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olfactory organ[äl′fak·trē ‚ȯr·gən]
the organ in man and animals that receives chemical stimuli that are interpreted as smells. The sense of smell is technically known as olfaction.
In man the olfactory organ is the superoposterior area of the nasal cavity, including part of the nasal septum and the two turbinate bones, which protrude from the lateral walls of the cavity. The entire olfactory organ is covered by the olfactory epithelium, whose surface area is about 5 cm2. In many macros-matics—mammals with highly developed olfaction—the olfactory area of the nose is enlarged because of the presence of supplementary turbinate bones in the bony wall of the nasal cavity. In reptiles and some mammals the nasal septum contains the vomeronasal organ, or Jacobson’s organ, in addition to the principal olfactory organs. In fish the olfactory organs are represented by paired nasal depressions or by nasal pouches, which are located on the head adjacent to the mouth. These olfactory organs include numerous connective-tissue membranes that are covered with olfactory epithelium. In insects the olfactory sen-silla—sensitive structures that are located mainly on the antennae—serve as olfactory organs. In a number of mollusks the osphradia serve as olfactory organs.
The spindle-shaped olfactory receptor cells, which are located in the olfactory organs, are the functional units of the olfactory apparatus. The rabbit has about 100 million such cells; man, about 10 million; and the male silkworm, about 40,000. Two outgrowths emerge from each olfactory receptor cell. The long, thin central outgrowth joins with the olfactory nerve; the second outgrowth extends to an external surface of the olfactory organ. In vertebrates the second outgrowth surfaces on the olfactory epithelium with a club-shaped thickening. Bundles of flagella that are from 0.25 to 0.3 microns thick and a few tenths of a micron long extend from these thickenings. In birds and reptiles the olfactory receptors are also equipped with short, finger-like outgrowths called microvilli.
In the olfactory sensilla of insects, each receptor cell has one flagellum. Outgrowths extend from each flagellum toward openings in the cuticle, which is the external protective layer of epidermis that covers the sensilla. In terrestrial and aquatic vertebrates and in insects the olfactory flagella are immersed in mucus that is secreted by glandular cells. This mucus is the interstitial medium into which molecules of odorous substances enter from the air or from water. It is assumed that the initial interaction of the receptor cells with the molecules of odorous substances occurs in the flagella of the olfactory receptors.
REFERENCESVinnikov, la. A., and L. K. Titova. Morfologiia organa obonianiia. Moscow, 1957.
Ivanov, V. P. “Ul’trastrukturnaia organizatsiia khemoretseptorov nasekomykh.” Trudy Vsesoiuznogo entomologicheskogo obshchestva, 1969, vol. 53.
Bronshtein, A. A. “Strukturnaia organizatsiia perifericheskikh otdelov oboniatel’nogo analizatora i oboniatel’noi lukovitsy.” In Fiziologiia sensornykh sistem, part 2. Leningrad, 1972.
A. V. MINOR