Mucus (adjective – mucous). The general name for the slimy secretion derived from mucous membranes. It is mainly composed of a substance called mucin, which varies according to the particular mucous membrane from which it is derived, and contains other substances, such as cells cast off from the surface of the membrane, enzymes, and dust particles. Mucin has the following characteristics: it is viscid, clear and tenacious; when dissolved in water it can be precipitated by addition of acetic acid; and when not in solution already, it is dissolved by weak alkalis, such as lime-water.

Under normal conditions the surface of a mucous membrane is lubricated by only a small quantity of mucus; the appearance of large quantities is a sign of inflammation.