568 SMELL. [BOOK in. 



and the sensations to which they give rise are always those 

 of smell. The mucous membrane of the nose is however also an 

 instrument for the development of afferent impulses other than the 

 specific olfactory ones. Chemical stimulation of the olfactory 

 membrane by pungent substances such as ammonia gives rise to a 

 sensation distinct from that of smell, a sensation which affords us 

 no information concerning the chemical nature of the stimulus, 

 and which is indistinguishable from the sensations produced by 

 chemical stimulation of other parts of the nasal membrane as well 

 as of other surfaces equally sensitive to chemical action. It is 

 probable that these two kinds of sensations thus arising in the ol- 

 factory membrane are conveyed by different nerves, the former by 

 the olfactory, the latter by the fifth nerve. 



For the developement of smell it appears necessary that the 

 odorous particles should be conveyed to the nasal membrane in a 

 gaseous medium, or at least that the surface of the membrane 

 should not be exposed at the same time to the action of fluids. 

 Thus, when the nostril is filled with rose-water, the odour of roses 

 is not perceived ; and simply filling the nostrils with distilled water 

 suspends for a time all smell, the sense returning gradually after 

 the water has been removed ; the water apparently acts injuriously 

 on the delicate olfactory cells. 



Each substance that we smell causes a specific sensation, and 

 we are not only able to recognise a multitude of distinct odours, 

 but also to distinguish individual odours in a mixed smell. 



As in the previous senses, we project our sensation into the 

 external world ; the smell appears to be not in our nose, but some- 

 where outside us. We can judge of the position of the odour how- 

 ever even less definitely than we can of that of a sound. 



The sensation takes some time to develope after the contact of 

 the stimulus with the olfactory membrane, and may last very long. 

 When the stimulus is repeated the sensation very soon dies out : 

 the sensory terminal organs speedily become exhausted. Mental 

 associations cluster more strongly round sensations of smell than 

 round any other impressions we receive from without. And reflex 

 effects are very frequent, many people fainting in consequence of 

 the contact of a few odorous particles with their olfactory cells. 



Apparently the larger the surface the more intense the sen- 

 sation; animals with acute scent having a proportionately large 

 area of olfactory membrane. The quantity of material required to 

 produce an olfactory sensation may be, as in the case of musk, 

 almost immeasurably small. 



When two different odours are presented to the two nostrils, an 

 oscillation of sensation similar to that spoken of in binocular vision 

 (p. 552) takes place. 



The assertion that the olfactory nerve is the nerve of smell has 

 been disputed. Cases have been recorded of persons who appeared 

 to have possessed the sense of smell, and yet in whom the olfactory 



