112 



ZOOLOGY. 



OF THE SENSE OF SMELL. 



217. Odours are produced by particles of extreme 

 tenuity, which escape from odorous bodies, and spread 

 through the air like vapours. The quantity requisite of 

 some of these odorous matters powerfully to affect the smell 

 is extremely small : a morsel of musk, for example, will per- 

 fume the air of a room for a considerable time without losing 

 its weight. Some bodies imbibe vapours and become odorous 



in their turn, such as clothes 

 and water ; but others resist 

 their passage altogether, such 

 as glass. Odours may be 

 perceived at a great distance, 

 but the odorous particles 

 must always come in contact 

 with the organs. For this, 

 the mechanism of smell is 

 analogous to that of taste 

 and touch, whilst in sight 

 and hearing it is quite other- 

 wise. 



218. As the air is the 

 ordinary vehicle of odours, 

 the organ perceiving them is placed at the entrance of the 

 respiratory tubes. In man, as well as in mammals, birds, 

 and reptiles, the nasal fossae are the seat of the sense of 

 smell. 



219. These fossae communicate with the exterior by the 

 nostrils, and open behind into the pharynx. They are separated 

 from each other by a vertical partition : their walls are 

 formed by various bones of the face, and by the cartilages of 

 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 rolled on themselves 

 they thus extend considerably the pituitary membrane, which 

 invests them. Upon this membrane the olfactory nerves are 



* Vertical section of the nasal fossae, 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. 



t That is, twisted in a spiral manner, like a univalve shell. 



Fig. 72. Nasal Fossae in Man.* 



