180 NATURE AND LIFE. 
By the aid of the muscles covering the lower part of the 
nostrils, the apparatus of smelling can be dilated or con- 
tracted, precisely like that of sight. This understood, the 
mechanism of olfaction is quite simple. It consists in the 
contact of odorous particles with the olfactory nerve. 
These particles are conveyed by the air to the inside of 
the nasal cavities, and there strike upon the sensitive 
fibres. If the access of air is prevented, or if the nerve is 
altered, no sensation is produced. Experiments in physi- 
ology, in fact, have settled that the olfactory nerves (or 
those of the first pair) are assigned exclusively to the per- 
ception of odors. Loss of the sense of smell oceurs when- 
ever the nerves are destroyed or injured by any process, 
or even whenever they are merely compressed. On the 
other hand, it is a matter of common observation that im- 
peding the passage of air into the nostrils is quite as 
effectual a way of making any sort of olfactory sensation 
impossible. Let us add that the region most sensitive to 
odors is that of the upper part of the nasal cavities. There 
are, as we Shall notice in proceeding, considerable differ- 
ences as regards the degree of sensitiveness in this sense 
_of smell, comparing one man with another. But it is a 
still more singular fact that sometimes, without apparent 
cause, the sense is utterly wanting. In other cases it is 
unaffected by the action of certain odors only, an analo- 
gous infirmity to that which students of the eye call dalton- 
ism, and which consists in the perception of certain colors 
only. We find in scientific annals the case of a priest who 
was insensible to all odors except that of a manure-heap, 
or that of decayed cabbage; and another, of a person to 
whom vanilla was entirely without scent. Blumenbach 
speaks too of an Englishman, with all his senses very acute, 
who perceived no perfume in mignonette. 
Olfaction is sometimes voluntary, sometimes involun- 
tary. In the former case, by an act which is called scent- 
