374 



THE PERIPHERAL NERVOUS SYSTEM 



gates, the primitive choanae become longer and form sKt-Kke openings in the 

 roof of the mouth cavity. By the development and fusion of the palatine proc- 

 esses (described on p. 147) the dorsal portion of the mouth cavity is separated 

 off and constitutes the nasal passages (cf. Figs. 371 and 372). The nasal pass- 

 ages of the two sides for a time communicate through the space between the 

 hard palate and the nasal septum. Later, the ventral border of the septum fuses 

 with the hard palate and completely separates the nasal passages (Fig. 372). The 

 nasal passages of the adult thus consist of the primitive nasal fossae plus a portion 



Olfactory epillt hum 

 V omero-nasal organ 



Inferior concha 

 Palatine process 



Dental lamina 



<>*-« > 



L ulihi'.ie of nasal 

 Lptiim 



Cartilage of vomero- 

 nasal organ 



X aso-larcimal duct 



Tongue 



-ileckel's cartilage 



Fig. 371. — Transverse section through the nasal passages and palatine processes of a 20 mm. human 

 embryo. In the nasal septum is seen a section of the vomero-nasal organ (of Jacobson). X 30. 



of the primitive mouth cavity which has been appropriated secondarily by the 

 development of the hard palate. The passages of the adult thus open caudaUy 

 by secondary choana into the cavity of the pharynx. 



Part of the epithelium which hnes the nasal fossae is transformed into the 

 sensory olfactory epithelium (Fig. 371). The remainder covers the conchae and 

 lines the vomero-nasal organ (of Jacobson), the ethmoidal cells, and the cranial 

 sinuses. 



The Vomero-nasal Organ {of Jacobson) is a rudimentary epithelial structure 

 which first appears in 8.5 to 9 mm. embryos on the median wall of the nasal fossa 



