Chap. XIX.] ORGANS OF SPECIAL SENSE. 



227 



while behind they open into the back of the pharynx by the 

 two posterior nares. Fourteen bones enter into the formation 

 of the nasal cavities : the floor 

 is formed by the palate and 

 part of the superior maxillary 

 bones ; the roof is chiefly 

 formed b}^ the perforated (crib- 

 riform) plate of the ethmoid 

 bone, and by the two small 

 nasal bones ; and in the outer 

 walls we find, in addition to 

 processes from other bones, 

 the three scroll-like turbinated 

 bones. The turbinated bones, 

 which are exceedingly light 



and spongy, project into the 



, .^. 1 T • 1 xi Fig. 133. — Vertical Longitudinal 



nasal cavities, and divide them section of Nasal Cavity. 1, olfactory 



into three incomplete passages nerve; w, branch of fifth nerve; h, hard 



from before backwards, — the 



superior, middle, and inferior meatus. The palate and superior 

 maxillary bones separate the nasal and mouth cavities, and the 

 cribriform plate of the ethmoid forms the partition between the 

 cranial and nasal cavities. 



The mucous membrane (sometimes called the Schneiderian ^ 

 membrane), which closely covers the nasal passages, is thickest 

 and most vascular over the turbinated bones. In some nasal 

 troubles it becomes much thickened and swollen, and occludes 

 the nasal passages to such an extent as to compel us to breathe 

 through the mouth. It contains numerous mucous glands which 

 secrete mucus for the purpose of keeping the membrane moist, 

 — a condition which is essential to perfection of the sense of 

 smell. 



The sense of smell is confined to the upper air passages of the 

 nose. Here the mucous membrane is remarkable in that it 

 contains nerve-cells. These cells have short, thick dendrones 

 which terminate in a bunch of short, hair-like projections pro- 

 truding beyond the surface of the mucous membrane, so that 



1 From Schneider, the first anatomist who showed that the secretions of the 

 nose proceeded from the mucous membrane, and not, as was formerly supposed, 

 from the brain. 



