630 THE SEXSES. 



THE SENSE OF SMELL. 



The sense of smell ordinarily requires, for its excitement 

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

 action produces certain changes in the olfactory nerve ; and 

 this nerve is susceptible of an infinite variety of states de- 

 pendent 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 pro- 

 duce 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 perceived. 

 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 subtle 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 dissolved in the 

 mucus of the mucous membrane before they can be imme- 

 diately 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 membrane 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 



