14 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



The structures formed from Meckel's cartilage are shown in 

 Figs. IOC, 10 D, and 12. 



Development and Ossification of the Lower Jaw.— In some 

 animals, such as the kangaroo, the two halves of the lower jaw, 

 each developed in its own mandibular process, never unite. In 

 man ossific union takes place early in the second year. In figure 

 12 are shown the manner of formation and ossification ot the 

 lower jaw, with the changes that take place with age. 



(relatiue length and angle_of_ / \ ( J y 

 asc. ramus in adult) ~7~ v_ -' / 



(centre for dentary, outer 

 aspect of Meckel's cart.) 



symphysis 

 at birth-} 



coronoid 

 condylar 



malleus 

 lat. lig. 

 lingula 

 angle 140" at birth 

 angle 110° 

 angular 



mental eminence of adult \ Meckel's cartilage 



centre for incisor segment I dentary 

 fJ aw (f or splenial on inner aspect 



of Meckel's cart.) 



Fig. 12.— Showing the Centres of Ossification and age changes in the Lower Jaw. 



The part of the lower jaw (A, Fig. 12) is developed directly 

 out of Meckel's cartilage. The dentary centre (2?) appears in the 

 membrane on the outer side of Meckel's cartilage, and forms the 

 body of the jaw from the mental foramen almost to the angle; 

 the splenial centre (C) appears on the inner aspect of the cartilage 

 and between it and the dentary, the canal for the dental nerves 

 and vessels is formed. The ascending ramus, developed in 

 mammals only, is formed from the coronoid and condylar centres 

 which appear in membrane. By the condylar process the lower 

 jaw comes to articulate directly with the skull (squamosal bone). 



The growth of the antrum of Highmore, by pushing 

 downwards the body of the lower jaw, leads to an elongation 

 of the ascending ramus, and to its assuming a more vertical 

 position to the body of the jaw (Figs. 11 and 12). In old a^e, 

 when the teeth drop out and the alveolar margins are absorbed, 



