The uppermost part of the respiratory tract, and the organ of smell. The nose is an air passage connecting the nostrils at its front to the nasopharynx (the upper part of the throat) at its rear. The nasal septum, which is made of cartilage at the front and bone at the rear, divides the passage into 2 chambers. The bridge of the nose is formed from 2 small nasal bones and from cartilage. The roof of the nasal passage is formed by bones at the base of the skull; the walls by the maxilla (upper jaw); and the floor by the hard palate. Three conchae (thin, downward-curving plates of bone) covered with mucous membrane project from each wall. Air-filled, mucous membrane-lined cavities known as paranasal sinuses open into the nasal passage. There is an opening in each wall to the nasolacrimal duct, which drains away tears. Projecting into the roof of the nasal passage are the hair-like endings of the olfactory nerves, which are responsible for the sense of smell. A main function of the nose is to filter, warm, and moisten inhaled air before it passes into the rest of the respiratory tract. Just inside the nostrils, small hairs trap large dust particles and foreign bodies. Smaller dust particles are filtered from the air by the microscopic hairs of the conchae. The mucus on the conchae flows inwards, carrying microorganisms and other foreign bodies back towards the nasopharynx to be swallowed and destroyed in the stomach. The nose detects smells by means of the olfactory nerve endings, which, when stimulated by inhaled vapours, transmit this information to the olfactory bulb in the brain. The nose is susceptible to a wide range of disorders. Allergies (see rhinitis, allergic), infections such as colds (see cold, common), and small boils are common. Backward spread of infection from the nose occasionally causes a serious condition called cavernous sinus thrombosis. The nose is also particularly prone to injury (see nosebleed; nose, broken). Obstruction of the nose may be caused by a nasal polyp (a projection of swollen mucous membrane). Noncancerous tumours of blood vessels, known as haemangiomas, commonly affect the nasal cavity in babies. Basal cell carcinoma and squamous cell carcinoma may occur around the nostril. The nose may also be invaded by cancers originating in the sinuses.


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