PACE, 



21.S 



is turned a little outwards, and in the adult 

 (onus nearly a right angle; in the infant and 

 in tin old person it is obtuse. This part of 

 the bone is prominent and separates the in- 

 sertion of the masseter aud internal pterygoid 

 muscles. 



On the upper part of each ramus stand 

 two processes, which are separated by the 

 sigmoid notch ; the anterior is the curimuiil, 

 which is of a triangular form, flattened laterally, 

 and sharp in front and behind ; its summit 

 is somewhat rounded : this process gives at- 

 tachment to the temporal muscles. The cun- 

 dyloid process is situated behind the sigmoid 

 notch, and arises from the ramus by a narrow 

 tuck, which is directed upwards and a little 

 inwards, swelling above into an oval head 

 or condyle, that has an articular surface on 

 its summit. Tins articular surface is trans- 

 \ei-ely oval, convex, covered in the recent 

 subject with cartilage, and inclines from within 

 outwards and a little forwards. The condyle, 

 from the direction of its neck, somewhat over- 

 liiiius the internal surface of the ramus; it 

 is articulated with the anterior division of the 

 (,'lenoid cavity of the temporal bone. The 

 direction and form of its articular surfaces 

 :ire calculated to facilitate the rotatory move- 

 ments of the lower jaw during mastication. 



inferioris, the levator menu, tht platysma, 



mylo-hyoideus, and the digastric. Tour p.urs 

 of ligaments are attached to it, viz. the < 

 ami tin- internal lateral ligaments of the t< m- 

 poro-maxillary articulation, tin- pd-ry^-i -maxil- 

 lary (or intermaxillary) ligament, and the stylo- 

 maxillary ligament. It forms the lower part 

 of the face and the cavity of the mouth ; it 

 protects the tonuiie, salivary gland, and pharynx ; 

 it differs from the upper jaw arid from all the 

 other bones of the head in its remarkable 

 mobility ; and it contributes essentially to 

 mastication as well as to deglutition ami 

 articulation. 



Development. The lower jaw at birth con- 

 sists of two lateral halves, which are united 

 vertically in front along the median line by 

 a piece of cartilage, forming what has been 

 improperly called a symphysis. A few months 

 after birth the removal of this cartilage com- 

 mences, and the two halves of the bone 

 become united below; but not unfrequently 

 a fissure remains above for several months. 

 At this period the alveolar border is, like that 

 of the upper jaw, very thick, and contains 

 some large irregular cavities which lodge the 

 first set of teeth. Besides the superior denial 

 foramen there is found in the foetus another, 



In front the neck of the condyle presents a which leads to a temporary canal that supplies 



depression for the attachment of the external 

 pterygoid muscle. 



Structure. The lower jaw is formed of two 

 complete plates, united by cancellous tissue, 



the first set of teeth, and behind the alveoli 

 of the incisors may be observed a row of holes 

 which are said to be connected with the de- 

 velopment of the second set of teeth. Some 



which is traversed by a long curved canal (the authors maintain that each side of the lower 



iiit'i riiir dental canal), which conveys the 

 vessels and nerves that supply the teeth. This 

 canal commences in a groove just above the 

 superior dental foramen, which is situated on 

 tin: internal surface of the ramus ; it then 

 enters the substance of the bone, taking the 

 course of the internal oblique line below, and 

 parallel to which it runs as far as the second 

 bicuspid tooth, where it divides into two 

 canals, one short and wide, which terminates 

 on the external surface of the bone at the 

 inferior dental foramen ; and another smaller 

 one, which continues onwards as far as the 

 middle incisor tooth, where it ceases. From 

 the upper side of this dental canal small tubes 

 arise, which proceed to the alveoli ; they 

 convey vessels and nerves to the fangs of the 

 teeth. The situation and size of the dental 

 canal vary according to the age of the individual. 

 At birth it runs near the lower border of the 

 bcnip, and is of considerable magnitude; after the 

 si (ond dentition it becomes placed just below 

 the mylo-hyoid ridge ; in the edentulous jaw 

 it runs along the alveolar border of the bone, 

 its size is much diminished, and the mental 



jaw is developed by four separate points of 

 ossification ; but this assertion wants confirma- 

 tion. It is certain that this bone is among 

 those which are the most early developed, and 

 in the embryo of two months it is already 

 of considerable size. Its alveolar border is 

 at first a mere groove, of which the internal 

 margin is defective, aud which gradually be- 

 comes hollowed into separate sockets as the 

 teeth are developed. The changes of form 

 which the lower jaw undergoes from birth till 

 old age depend chiefly upon the development 

 and decay of the treth. Some of these changes 

 have been already noticed, and will be found 

 to correspond with those which occur in the 

 alveolar border of the upper maxilla; the 

 varying form and direction of the rami and 

 angles of the lower jaw we have noticed, 

 and for the more detailed account of the de- 

 velopment of this bone as connected with 

 dentition, we refer to the article TEETH. 



()/' tin J'nce in general. Dimentimu. The 

 \-i-rtical t/iiniieter of the face is the greatest, 

 and extends in front from the nasal eminences 

 of the frontal bone to the lower border of the 



foramen is found close upon the upper border symphysis menti ; this diameter decreas, - as 



of the bone. 



Cormenoni uml s. The lower jaw is arti- 

 culated with the temporal bones, and receives 

 the sixteen inferior teeth. It gives attachment 

 to fourteen pairs of muscles, viz. the temporal, 

 the masseter, the two pterygoids, the bucci- 

 nator, the superior constrictor of the pharynx. 

 ilie ilepic.-Kir anguli ons, the depressor labit 



we trace it backwards. The transverse dia- 

 nntir is next in length if measured at the 

 level of the malar bone, where it is most con- 

 siderable; below and above this it gradually 

 diminishes. The atterv-potterior diameter is 

 greatest at the level of the i lurk-hone-, where it 

 extends from the cuneiform process of th. 

 pital bone to the anterior nasal -pine of the upper 



