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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