1170 



EMBRYOLOGY. 



the midfrontal process between the globular processes forms the lower part of the 



nasal septum, while above this is 

 seen a prominent angle which be- 

 the future point, and still 



Hyo-mandibiilar 

 cleft. 



.Cerebral hemisphere. 



al nasal process. 

 Olfactory pit. 

 ,-Processus globularis. 

 Maxillary process. 



-Mandibular arch. 



comes tne future point, and 

 higher, a flat area, the future bridge 

 of the nose (Figs. 717, 718). The 

 alae of the nose are developed from 

 the lateral nasal processes. 



The maxillary processes de- 

 scend 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 nasal process, form the 

 floor of the orbit, and shut it oft 

 from the rest of the face ; con, 

 tinuing their course downward and 

 inward, they join the globular proc- 

 esses, and with them complete the alveolar arch and upper lip. Finally a pair 

 of palatal processes are formed by inward extensions of the maxillary processes ; 

 these coalesce with each other in the median line, thus separating the cavity of the 

 mouth from the nasal fossae, and completing the palate (Fig. 717). In front the 

 palatal processes join with the premaxillae, except in the middle line, where a cleft 

 remains which constitutes the naso-palatine canal. 



The mandibular arch, by its junction with the corresponding process on the other 

 side, forms the lower jaw or mandible. The cartilaginous rod which it contains has 

 long been known as the ''cartilage of Meek el." The proximal end of this cartilage 

 is in contact with the periotic capsule, and from it are developed two of the ossicles 



FIG. 715. Under surfrce of the head of a human embryo, 

 about thirty days old. (After His.) 



Mouth of olfactory pit- 

 External nostril). 



Median part of fronto- 



nasal process. 



Processus globularis. 



Pituitary body.- 

 Hyo mandibular pouch. 



Sinus prsecervicalis. 



Larynx. 



Lung. 



Eye. 



Maxillary process. 



Mandibular arch. 



Membrane closing 

 hyomandibular cleft 

 (tympanic membrane). 



Hyoid arch. 

 Thyro-hyoid arch. 



Fourth post-oral arch. 



FIG. 716. The head and neck of a human embryo thirty-two days old, seen from the ventral surface The 

 floor of the mouth and pharynx have been removed. (His.) (From Marshall's Vertebrate Embryology.) 



of the middle ear, the malleus and incus * (Fig. 719). The remainder of the rod 

 is associated with the formation of the lower jaw, though the greater part of that 

 bone is developed from membrane. The second visceral arch is named the hyoid 

 arch : from it are 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 

 1 The incus is by some regarded as arising from the proximal end of the hyoid bar. 



