DEVELOPMENT OF THE ALIMENTARY CANAL. 679 



The lips and cheeks arise by folds of the integument and subjacent mus- 

 cular layers, which, projecting downward, upward, and forward, form 

 the permanent borders of the mouth. The upper lip in man presents a 

 median furrow, bordered by two slightly elevated ridges, corresponding 

 with the union of the maxillary and intermaxillary processes. The 

 lower lip, like the inferior maxilla, is consolidated on the median line, 

 and shows no trace of its double origin. 



In some instances, the superior maxillary and intermaxillary pro- 

 cesses fail to unite with each other, giving rise to the malformation 

 known as hare-lip. The fissure, in cases of hare-lip, is consequently 



FIG. 227. FIG. 228. 



HEAD OF HUMAN EMBRYO at about HEAD OF HUMAN EMBRYO, about the 



the sixth week. From a specimen end of the second month. From a 



in the author's possession. specimen in the author's possession. 



situated, as a rule, not in the median line, but a little on one side, cor- 

 responding with the outer edge of the intermaxillary process. Some- 

 times the deficiency exists on both sides at once, forming " double hare- 

 lip ;" in which case, if the fissure extend through the bony structures, 

 the central piece of the superior maxilla, detached from the remainder, 

 contains the upper incisor teeth, and corresponds with the intermaxillary 

 bone of the lower animals. In one instance, observed by Wyman,* 

 the fissure of hare-lip was situated in the median line, the two inter- 

 maxillary bones not having united with each other. 



The eyes at an early period are upon the sides of the head (Fig. 226). 

 As development proceeds, they come to be situated farther forward 

 (Fig. 221), looking obliquely outward. At a still later period they are 

 placed on the anterior plane of the face (Fig. 228), wit-h their axes 

 nearly parallel and looking forward. This change is effected by the 

 more rapid growth of the posterior and lateral portions of the head, 

 which enlarge in such a manner as to alter the relative position of the 

 parts in front. 



The palate is formed by the growth of a horizontal partition between 

 the mouth and nares. which arises on each side as an offshoot from the 

 superior maxilla. The two plates afterward unite on the median line, 



* Transactions of the Boston Society for Medical Improvement, March 9th, 1863. 



