contained in the air, with the fibers of the
olfactory nerves, which are distributed over the delicate surface of
the upper parts of the nasal cavities. In the lower parts are the endings
of nerves of ordinary sensation. These latter nerves may be irritated by
some substance like ammonia, resulting in a powerfully pungent sensation.
This is not a true sensation of smell, but merely an irritation of a nerve
of general sensation.

In ordinary quiet breathing, the air simply flows along the lower nasal
passages into the pharynx, scarcely entering the olfactory chamber at all.
This is the reason why, when we wish to perceive a faint odor, we sniff up
the air sharply. By so doing, the air which is forcibly drawn into the
nostrils passes up even into the higher olfactory chamber, where some of
the floating particles of the odorous material come into contact with the
nerves of smell.

One of the most essential conditions of the sense of smell is that the
nasal passages be kept well bathed in the fluid secreted by the lining
membrane. At the beginning of a cold in the head, this membrane becomes
dry and swollen, thus preventing the entrance of air into the upper
chamber, deadening the sensibility of the nerves, and thus the sense of
smell is greatly diminished.

The delicacy of the sense of smell varies greatly in different individuals
and in different animals. It is generally more acute in savage races. It
is highly developed in both the carnivora and the herbivora. Many animals
are more highly endowed with this sense than is man. The dog, for example,
appears to depend on the sense of smell almost as much as on sight. It is
well known, also, that fishes have a sense of smell. Fragments of bait
thrown into the water soon attract them to a fishing ground, and at depths
which little or no light can penetrate. Deer, wild horses, and antelopes
probably surpass all other animals in having a vivid sense of smell.

Smell has been defined as "taste at a distance," and it is obvious that
these two senses not only form a natural group, but are clearly
associated in their physical action, especially in connection with the
perception of the flavor of food. The sense of odor gives us information
as to the quality of food and drink, and more especially as to the quality
of the air we breathe. Taste is at the gateway of the alimentary canal,
while smell acts as the sentinel of the respiratory tract. Just as taste
and flavor influence nutr