OF SMELL. 61 



mammals, the outer walls of the nose are composed of carti- 

 lage ; but internally, the nostrils communicate with bony cav- 

 ities situated in the bones of the face and forehead. These 

 cavities are lined by a thick membrane, the pituitary mem- 

 brane, on which are expanded the nerves of smell, namely, 

 the olfactory nerves, and some filaments of the nerve which 

 goes to the face. 



108. The process of smelling is as follows. Odors are 

 particles of extreme delicacy which escape from very many 

 bodies, and are diffused through the air. These particles 

 excite the nerves of smell, which transmit the impressions 

 made on them to the brain. To facilitate the perception of 

 odors, the nostrils are placed in the course of the respiratory 

 passages, so that all the odors which are diffused in the air 

 inspired, pass over the pituitary membrane. 



109. The acuteness of the sense of smell depends on the 

 extent to which the membrane is developed. Man is not so 

 well endowed in this respect as many animals, which have 

 the internal surface of the nostrils extremely complicated, as 

 it is especially among the beasts of prey. 



110. The sense of smell in Reptiles is less delicate than 

 in the mammals ; the pituitary membrane, also, is less de- 

 veloped. Fishes are probably still less favored in this 

 respect. As they perceive odors through the medium of 

 water, we should anticipate that the structure of their 

 apparatus would be different from that of animals which 

 breathe in the air. Their nostrils are mere superficial pouch- 

 es, lined with a membrane gathered into folds which gen- 

 erally radiate from a centre, but are sometimes arranged 

 in parallel ridges on each side of a central band. As the 

 perfection of smell depends on the amount of surface 

 exposed, it follows that those fishes which have these 

 folds most multiplied are also those in which this sense is 

 most acute. 



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