1430 



HUMAN ANATOMY. 



Fore-brain 



Nasal area 



Fore-brain 



The first step in the production of the latter is the appearance, about the ninth 

 week, of the palatal ridges, wedge-shaped elevations that grow downward and in- 

 ward from the maxillary processes. In front these ridges begin at the primitive 

 choanae, where they are continuous with the primitive palate, and extend backward 

 as far as the tympanic pouches. At first almost sagittal in their plane, the palatal 

 ridges become gradually converted into horizontal plates that come into contact 



and finally unite along their 



FIG. 1188. opposed median edges to com- 



Fore-brain plete the roof of the mouth and 



the floor of the nasal fossae and 

 the definite or secondary choautf, 

 this fusion being accomplished 

 by the end of the third month. 

 Coincidently with these 

 changes the primitive choanae 

 elongate and come to lie on 

 either side of the posterior por- 

 tion of the nasal septum to 

 which the frontal process has 

 now become reduced. The 

 union of a pair of outgrowths 

 from the palatal plates, beyond 

 their point of fusion beneath 

 the choanae, produces the uvula, 

 while the remaining ununited 

 portions of the ridges give rise 

 to the palato-pharyngeal arches. 

 For a time the nasal sep- 

 tum is still incomplete, since it 

 has not yet reached the palate, 

 and the nasal fossae communi- 

 cate by means of a cleft between 

 the septum and the palate. 

 With the downward growth of 

 the partition this communica- 

 tion is obliterated, the septum 

 joining the palate along the line 

 of the median suture. 



The formation of the ante- 

 rior part of the floor of the nasal 

 fossae is more complex since, 

 according to Peter, 1 in this 

 region the palatal processes do 

 not come in contact with each 

 other owing to the interposi- 

 tion of a portion of the partition 

 that separates the primitive 

 choanae. The palatal plates, 

 however, fuse with this wedge 

 of tissue along the line of appo- 

 sition except at one point on 

 each side, where the epithelium persists as a solid strand leading downward and inward 

 from the fore part of the floor of the nasal fossa to the roof of the oral cavity. These 

 strands acquire a lumen and become the incisive canals (page 1413) that may persist 

 throughout life and establish communication between the nasal and oral chambers. 



The further differentiation of the nasal fossae of man follows the same funda- 

 mental plan that applies to other mammals, but is modified by the reduction that 



1 Anatom. Anzeiger, Bd. xx., 1902. 



Lateral nasal 

 process 



Nasal fossa 



Fore-brain 



Nasal fossa 





Naso-frontal process Processus globularis 



Frontal sections of fore-brain of rabbit embryos, illustrating early 



opm 



of ectoblasti in B, nasal area is slightly depressed ; in C and I), nasal 

 ' 



tng 



stages in development of nose; in A. nasal aivi shows as thickening 

 of ectoblasti in B, 

 fossae are forming. 



' X 30. 



