In connection with mouth and stomach secretions, mention has been made of glands which are elongated saccules lined with clear cells with highly refracting contents (Fig. 165). They are distributed over all mucous membranes, and are the chief source of the thick, tenacious, clear, alkaline, and tasteless secretion called mucus.

This material contains about five per cent, of solid matters, of which the chief is mucin, the characteristic material of mucus, which swells up in water and gives the peculiar tenacity to the fluid. It is precipitated by weak mineral and acetic acids; and, as the precipitate with the latter does not redissolve in an excess, this acid becomes a good test to distinguish it from its chemical allies. Mucin is not precipitated by boiling. Mucus also contains traces of fat and albumin, and inorganic salts, viz., sodium chloride, phosphates and sulphates, and traces of iron.

The fluid is secreted either by the special mucous glands, or it may be produced by the epithelium of the mucous surfaces. The cells produce in their protoplasm a quantity of the secretion, which may often be seen to swell them out to a considerable extent. This clear fluid is then expelled, and the altered cells are repaired or replaced. Many form elements, like the remains of epithelial cells, are found in the secretion; and also round nucleated masses of protoplasm similar to white blood corpuscles after the imbibition of water. In the abnormal secretion of a mucous surface during inflammation these mucous corpuscles are, as well as the general amount of secretion, greatly increased, so that the secretion may become opaque, and may appear to be purulent.

The chief object of the secretion seems to be to protect the mucous surfaces, which are rich in delicate nerves and vessels, and are subjected to many injurious influences of a chemical or mechanical nature. It is analogous to the keratin of the epidermis, and may be regarded as an excretion, since it is not absorbed, but is cast out from the mucous passages, and passes from the intestinal tract with the faeces, and from the air passages as sputum, etc.

Section of the Mucous Membrane of the upper part of nasal cavity.

Fig. 165. Section of the Mucous Membrane of the upper part of nasal cavity showing "numerous Mucous Glands cut in various directions, a, Surface epithelium; b, gland saccule lined with secreting cells; c, connective tissue. (Cadiat).