Mucus, a transparent, colorless, and glairy or viscid fluid, exuded upon the free surface of the mucous membranes of the living body. It is the secretion of the numerous glandulae or follicles with which these membranes are provided, and varies in the details of its composition and the degree of its viscidity with the particular region in which it is produced and the special function which it is destined to perform. It does not readily mix with water, but when agitated with that liquid is broken up into floating shreds or flakes, which are apt to become frothy from the entanglement of bubbles of air. It consists of water combined with a small quantity of the mineral salts, and a peculiar variety of animal or organic matter termed mucosine; to this last ingredient the glairy or viscid consistency of mucus is mainly due. The office of mucus is to lubricate the mucous canals and thus facilitate the passage of their contents, as in the mouth, oesophagus, and genito-urinary passages; to protect their surfaces from injury by desiccation, as in the nares, trachea, and bronchial tubes; or to take part in the chemical changes going on in their cavities, as in the small intestine.

In the cervix uteri, during gestation, the mucus has so great a degree of viscidity as to be semi-solid like gum or strong paste; its office is to block up the cavity of the cervix uteri and prevent the escape or injury of the foetus.