THE SPECIAL SENSES. 



193 





upper opening of the throat, and rise as high as the junc- 

 tion of the nose with the forehead. The inner wall of 

 each cavity is straight and smooth ; but from the outer 

 wall there jut into each cavity three small scroll-like 

 bones. The structure of these bones is very light, and 

 hence they have been called the " spongy" bones of the nose. 

 In this manner, while the extent of surface is greatly in- 

 creased by the formation of these winding passages, the 

 cavities are rendered extremely narrow; so much so, in fact, 

 that a moderate swelling of the mucous membrane which 

 lines them, as from a cold, is sufficient to obstruct the pass- 

 age of air through theni. 



40. The Nerve of Smell. The internal surface of 

 the nasal passages is covered by a delicate and sensitive 

 mucous membrane. Its surface is quite extensive, follow- 

 ing as it does, all the inequalities produced by the curved 

 spongy bones of the nose. The upper portion of it alone 

 is the seat of smell, since that part alone receives branches 

 from the "first pair" 

 of cranial nerves, or 

 the olfactory nerve, 

 which is the special 

 nerve of smell (see 

 Fig. 43). In Fig. 40 

 is shown the distri- 

 bution of this nerve, 

 in the form of an 

 intricate network 

 upon the two upper 



Spongy bones. The FlG ' 46 -- SECTION OF THE RIGHT NASAL CAVITY. 



nerve itself (1) does not issue from the skull, but rests 

 upon a thin bone which separates it from the cavity of the 

 nose; and the branches which proceed from it pass through 

 this bone by means of numerous small openings. The en- 



4O. The special nerve of smell ? Its location ? 



9 



