782 



DEVELOPMENT OF THE ALIMENTARY CANAL 



Fi<r. 290. 



the sides, unite with the intermaxillary process, to form the upper jaw. 

 In quadrupeds the intermaxillary bones, containing the upper incisor 

 teeth, remain distinct from those of the superior maxilla, the line of 

 demarcation between them being indicated by a suture. In man, as a 

 general rule, they are consolidated with each other, the only permanent 

 suture being that on the median line, between the right and left halves 

 of the upper jaw. According to Geoffroy Saint-Hilaire, 1 a permanent line 

 of suture sometimes remains between the intermaxillary and the superior 

 maxillary bones. 



The two inferior maxillary processes unite with each other, making 

 the lower border of the cavity of the mouth, and form, by their union 

 upon the median line, the inferior maxilla. In quadrupeds the two 



inferior maxillary bones remain perma- 

 nently divided by a median suture; but in 

 man they are consolidated into a single 

 piece during the first year after birth. 



As the intermaxillary process grows 

 from above downward, it becomes double 

 at its lower extremity, and at the same 

 time gives origin to two lateral offshoots, 

 which curl round and inclose two circular 

 orifices, the anterior nares (Fig. 290); the 

 offshoots themselves becoming the alse 

 nasi. The external border of the ala nasi 

 subsequently adheres to the superior maxil- 

 lary process, leaving only a curved crease 

 or furrow at the side of the nose, which 



marks the line of union between them. In many of the quadrupeds, 

 this furrow remains partially open, extending, as a curvilinear cleft, out- 

 ward and upward from the orifice of the nostril. 



The mouth at this time is wide and gap- 

 ing, owing to the incomplete development 

 of the upper and lower jaw and the com- 

 parative insufficiency of the lips and 

 cheeks. The soft parts afterward increase 

 in growth, and thus gradually diminish 

 the size of the oral orifice (Fig. 291). The 

 lips and cheeks arise by folds of the in- 

 tegument and subjacent muscular layers, 

 which, projecting respectively from above 

 downward, from below upward, and from 

 behind forward, form the permanent bor- 

 ders of the opening of the mouth. The 

 upper lip in man presents a median furrow, 

 possession. bordered by two slightly elevated ridges, 



HEAD OP HUMAN EMBRYO 

 at about the sixth week. From a 

 specimen in the author's possession. 



1 Histoire des Anomalies de 1'Organization. Paris, 1832, tome i. p. 581. 



