482 



HISTOLOGY 



maxillary, and frontal sinuses, and the ethmoidal cells. The boundary be- 

 tween the epithelium of the nasal pit and that of the pharynx early disap- 

 pears, and the extent of each in the adult is uncertain. Presumably the ol- 

 factory neuro-epithelium is derived from the nasal pit. In man the olfac- 

 tory region is limited to the upper third of the nasal septum and nearly the 

 whole of the superior concha (Read). This regio olfactoria is covered by a 

 yellowish-brown membrane, which may be distinguished macroscopically 

 from the reddish mucosa of the regio respiratoria. The latter includes the 

 remainder of the nose. The two regions 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 



Cartilaginous 

 nasal septum. 



Dental ridge 



of the 

 upper iaw. 



Nasal cavity. 



Maxilla. 



Oral cavity. 



Tongue. 



Mandible. 



FIG. 488. FRONTAL SECTION OF THE HEAD OF A 40-MM. SHEEP EMBRYO, x is. 



The palate processes have united with the nasal septum. The concha are developing along the lateral 

 walls of the nasal cavity. In the lower part of the nasal septum the vomero-nasal organs are seen as a 

 pair of tubes, each of which is partly surrounded by a crescentic cartilage. 



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 (vibrissce) 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-stratified 

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

 goblet cells. The tunica propria is well developed, being even 4 mm. 

 thick on the inferior concha (Fig. 489). It consists of fibrillar tissue with 

 many elastic elements, especially abundant in its deeper layers. Beneath 

 the epithelium, it is thickened to form a homogeneous membrana propria, 



