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Bronchial cilia. Coloured scanning electron micrograph (SEM) of a normal human bronchial epithelium, the mucus membrane lining the major airways of the lung. Clumps of hair-like cilia (green/pink) protrude from the tops of specialised epithelial cells (brown). Rhythmic movements of the cilia serve to move bacteria and other particles away from the gas-exchanging parts of the lung and towards the throat, where they can be expelled. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)