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Trachea. Coloured 3D computed tomography (CT) scan of the inside of the trachea (windpipe), looking down to the point where it divides into the two primary bronchi to enter the lungs. The trachea is held open by rings of cartilage, the shapes of which are visible here. This scan is of the trachea of a 45-year-old man. (KEYSTONE/SCIENCE PHOTO LIBRARY/ZEPHYR/SCIENCE PHOTO LIBRARY)