Nasal cavity. Coloured scanning electron micrograph (SEM) of the surface of the nasal cavity. It is covered in epithelial hair-like structures known as cilia (orange), which line the respiratory tract. They are covered with a wet, sticky mucus that humidifies inhaled air and traps dust particles and other air pollutants. Co-ordinated, wave-like beating of the cilia propels the mucus to the back of the nose (pharynx), where it is swallowed. Magnification: x1800 at 6x7cm size.