To increase the area of the air passages, the two light, spongy turbinated bones, one on each side, form narrow, winding channels.  The mucous membrane, with the branches of the olfactory nerve, lines the dividing wall and the inner surfaces of these winding passages.  Below all these bones the lower turbinated bones may be said to divide the olfactory chamber above from the ordinary air passages.

[Illustration:  Fig. 126.—­Distribution of Nerves over the Interior of the Nostrils. (Outer wall.)

  A, branches of the nerves of smell—­olfactory nerve, or ganglion;
  B, nerves of common sensation to the nostril;
  E, F, G, nerves to the, palate springing from a ganglion at C;
  H, vidian nerve, from which branches
  D, I, and J spring to be distributed to the nostrils.
]

The nerves which supply the nasal mucous membrane are derived from the branches of the fifth and the first pair of cranial nerves,—­the olfactory.  The latter, however, are the nerves of smell proper, and are spread out in a kind of thick brush of minute nerve filaments.  It is in the mucous membrane of the uppermost part of the cavity of the nostril that the nerve endings of smell proper reside.  The other nerves which supply the nostrils are those of common sensation (sec. 271).

323.  The Sense of Smell.  The sense of smell is excited by the contact of odorous particles 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.