscles, viz. 

 the two superior genial processes to the genio- 

 hyo-glossi, the two inferior to the genio- 

 hyoidei : below and to the outer side of these 

 processes, on the lower border of the bone, 

 are two oval rough depressions, one on each 

 side of the symphysis, for the attachment of 

 the anterior bellies of the digastric muscles. 

 From the genial processes proceeds obliquely 

 upwards and backwards, to join the anterior 

 border of the ramus of the jaw, the internal 

 oblique line, or the mylo-hyoid ridge. It is 

 distinctly marked and very prominent oppo- 

 site the last molar tooth ; like the external 

 oblique line it divides the bone diagonally into 

 two triangular portions, the anterior of which, 

 situated above and in front of the ridge, 

 is smooth, concave, and to the outer side of 

 the genial processes presents a depression 

 ( sublinguul fossa) for the reception of the sub- 

 lingual gland : elsewhere this surface is lined 

 by the gums, and forms the inner wall of the 

 alveolar cavities; but it is destitute of the 

 ridges and depressions which are seen on the 

 outer surface of the bone. The triangular 

 surface below the oblique line is marked by 

 numerous small holes for the passage of nu- 

 tritious vessels, and by a large depression 

 (the ntbmaiitttuy fossa) for the reception 

 of the submaxillary gland. The two oblique 

 maxillary lines which have been just described 

 divide the body of the jaw into two portions, 

 one superior or alveolar, the other inferior or 

 basilar: in the fcetus the former predominates 

 considerably; in the adult they are nearly 

 equal, and in the edentulous jaw of old age 

 the body almost entirely consists of the basilar 

 portion. 



The upper or alveolar border forms a lesser 

 curve than that of the alveolar border of the 

 superior maxilla : like it, however, it presents 



sockets for the reception of sixteen teeth, which 

 vary also in form and depth in correspondence 

 with the fangs of the teeth which they lodge. 

 The orifices of the sockets, however, take a 

 direction different from those of the upper jaw, 

 for while the sockets of the upper incisors look 

 downwards and forwards, those of the lower 

 are directed upwards and backwards; and 

 again the alveoli of the upper canine and 

 molar teeth look downwards and outwards, 

 whereas those of the lower are directed up- 

 wards and inwards : hence, from this different 

 inclination of the teeth in the two jaws, and 

 from the larger curve described by the alveolar 

 border of the superior maxilla, we find that 

 when the mouth is closed the upper front teeth 

 cover the lower and at the sides overhang 

 them a little. This arrangement is favourable 

 to the division and mastication of the food. 



The lower border or base is smooth and 

 thick, and forms a larger curve than the upper, 

 so that the surfaces of this jaw have an in- 

 clination from above downwards and forwards: 

 it forms the oval border of the lower part 

 of the face, and is the strongest portion of the 

 bone. 



The rami are flat, quadrilateral processes, 

 which stand up from the body of the jaw at 

 almost a right angle : in the child and old 

 person this angle is much more obtuse. Each 

 ramus presents two surfaces and four borders. 

 The external or masseteric surface has an 

 inclination from above downwards and more 

 or less outwards : it is rough, especially below, 

 where it presents some irregular oblique ridges 

 and depressions for the attachment of the 

 masseter: in front of these marks, near the 

 lower border of the bone, there is often a 

 slight groove, which indicates the course of 

 the facial vessels. 



Tlie internal or ptert/goid surface is also 

 rough below for the attachment of the internal 

 pterygoid muscle. In its centre is the spreading 

 superior orifice (superior dental foramen) of 

 the lower dental canal, marked and partly 

 hidden internally by a spine, which gives 

 attachment to the internal lateral ligament of 

 the temporo-maxillary articulation : from this 

 hole, taking a direction downwards and for- 

 wards is a groove (the mylo-hyoid groove), 

 which lodges the branch of the inferior dental 

 artery and nerve. 



The borders of the rami are, an anterior 

 or buccal, grooved below, where it corre- 

 sponds with the alveolar border of the bone ; 

 the margins of this groove, which are con- 

 tinuous with the oblique lines of the bone, 

 unite above and form a sharp convex edge. 

 The posterior or parotid border is round and 

 thick above, and narrow below, and is em- 

 braced by the parotid gland : inferiorly and 

 internally it gives attachment to the stylo- 

 maxillary ligament. The superior or zygomatic 

 border is sharp and concave, forming a notch 

 (the sigmoicl notch), which looks upwards. 

 The inferior border is rounded, and is con- 

 tinuous with the lower border of the body. 



The ales of the lower jaw are formed 

 by the union of the body and rami ; each 



