OF THE SENSE OF SMELL. 107 



the nose. On the external wall of each may be seen three 

 prominent laminae, curved on themselves; they are formed by 

 the turbinated bones ; and by being thus rolled on themselves, 

 they thus extend considerably the pituitary membrane, which 

 invests them. Upon this membrane the olfactory nerves are 

 distributed. Finally, between these turbinated bones there 

 are longitudinal grooves, 

 called meatuses. These 

 fossae communicate with 

 cavities, called sinuses, hol- 

 lowed out in the thickness 

 of the frontal bone, the su- 

 perior maxillary, &c. The 

 pituitary membrane is 

 thick, and has a velvety 

 appearance; on its surface 

 may be observed a vibratile 

 movement, produced by 

 microscopic cilia ; finally, it 

 is continually moistened by Fig. 60 bis, Nasal Foss.* 



a viscous liquid, called nasal 



mucus, and it receives many nervous filaments from the 

 olfactory and fifth pair of nerves. 



220. The membrane must be moist, in order that odours 

 may be perceived ; and the sense is strongest in the upper 

 part of the nostril. The extent of membrane seems greatly 

 to influence the power of the organ ; and in this respect the 

 carnivora, the ruminants, and some pachydermata, are re- 

 markable for the development of the turbinated bones, and 

 consequently for the extent of mucous membrane covering 

 them. In reptiles the organ is extremely simple. 



221. In fishes, the nasal fossae do not communicate 

 with the pharynx, but are merely cavities shut in at the 

 back, and the pituitary membrane presents in them a num- 

 ber of folds, arranged like rays around a central point, or 

 in parallel rows like the teeth of a comb, on each side of a 

 median band. 



But there are many animals which have a fine sense of 



* Vertical section of the nasal fossse, representing the outer wall of the 

 left nasal fossa. a, the mouth ; d, portion of the base of the cranium ; 

 e, forehead ; m, sphenoidal sinus ; n, opening of the Eustachian tube ; o, pen- 

 dulous palate. The frontal sinuses do not exist in young persons, but with 

 age they often acquire considerable dimensions, as do the sphenoidal sinuses. 

 They are generally small in women. 



