The olfactory membrane is a thick yellow-brown structure, about one inch square, located in the upper part of each nasal cavity. It consists of about one hundred million smell receptor cells, which are surrounded by supporting cells. The smell receptor cells have an olfactory vesicle bearing cilia, which project into the mucus that covers the smell membrane. Chemicals in the air react with the cilia and stimulate the receptor cells. The smell information is passed by the receptor cell axons, which leave the membrane as the first cranial nerve, and which relay with mitral cell axons to the olfactory cortex. The supporting sustentacular cells contain a pigment that colors the membrane yellow. The membrane also contains Bowman's glands, which secrete mucus. Their function is to keep the membrane moist so that chemicals can dissolve and stimulate the cilia.
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