Tracheal epithelium. Scanning electron micrograph (SEM) of the internal lining, or epithelium, of the human trachea. The numerous fine hair-like projections, called cilia, protrude from the tops of specialised epithelial cells. Between the epithelial cells lie goblet cells whose function is to release mucus onto the surface of the epithelium. Rhythmic movements of the cilia serve to move bacteria and other particles caught in the mucus away from the gas-exchanging parts of the lungs, up the trachea and towards the throat where they can be swallowed or coughed up. Magnification: x4000 at 10x8 inch size.