37-" 



THE PERIPHERAL NERVOUS SYSTEM 



cesses (described on p. 156) the dorsal portion of the mouth cavity is separated 

 off and constitutes the nasal passages (compare Figs. 352 and 353). The nasal 

 passages 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. The nasal 

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

 primitive mouth cavity which has been separated off secondarily by the develop- 

 ment of the hard palate. The passages of the adult thus open caudally by sec- 

 ondary choana; into the cavity of the pharynx. 



The epithelium which lines the nasal fossae is, a portion of it, transformed into 



Olfactory epithelium 



Fig. 353. — Transverse section through the nasal passages of a 65 mm. embryo. X 14. 



the sensory olfactory epithelium (Fig. 352). It is also differentiated by the de- 

 velopment of the so-called organ of Jacobson, of the concha?, of the ethmoidal 

 cells and of the cranial sinuses. 



The Organ of Jacobson is a rudimentary epithelial structure which first ap- 

 pears in 8.5 to 9 mm. embryos on the median wall of the nasal fossa (Fig. 350 

 C, E). The groove deepens and closes caudally to form a tubular structure in 

 the cranial portion of the nasal septum (Fig. 352). At two and a half months 

 it attains a length of 0.42 mm. It is supplied by nerve fibers which arise from 

 cells in its epithelium and in part by the n. tcrminalis. In late fetal stages it often 

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



