830 



EMBRYOLOGY 



DEVELOPMENT OF THE FACE 



The anterior portion of the embryo remains open in front long after 

 the medullary plates have met at the back and enclosed the neural canal 

 The common cavity of the thorax and abdomen is closed by the growth 

 of the visceral plates, which meet in front. At the time that the vis- 

 ceral plates are closing over the 

 thorax and abdomen, four distinct 

 tongue-like projections appear, one 

 above the other, by the sides of the 

 neck. These are called the visceral 

 arches, and the slits between them 

 are called the visceral clefts. The 

 first three arches, enumerating them 

 from above downward, correspond, 

 in their origin, to the three primitive 

 cerebral vesicles. The fourth arch 

 corresponds to the superior cervical 

 vertebrae. Of these four arches, the 

 first is the most important, as its de- 

 velopment, in connection with that of 

 the frontal process, forms the face 

 and the malleus and incus of the 

 middle ear. The second arch forms 

 the lesser cornua of the hyoid bone, 



median, or fron.al process, the inferior ^ ^^ ^ ^ ^ ligament 



Fig. 237. Mouth of a human embryo of 

 twenty-five to twenty-eight days, x 15 (Coste). 





2, right nostril; 3, left nostril; 4,4, inferior The third arch forms the body and 



maxillary processes, already united in the , . , ,-p,, 



median line; 5, 5, superior maxillary pro- the greater COmua of the hyoid. 1 he 



cesses, which have become quite prominent f ou rth arch forms the larynx. The 



and nave descended to the level of the slope 



of the frontal process; 6, mouth; 7, first vis- first cleft, situated between the first 



and the SeCOnd arch ' ls fillall y Cl Sed 



in front ; but an opening remains by 

 the side, which forms, externally, the external auditory meatus, and in- 

 ternally, the tympanic cavity and the Eustachian tube. The other clefts 

 become obliterated as the arches advance in their development. 



From the above sketch it is seen that the face and the neck are 

 formed by the advance and closure in front of projections from behind. 

 in the same way as the cavities of the thorax and abdomen are closed ; 

 but the closure of the first visceral arch is complicated by the projec- 

 tion, from above downward, of the frontal, or intermaxillary process, 

 and by the formation of several secondary projections, which leave cer- 

 tain permanent openings, forming the mouth, nose etc. 



