SENSE OF SMELL. 401 



508. Besides receiving the Olfactory nerve, the mucous mem- 

 brane of the nose is supplied by branches of the Fifth pair ; 

 this nerve endows it with common sensibility, and also receives 

 the impressions produced by acrid or pungent vapours, which 

 act upon it in the same way as the corresponding fluids do 

 upon the tongue. Such vapours are felt by the irritation they 

 produce, rather than smelt; and the impression they occasion 

 gives rise to the reflex action of sneezing, by which they are 

 driven from the nose ( 342). Hence this action may be 

 excited by an irritating agent (such as snuff) after the olfac- 

 tory nerve has been divided, if the branches of the fifth pair 

 be entire : whilst it does not take place when the fifth pair is 

 paralysed, even though the sense of smell may be retained. 

 This sense loses much of its acuteness, however, when the 

 branches of the fifth pair supplying its organ can no longer 

 discharge their functions ; for the membrane then becomes 

 dry from the want of its proper secretion, and the odoriferous 

 particles are consequently not properly applied to it. 



509. Among animals that live in water, the olfactory organs 

 cannot act to the like advantage ; and we do not find much 

 provision made for this sense. In the Whale tribe, the nostrils 

 serve as the channels by which the water is expelled, that has 

 been drawn-in through the mouth ( 185) ; they are situated 

 at the top of the head, and are known as blow-holes. In 

 Fishes, the nasal cavity has no posterior opening; but the 

 surface of its lining membrane is very much extended by its 

 arrangement in folds, which are sometimes disposed in a 

 radiated manner around a centre, and sometimes parallel like 

 the teeth of a comb. There are many Invertebrated Animals, 

 from whose actions it may be judged that they possess a deli- 

 cate sense of smell, although the precise seat of it cannot be 

 assigned. This is the case especially with Insects, Crustacea, 

 and the higher Mollusca. The lining membrane of the air- 

 tubes of Insects appears to be delicately sensitive to irritating 

 vapours ( 443) ; but we have no evidence that it ministers 

 to the sense of Smell properly so called. 



Sense of Hearing. 



510. By this sense we become acquainted with the Sounds 

 produced by bodies in a certain state of vibration. The vibra- 

 tions which sonorous bodies undergo, are communicated by 



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