THE NASAL MUCOUS MEMBRANE. 1413 



mucous membrane stretches. The lower part of the naso-frontal division, below the 

 nasal bone, is cutaneous and cartilaginous. Anteriorly the roof is reduced to little 

 more than a groove on account of the approximation Qf the lateral and median 

 walls, but posteriorly broadens towards the choana. The median part of the roof, 

 formed by the cribriform plate of the ethmoid, is very thin and makes a sharp angle 

 with the steeply descending sphenoidal division. Between the latter and the superior 

 turbinate bone lies the spheno-ethmoidal recess. 



The floor of the nasal fossa, much broader than the roof and supported by 

 the palatal process of the maxilla and the horizontal plate of the palate bone, from 

 before backward is approximately horizontal, but from side to side is distinctly con- 

 cave. Anteriorly this wall is robust, but rapidly diminishes in thickness as it passes 

 backward. About 2 cm. behind the posterior margin of the nostril and close to the 

 septum, the floor of each nasal fossa presents a slight depression, sometimes narrow 

 and funnel-shaped, that leads into a small canal lined with a prolongation of mucous 

 membrane. This canal converges towards the septum with its fellow of the opposite 

 fossa, descends almost vertically, and passes through the incisive foramen in the hard 

 palate to end on the roof of the mouth as a minute slit at the side of the incisive pad 

 or papilla palatina. Although the two tubes of mucous membrane may join to form 

 a single incisive canal, they usually retain their independence (Leboucq, Merkel). 

 They are often closed and impervious ; sometimes, however, even in the adult 

 communication is retained between the nasal and oral cavities. 



The posterior nares or choanae, the apertures through which the nasal fossae 

 communicate with the naso-pharynx, one on either side of the septum, resemble in form 

 somewhat a Gothic arch (Fig. 1354). They are relatively much lower in the new- 

 born child than in the adult, in which they measure about 3 cm. in height and 

 1.5 cm. in breadth (Zuckerkandl), although individual variation is considerable. 

 Each opening is bounded below by the horizontal plate of the palate bone ; laterally 

 by the inner surface of the internal pterygoid plate of the sphenoid ; above by the 

 vaginal process of the sphenoid and the ala of the vomer ; and mesially by the 

 \'ertical posterior borders of the vomer. Over this bony arch the nasal mucous 

 membrane is continuous with that lining the pharynx. Laterally the posterior 

 limit of the nasal fossa in the recent condition is indicated by a furrow (sulcus nasalis 

 posterior) that extends from the under surface of the sphenoid downward to about 

 the junction of the hard and soft palates. Behind this furrow, about on a levd with 

 the lower border of the inferior turbinate, lies the opening of the Eustachian tube 

 (Fig. 1 174). Since the turbinates end approximately 12 mm. in advance of the 

 choanae, the outlines of these openings are unbroken by the scrolls that model the 

 lateral wall of the nasal fossae, all three conchae, however, being visible through the 

 posterior nares. 



THE NASAL MUCOUS MEMBRANE. 



Beyond the limen that marks the limit of the integument clothing the vestibule 

 (page 1409), the nasal fossa is lined by mucous membrane continuous with that of the 

 naso-pharynx through the choanae. Since in addition to lining the tract over which 

 the respired air passes the nasal mucous membrane contains the cells receiving the 

 impressions giving rise to the sense of smell, it is appropriately divided into a respir- 

 atory and an olfactory part. 



The Olfactory Region, — The highly specialized regio olfactoria is quite 

 limited in extent and embraces an area situated over the middle of the upper tur- 

 binate and the corresponding part of the septum. According to Brunn, ^ whose 

 conclusions are here presented, the olfactory area of each fossa includes only about 

 250 sq. mm. , the septum contributing something more than one-half of the entire surface. 

 Accordingly the specialized field is by no means coextensive with the upper turbinate 

 bone, as it reaches neither its lower nor posterior border (Fig. 1177). The anterior 

 margin of the area, which lies about i cm. behind the front wall of the nasal fossa, is 

 irregular in outline owing to the invasion of the specialized region by the adjacent 



^ Archiv f. mikros. Anat., Bd. 39, 1S92. 



