THE SENSE OF SMELL. 631 



THE SENSE OF SMELL. 



The sense of smell ordinarily requires, for its excitement 

 to a state of activity, the action of external matters, which 

 action produces certain changes in the olfactory nerve ; 

 and this nerve is susceptible of an infinite variety of states 

 dependent on the nature of the external stimulus. 



The first condition essential to the sense of smell is the 

 existence of a special nerve, the changes in whose condition 

 are perceived as sensations of odour ; for no other nerve is 

 capable of these sensations, even though acted on by the 

 same causes. The same substance which excites the sen- 

 sation of smell in the olfactory nerves may cause another 

 peculiar sensation through the nerves of taste, and may 

 produce an irritating and burning sensation on the nerves 

 of touch ; but the sensation of odour is yet separate and 

 distinct from these, though it may be simultaneously per- 

 ceived. The second condition of smell is a peculiar state 

 of the olfactory nerve, or a peculiar change produced in it 

 by the stimulus or odorous substance. 



The material causes of odours are, usually, in the case of 

 animals living in the air, either solids suspended in a state 

 of extremely fine division in the atmosphere ; or gaseous 

 exhalations often of so subtile a nature that they can be 

 detected by no other re-agent than the sense of smell 

 itself. The matters of odour must, in all cases, be dis- 

 solved in the mucus of the mucous membrane before they 

 can be immediately applied to, or affect the olfactory 

 nerves; therefore a further condition necessary for the 

 perception of odours is, that the mucous membrane of 

 the nasal cavity be moist. When the Schneiderian mem- 

 brane is dry, the sense of smell is impaired or lost ; in the 

 first stage of catarrh, when the secretion of mucus within 

 the nostrils is lessened, the faculty of perceiving odour is 

 either lost, or rendered very imperfect. 



In animals living in the air, it is also- requisite that the 



