THE NASAL CAVITIES. 395 



NOSE. 



The nasal cavities are formed by the invagination of a pair of epi- 

 dermal thickenings similar to those which give rise to the lens and auditory 

 vesicle. The pockets thus produced in the embryo are called "nasal pits" 

 (Fig. 187, n, p. 166). Their external openings remain as the nares of the 

 adult. Temporarily, from the third to the fifth month of fetal life, they are 

 closed by an epithelial proliferation. Each nasal pit acquires an internal 

 opening, choana, in the roof of the pharynx. The choanae are at first situ- 

 ated near the front of the mouth, separated from one another by a broad 

 nasal septum (Fig. 445). As the latter extends posteriorly it is joined by 

 the palate processes which grow toward it from the sides of the maxillae. 

 Thus the choanae recede toward the back of the mouth while the em- 

 bryonic condition of cleft palate is being removed. 

 The lateral walls (not the medial) of the nasal 

 cavities produce three curved folds one above 

 another; they are concave below, and in them the 

 conchae [turbinate bones] develop. The nasal 

 mucosa covers these and extends into the sphe- 

 noid, maxillary, and frontal sinuses, and the 

 ethmoidal cells. The boundary between the 

 epithelium of the nasal pit and that of the 

 pharynx early disappears, and the extent of 

 each in the adult is uncertain. Presumably the FlG ' MOUTH? 

 olfactory neuro-epithelium is derived from the fAfterK 8 iimann.)' 



nasal pit. In man the olfactory region is limited na> : i* Ia p is; a nd''pa h< p!raiveoiai : ! 

 to the superior and middle concha and the part proceTses. illa 

 of the septum which is opposite them. This 



regio olfactoria is covered by a yellowish-brown membrane which may be 

 distinguished macroscopically from the reddish mucosa of the regio res- 

 piratoria. The latter includes the remainder of the nose. The two re- 

 gions may be considered in turn. 



The vestibule or cavity of the projecting cartilaginous portion of the 

 nose is a part of the respiratory region which is lined with a continuation 

 of the skin. Its stratified epithelium has squamous outer cells and rests 

 upon a tunica propria with papillae. It contains the sheaths of coarse 

 hairs (vibrissae) together with numerous sebaceous glands. The extent 

 of the squamous epithelium is variable; frequently it is found on the 

 middle concha, less often on the inferior concha. 



The remainder of the respiratory mucosa consists of a pseudo-stratiried 

 epithelium with several rows of nuclei. It may contain few or many 



