98 



THE SKELETON 



Ossification. — The mandible is mainly formed by ossification in the fibrous tissue investing 

 the cartilage of the first branchial arch or Meckel's cartilage, although a small portion of the 

 cartilage itself is directly converted into bone. 



It is now generally admitted that the lower jaw is developed in membrane as a single skel- 

 etal element. The centre of ossification appears in the outer aspect of Meckel's cartilage and 

 gives rise to the bony plate known as the dentary. This plate extends forward right up to the 

 middle line in front, and from it a shelf grows upward for the support of the tooth germs. 



Fig. 123. — The Mandible at Birth. 



Outer view 



Inner view 



Meckel's cartilage lies below and medial to the dentary plate, and the inferior alveolar nerve 

 passes forward between the two structures. Meckel's cartilage itself takes some small part in 

 the formation of the lower jaw. Ossification from the primary nucleus invades the cartilage 

 at a point opposite the interval between the first and second tooth germs, and the resulting 

 bone contributes to the formation of the alveolar margin opposite these two teeth. Behind 

 this point the cartilage atrophies except in so far as it helps to form the spheno-mandibular 

 ligament and the malleus and incus. Behind the symphysis the anterior extremity of the 

 cartilage does not enter into the formation of the jaw, but it usually persists throughout foetal 



Fig. 124.- 



-The Skull of a Woman Eighty-three Years Old, to show the Changes in 

 THE Mandible and Maxilla. 



life as one or two small, rounded, cartilaginous masses. Occasionally they become ossified 

 and give rise to accessory o.ssicles in this situation. The lamella of bone situated on the medial 

 side of Meckel's cartilage, corresponding to the distinct splenial element in some animals, arises 

 in man as an extension from the dentary element. 



In connection with the condylar and coronoid processes, cartilaginous masses are developed. 

 These do not, however, indicate separate elements, but are adaptations to the growth of the 

 lower jaw. "They are ossified by an extension from the surrounding membrane bone. 



