OF SMELL. 37 



nostrils communicate with bony cavities situated in the 

 bones of the face and forehead. These cavities are lined by 

 a thick membrane, the pituitary membrane, on which are 

 expanded the nerves of smell, namely, the olfactory nerve, 

 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 par- 

 ticles are recognized by the nerves of smell only, which 

 transmit the impressions made by them to the brain. 

 Smell differs, consequently, from sight and hearing, in being 

 produced by a material body, and not by a simple undulatory 

 movement. 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 perfection 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 air. Their nostrils are mere superficial pouches, 

 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. The 

 perfection of smell depending on the amount of surface 

 exposed, it follows that those fishes which have these 



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