The numerous delicate nerves which pass from the olfactory bulb to the mucous membrane of the upper and part of the middle meatus of the nose form the special nerves of smell. When certain subtle particles we call odors come in contact with the terminals of these nerves they excite impulses which, on arriving in the special centres of the brain, give rise to the impressions of smell.
Anatomically, the relations of the olfactory region are well defined. Its. mucous membrane is not covered with motile cilia, as is that of the rest of the nasal cavity, and it is less vascular and peculiarly pigmented, looking yellow to the naked eye when compared with the neighboring membrane. The epithelial cells are elongated into peculiar cylinders, between which lie long thin rods, ending on the surface in free hair-like processes. The deeper extremities of these rod-shaped filaments expand to surround a nucleus, and are then continued into a network of filaments, into which prolongations of the epithelial cells also seem to pass, and in which the delicate fibrils of the olfactory nerve can be traced. The existence of direct communication between the nerves and the rod-shaped filaments and the epithelial cells is satisfactorily established in some animals.
The odorous particles must be in the form of gases, in order to be carried by the air into the olfactory region, and the air must be kept in motion, by sniffing it in and out of the nasal cavity, in order to excite the nerve terminals, which are not influenced by the odors of air absolutely at rest, though it be in contact with the mucous membrane of the olfactory tract.
The extreme delicacy of appreciation of odors by the olfactory nerve terminals is very remarkable. Even in human beings, whose sense of smell is but poorly developed when compared with that of animals, an amount of odorous substance can be perceived which the finest chemical tests fail to appreciate. Thus, Valentin has estimated that the two-millionth of a milligram of musk is sufficient to excite the specific energy of a man's olfactory apparatus.
Fig.215. Section through the mucous membrane of the nasal fossa in the level of the olfactory region. a, Epithelial cells and bundles of nerves; b, Glands separated from each other by bundles of nerves, c. (Cadiat).
No satisfactory classification of odors has been made out. The common division into agreeable and disagreeable smells, or scents and stinks, is dissimilar in different individuals, and therefore cannot have a physiological basis.
With smell, as with taste, no degree of intensity of stimulation can be said to produce pain, though disgust, nausea, vomiting, and many other nervous operations, may be induced by various smells. The appetites are either excited or annulled by different excitations of the olfactory nerves.