CHAPTER V. 



THE DEVELOPMENT OF THE JAWS THE ERUPTION AND THE 

 ATTACHMENT OF THE TEETH. 



AT an early period in the development of the embryo 

 there is a single primitive buccal cavity, which is subse- 

 quently divided into a nasal and an oral cavity by the 

 palatine plates growing horizontally across it ; the pharynx 

 behind the hinder end of the primitive buccal cavity 

 remains undivided. Both upper and lower jaws make their 

 appearance about the twentieth day as little buds from the 

 first visceral arch, and grow inwards towards the middle 

 line : those which form the lower jaw reach to the middle 

 and there coalesce, those for the upper jaw stop short, and 

 the gap left between them is filled by a double downward 

 sprouting process from the forehead, which afterwards forms 

 the intermaxillary bone. A failure in the coalescence of the 

 maxillary processes with this intermaxillary process, on 

 one or both sides, results in a single or double hare-lip. 



In the lower jaw or mandibular processes there appears, 

 about the end of the first month, a dense cord of cartilaginous 

 consistence, Meckel's cartilage, which seems to serve as a 

 scaffolding, giving form and consistency to the lower jaw 

 prior to the occurrence of calcification. Meckel's cartilage, 

 formed as two distinct halves, soon unites in the middle, and 

 then forms a continuous curved bar, the hinder ends of 

 which reach up to the tympanum. 



About the fortieth day a centre of ossification appears in 

 the mandibular process, which, spreading rapidly, soon forms 



