AND ITS APPENDAGES. 781 



such an extent that they become more or less prominent folds, and have 

 received the name of the " visceral folds." The first visceral fold grows 

 rapidly forward, and divides into two somewhat diverging processes or 

 offshoots, which continue to become more and more prominent. The 

 corresponding processes from the right and left sides tend to approach 

 each other, and to unite upon the median line. Those of the lower pair 

 do so unite, and thus form the inferior maxilla. Those of the tipper 

 pair, which form the superior maxilla, unite, not with each other, but 

 with an intervening process which grows from above downward, upon 

 the median line, between them. 



By this growth of folds or processes in an anterior direction, and by 

 their union, above and below, upon the median line, there is included 

 between them a depressed space, lined with a continuation of the exter- 

 nal blastodermic layer, and situated immediately in front of the extremity 

 of the pharynx. This excavation is the cavity of the mouth, inclosed 

 on each side by the processes of the superior and inferior maxillae, widely 

 open in front, but terminating at its bottom by a blind pit ; there being 

 as yet no communication between it and the interior. 



Subsequently an opening is formed between the bottom or back part 

 of the mouth and the cavity of the pharj-nx, by a perforation through 

 the substance of both blastodermic layers at that point. This perfora- 

 tion takes place in the human embryo, according to Burdach, 1 during 

 the sixth week. The opening thus formed marks the situation of the 

 fauces; and the alimentary canal is thus made to communicate with 

 the exterior. The lining membrane of the mouth is consequently de- 

 rived from the external blastodermic layer, is a continuation of the 

 external integument, and the muscles sur- 

 rounding it are voluntary muscles ; while the Fig. 289. 



mucous membrane of the pharynx and oeso- 

 phagus is derived from the internal blasto- 

 dermic layer, and is surrounded by involun- 

 tary muscles. 



The completion of the component parts 

 of the face about the mouth is accomplished 

 by the continuous development of the five 

 buds or processes, above described, which 

 grow together in such a way as to diminish 

 the size of the originally wide oral orifice, HUMAN EMBRYO, about one 

 and to modify its form in various directions, month old: showing the growth 



/TT c\nf\ \ mi T of the frontal process downward, 



(Fig. 289.) The process which grows di- and that of th ; 8uperior anrt in ! 

 rectly downward in the median line from the ferior maxillary processes from 

 frontal region, is called the frontal or inter- ^ .2SET" '" '" 

 maxillary process, because it afterward con- 

 tains, in its lower extremity, the intermaxillary bones, with the four 

 upper incisor teeth. The superior maxillary processes, coming from 



1 TraitS de Physiologic ; traduit par Jourdan. Paris, 1838, tome iii. p. 468. 



