232 



Anatomy of Skeleton 



In the tenth week the ossification extends inwards and backwards above the teeth 

 germs into the palate folds, which have met in the ninth week (Fig. 186). From this 

 extension the inner walls of the alveoli are formed as secondary down-growths : before 

 this takes place there may be a cartilaginous change here and there in the alveolar 

 condensation. 



The down-growth that comes from the premaxillary region to form the inner walls 

 of the incisive alveoli are separated from the rest of the palatine in-growth by the 

 position of the anterior palatine foramen, which marks the situation of junction of 

 the paired palate-folds with the fronto-nasal process (Fig. 186), and these alveolar walls 

 may not completely fuse with each other or with the canine alveolus, giving rise to the 



fronto- nasal pneess 



FIG. 1 86. To illustrate the formation of the palate. On the left the roof of 

 the mouth in an embryo of seven weeks is seen from below, the lower jaw 

 being removed. The palate is represented by a pair of folds which 

 project from the inner side of each maxillary process and, with that 

 process, reach the fronto-nasal process in front. In the next figure the 

 palate folds are shown closed, having met behind the fronto-nasal pro- 

 cess or premaxilla ; thus the whole palate, though mainly formed from 

 the folds, has a small piece of fronto-nasal process in its extreme front 

 part. When the folds do not meet a cleft palate results, and the cleft may 

 extend forward between the maxillary and fronto-nasal processes. 



incisive sutures frequently seen in this region of the hard palate, and formerly supposed 

 to mark off the separate premaxilla from the true maxilla. 



About the time that the palatine process grows inwards a secondary centre appears 

 in the upper part of the premaxillary region, from which the " vomerine crest " and 

 front wall of the canals here are formed. 



The infraorbital nerve and vessels are at first in a groove on the upper aspect of 

 the body, but are covered in in part after the fourth month by a growth inwards of 

 the outer part of the body : a suture usually persists in the adult above and internal 

 to the canal, marking the extent of this inward growth. 



The antrum is represented by a depression on the outer wall of the nose in the fourth 

 month, and at birth it forms a fair-sized cavity in the inner part of the maxilla (Fig. 214) : 

 it reaches the situation of the infraorbital canal during the first year, and after this 

 grows fairly rapidly until it attains its full size ; its growth is of course conditioned 

 by the size of the bone, and cannot be completed until the last molar comes into position. 

 (For further account see later, p. 250.) 



