Illustration of the structure of the nose. The nose is the organ of smell, and uppermost part of the respiratory tract. It also acts as a resonator for the voice. Plates of cartilage form the exter- nal nose, opening at nostrils which are haired to prevent foreign objects entering. The inner nasal cavity contains projecting bones called conchae (three curling plates), covered with a mucous membrane. This membrane has blood vessels (here, coloured red and blue) which warm the air; and nerves (yellow, leading to the olfactory bulb of the brain) responsible for smell. Once smelled, the air is warmed, moistened, and filtered of dust by mucous, before it passes to the lungs.