374 



THE PERIPHERAL NERVOUS SYSTEM 



septum. Later, the ventral border of the septum fuses with the hard 

 palate and completely separates the nasal passages (Pig. 372). The 

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

 portion of the primitive mouth cavity- which has been appropriated second- 

 arily by the development of the hard palate. The passages of the adult 

 thus open caudally by secondary choanaz into the cavity of the pharynx. 



Olfactory epithelium 

 V omero-nasal organ 



Inferior concha4 



Palatine process ~-*~, .:_ 



Dental lamina 



Cartilage of nasal 

 septum 



.Cartilage of wmero- 

 nasal organ 



Naso-lacrimal duct 



- - -^ Tongue 



Meckel's cartilage 



FIG. 371. Transverse section through^the nasal passages and palatine processes of a 20 mm. 

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

 X 3. 



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

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

 conchas 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 (Fig. 369 C, E). The groove deepens and closes cau- 

 dally to form a tubular structure in the cranial portion of the nasal septum 

 (Fig. 371). During the sixth month it attains a length of 4 mm. Nerve 

 fibers, arising from cells in its epithelium, join the olfactory nerve, and it 

 also receives fibers from the n. terminalis. In late fetal stages it often 

 degenerates, but may persist in the adult (Merkel, Mangakis). Special 



