THE FACE. 



119 



first groove, and the last behind the last groove, while the second, third, and 

 fourth are between successive grooves. These are the branchial arches, the first 

 of which has its upper end bent so as to lie at an angle with the lower end. each 

 half of the arch being thus <-shaped. The upper limb of the < is termed the 

 maxillary, and the lower, the mandibular process, and between the two there 

 lies a depression, the oral $inm. The outline of this depression is pentagonal, 

 since the ends of the two maxillary processes do not unite, but have projecting 

 down between them a broad plate, the fronto-nasal process. In the mesoblast 

 which occupies the axis of each branchial 

 arch a cartilaginous bar develops, serv- 

 ing as a support for the arch. 



The maxillary processes unite with the 

 fronto-nasal process. The latter consists of 

 three plates, a central single one and two 

 lateral ones. The central is called the 

 " mil-frontal " process. It is free in front 

 and below, but behind it is united with the 

 coalesced portion of the trabeculse cranii, 

 which therefore probably assists in the for- 

 mation of the septum nasi, and. in addition, 

 of the prominent part of the future nose. 

 The lateral plates of the fronto-nasal pro- 

 cess are separated from the central one by 

 a depression or furrow on either side ; 

 these furrows form the primary nasal pits 

 or fossae. The lateral plates project down- 

 ward parallel to the mid-process for a cer- 

 tain distance, and then, curving inward, 

 unite with it. thus shutting off the nasal 

 fossae from the rest of the face. The 

 lateral masses of the ethmoid and lachry- 

 mal bones are developed in the lateral 

 plates, and by their union with the mid- 

 frontal process form the intermaxillary bone and the lunula, or central part 

 of the upper lip. 



The maxillary processes descend for a short distance, forming the outer wall 

 of the orbit, in which the malar bone is developed ; they then incline inward, 

 and, meeting the lateral plate of the fronto-nasal process, form the floor of the 

 orbit, and shut it off from the rest of the face ; then, continuing their course 

 downward and inward, they join the mid-frontal process, and with it complete the 

 alveolar arch and the superior maxillary bone. Finally, palatal processes are 

 formed by an extension of the inner sides of this arch ; these coalesce with each 

 other in the median line, thus separating the cavity of the mouth from the nasal 



e, and completing the palate. In front, however, the palatal processes do not 

 join with the mid-frontal process, but a cleft is left which constitutes the naso- 

 palatine canal. 



The maudibular process forms the lower jaw or mandible, the cartilage which 

 it contains being known as Meeker s cartilage. This becomes ossified, and unites 

 with membrane-bones, developing in the mesoblastic tissue around it, to form the 

 mandibular bone. Its upper end is in contact with the periotic capsule, and 

 from it two portions are separated and ossify to form two of the bones of the 

 middle ear. the malleus and incus. The second arch is named the hyoid arch ; 

 from it is formed the styloid process, the stylo-hyoid ligament, and the lesser 

 cornu of the hyoid bone. The third, or thyro-hyoid arch, gives origin to the great 

 cornu of the hyoid bone, while the body of this bone is formed between the 

 second and third arches. The fourth and fifth arches do not reach so great a 

 development as the others, and their cartilages likewise only partially develop. 



FIG. 92. Face of an cmbrvo of 25 to 28 days. 

 Magnified 15 times. 1. Frontal prominence. 2,3. 

 Right and left olfactory fossa. 4. Inferior max- 

 illary tubercles, united in the middle line. 5. 

 Supe'rior maxillary tubercles. 6. Mouth of fau- 

 ces. 7. Second pharyngeal arch. 8. Third. 9. 

 Fourth. 10. Primitive ocular vesicle. 11. Prim- 

 itive auditory vesicle. 



