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MANUAL OF PHYSIOLOGY. 



The spheno-ethrnoidal portion develops the anterior part of 

 the sphenoid together with the ethmoid bones and the cartilage 

 of the septum of the nose, the first arising from the back part is 

 developed from membrane. The trabeculse are carried forward, 

 and bending down at the nasal depression form the lateral nasal 

 cartilages and the anterior part of the septal cartilage. 



The face is developed in connection with ridges known as the 

 visceral folds or arches, between which are a number of clefts, the 

 visceral clefts. 



The eyes and the openings of the nose are in the face ; whilst 



FIG. 300. 



B 



Different stages of the development of the head and face of a human embryo. A. 

 Embryo of four weeks. (Allen Thomson.) B. Embryo of six weeks. (Ecker.) C. 

 Embryo of nine weeks. (Allen Thomson.) a. Auditory vesicle. 1. Lower jaw. 1'. 

 First pharyngeal cleft. 



the ear arises at the side of the face, in connection with one of the 

 visceral clefts. 



The nasal depressions or pits appear in the wall of the head, 

 covering the anterior part of the brain. 



Just above the first visceral arch or fold is the depression which 

 ultimately becomes the buccal cavity, and unites with the alimen- 

 tary tract to form the mouth. 



The first fold is called the mandibular ; this gives off at either 

 end a process which grows upward and inward, forming the rudi- 

 ment of the superior maxillary bone and side of the face. 



Between these is a median process, the fronto-nasal, which gives 



