260 Anatomy of Skeleton 



External Pterygoid. The hollow below the neck internally is separated from the long 

 ligament by an interval in which lie, between the bone and ligament, from above 

 down (Fig. 207), the auriculo- temporal nerve, origin of middle meningeal, internal 

 maxillary and sometimes a process of parotid, the inferior dental artery, the inferior 

 dental nerve. The mylo-hyoid nerve and vessels must pierce the ligament to reach 

 the groove, for the ligament is fastened to the whole length of the margin of the 

 foramen. 



Looking at the bone as a whole, it is noticeable that its alveolar part is more 

 sharply curved than the rest of the body, with the result that its hinder portion forms 

 a bracket-like projection on the buccal aspect of the bone. The dental arcade forms a 

 parabolic curve, thus differing from the semi-ellipse of the upper arcade. The result 

 is reflected in the disposition of the teeth : the parabolic curve being narrower in front 

 than the ellipse, and broader further back, the teeth in the lower jaw will tend to be 

 behind the others anteriorly and external to them behind. So we find the direction of 

 the lower front teeth to be upward and forward while the back teeth are directed 

 upwards and inwards to meet their opposers, and the same tendency can be observed 

 in the direction of the sockets. The comparative shortness of the jaw also has effect 



in causing the direction of the front teeth. 



The inferior dental canal runs, inside the bone, between 

 the splenial and alveolar portions. Openings exist for vessels 

 and nerves between the canal and the dental sockets, and 

 the mental foramen carries the mental vessels and nerve to 

 the surface. This foramen is situated as a rule below the 

 jugum between the two bicuspids, about half-way between 

 FIG. 210. The muscles the upper and lower margins of the body ; but this level is 



th e S direcol a Jan U d 1 are onl Y true in the adult tooth-bearing jaw, for, as shown in 



resisted by the upper Fig. 209, it is different at other periods of life. In the same 



Brthis'vrouUi tend' to figure the differences between the angles formed by the axes 



open out the angle o f ramus and body are apparent, and the probable explanation 



between ramus and , ., , ,, ... 



body, an effect coun- of the occurrence of the angle is given. 



teracted by the strong The general build of the bone is in accord with the 



principle of resistance to the forces tending to destroy its 



form. Thus, with powerful muscles acting on its ramus and lifting the long arm of 

 the body against the upper jaw, it is evident that the tendency of such action would be 

 to open out the angle between body and ramus, and we find a thick and strong bar 

 thrown downwards and forwards to subtend the angle and render it secure (Fig. 210). 

 Secondarily there is the thickening of the splenium, which resists the downward push 

 of the alveolar teeth. 



The mandible is composed of a thick outer shell of dense bone with a small amount 

 of cancellous tissue contained within it, so that its body is cut with difficulty by bone 

 forceps : it resists decay longer than other parts of the skeleton, which probably 

 accounts for it being often found as the sole representative of the bones of the face or 

 skull in specimens from strata of past geological periods. Such ancient jaws show that 

 the presence of a mental prominence has been a human characteristic for ages, but 

 the earliest known jaws of the early Pleistocene or late Pliocene periods do not 

 possess the prominence. It is accounted for by assuming that a recession of the 

 alveolar part has taken place in man at the same time that the splenial portion 

 was comparatively well preserved, perhaps in connection with specialisation of the 

 tongue and floor of the mouth in association with the function of speech. 



The direction of the condyles is peculiar. Their long axes are directed inwards 



