MUCUS

viscous fluid that moistens, lubricates, and protects many of the passages of the digestive and respiratory tracts in the body. Mucus is composed of water, shed epithelial (surface) cells, dead leukocytes, mucin, and inorganic salts, all held in suspension. Mucus is produced by small cells called mucous cells. These are frequently clustered into small glands, which in turn are located on surface tissues, called mucous membranes (q.v.), which line virtually the entire digestive tract. Great numbers of mucous cells occur in the mouth, where mucus is used both to moisten food and to keep the oral membranes moist while they are in direct contact with the air. Mucus in the nose helps to trap dust, bacteria, and other small particles breathed in with the air. The stomach also has large numbers of mucous cells. There gastric mucus forms a layer about one millimetre thick that lines the stomach walls, protecting them from the highly acidic gastric juice and preventing the juice from digesting the stomach itself.