628 Jacob Parsons Schaeffer. 



above the tongue, and fuse from before backwards along the 

 opposed edges. In this manner the separation of the nasal fossae 

 from the mouth cavity is made complete and permanent. How- 

 ever in the ventral portion of the palate, where the palatal processes 

 do not come in contact with each other (due to the interposition 

 of the tissue separating the primitive choanae) the separation 

 of the nasal cavity from the mouth cavity is not entirely com- 

 plete. This is brought about by the medial nasal processes unit- 

 ing and extending dorsally in the roof of the mouth cavity as the 

 inter-maxillary process. The latter process projects farthest 

 dorsally in the median plane, and on its lateral borders it is met 

 by the ventral ends of the palatal processes. At these contact 

 points the incisive canals are formed, and the latter at times serve 

 as a means of permanent communication between the oral and 

 nasal cavities. The lumina of the incisive canals are, however, 

 generally obliterated early in life. 



The permanent choanae and the permanent riasal fossae 



Coincidently with these changes in the formation of the nasal 

 fossae, the primitive choanae elongate and ultimately occupy their 

 definitive position, and thus become the permanent or secondary 

 choanae. The changes in the formation of the permanent choanae 

 also aid materially in increasing the ventrodorsal extent of the 

 nasal cavity. The latter now begins to take on its adult form. 

 The nasal septum which already has separated the nasal fossae 

 ventrally, rapidly fuses with the palate in the median plane, 

 i.e., along the line of fusion of the palatal processes. In this 

 manner the cavum nasi is divided into the nasal fossae. This divi- 

 sion takes place from before backwards. 



Before the first appearance of the palatal processes as inferior 

 boundaries of the nasal fossae, the lateral walls of the fossae have 

 begun in a simple manner to form the complex configuration 

 which characterizes the adult lateral nasal walls. 



