THE SENSE OF SMELL. 635 



destruction of the olfactory nerves, that Magendie was led 

 to believe the fifth nerve might exercise the special sense. 



Animals do not all equally perceive the same odours ; 

 the odours most plainly perceived by an herbivorous animal 

 and by a carnivorous animal are different. The Oarnivora 

 have the power of detecting most accurately by the smell 

 the special peculiarities of animal matters, and of track- 

 ing other animals by the scent ; but have apparently very 

 little sensibility to the odours of plants and flowers. 

 Herbivorous animals are peculiarly sensitive to the latter, 

 and have a narrower sensibility to animal odours, especially 

 to such as proceed from other individuals than their own 

 species. Man is far inferior to many animals of both 

 classes in respect of the acuteness of smell ; but his sphere 

 of susceptibility to various odours is more uniform and 

 extended. The cause of this difference lies probably in 

 the endowments of the cerebral parts of the olfactory 

 apparatus. 



Opposed to the sensation of an agreeable odour is that 

 of a disagreeable or disgusting odour, which corresponds 

 to the sensations of pain, dazzling and disharmony of 

 colours, and dissonance, in the other senses. The cause 

 of this difference in the effect of different odours is un- 

 known ; but this much is certain, that odours are pleasant 

 or offensive in a relative sense only, for many animals 

 pass their existence in the midst of odours which to us are 

 highly disagreeable. A great difference in this respect is, 

 indeed, observed amongst men : many odours, generally 

 thought agreeable, are to some persons intolerable ; and 

 different persons describe differently the sensations that 

 they severally derive from the same odorous substances. 

 There seems also to be in some persons an insensibility to 

 certain odours, comparable with that of the eye to certain 

 colours ; and among different persons, as great a difference 

 in the acuteness of the sense of smell as among others in 

 the acuteness of sight. We have no exact proof that a 



