DEVELOPMENT OF THE FACE. 923 



auditory meatus, and internally, 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 projection, from above downward, of the frontal, or intermaxillary 

 process, and by the formation of several secondary projections, which leave certain per- 

 manent openings, forming the mouth, nose, etc. These processes of development, we 

 shall now attempt to follow. 



In the very first stages of development of the head, there is no appearance of the 

 face. The cephalic extremity consists simply of the cerebral vesicles, the surface of this 

 enlarged portion of the embryon being covered, in front as well as behind, by the exter- 

 nal blastodermic membrane. During the sixth week, after the cavity of the pharynx 

 has appeared, the membrane gives way in front, forming a large opening, which may be 

 called the first opening of the mouth. At this time, however, the face is entirely open 

 in front as far back as the ears. The first, or the superior visceral arch, now appears 

 as a projection of the middle blastodermic layer, extending forward. This is soon marked 

 by two secondary projections, the upper projection forming the superior maxillary por- 

 tion of the face, and the lower, the inferior maxilla. The two projections which form 

 the lower jaw soon meet in the median line, and their superior margin is the lower lip. 

 At the same time there is a projection from above, extending between the two superior 

 projections, which is called the frontal, or intermaxillary process. This extends from 

 the forehead (that portion which covers the front of the cerebrum) downward. The 

 superior maxillary projections then advance forward, gradually passing to meet the 

 frontal process, but leaving two small openings on either side of the median line, which 

 are the openings of the nostrils. The upper portion of the frontal process thus forms 

 the nose ; but below, is the lower end of this process, which is at first split in the median 

 line, projects below the nose, and forms the incisor process, at the lower border of which 

 are finally developed the incisor teeth. As the superior maxillary processes advance 

 forward, the eyes are moved, as it were, from the sides of the head and present anteriorly, 

 until finally their axes become parallel. These processes advance from the two sides, 

 come to the sides of the incisor process beneath the nose, unite with the incisor process 

 on either side, and their lower margin, with the lower margin of the incisor process, 

 forms the upper lip ; but, before this, the two lateral halves of the incisor process have 

 united in the median line. At the bottom of the cavity of the mouth, a small papilla 

 makes its appearance, which gradually elongates and forms the tongue. 



While this process of development of the anterior portion of the first visceral arch 

 is going on, at its posterior portion, we have developing, the malleus and incus, the 

 former being at first connected with the cartilage of Meckel, which extends along the 

 inner surface of the inferior maxilla, the cartilages from either side meeting at the chin. 

 The cleft between the first and the second visceral arch has closed, except at its posterior 

 portion, where an opening is left for the external auditory meatus, the cavity of the 

 tympanum, and the Eustachian tube. 



At the same time, the second visceral arch advances and forms the stapes, the styloid 

 ligament, and the lesser cornua of the hyoid bone. The third arch advances in the same 

 way ; and the arches from the two sides meet, become united in the median line, and 

 form the body and the greater cornua of the hyoid. The clefts between the second and 

 the third and between the third and fourth arches become obliterated by the deposition 

 of plastic matter. 



The fourth arch forms the sides of the neck and the larynx, the arytenoid cartilages 

 being developed first. In front of the larynx and just behind the tongue, is a little ele- 

 vation, which is developed into the epiglottis. The openings of the nostrils appear during 

 the second half of the second month. A little elevation, the nose, appears between these 



