This is characterized by the presence in the tissues of Mucin. This is a normal secretion of certain glands, and is a body of definite chemical reactions. It is closely allied to albumen, but it is precipitated by dilute mineral acids, and by organic acids (acetic acid), and is not re-dissolved by excess of acid. With alcohol it gives a membranous and fibrous coagulum, which is partly re-dissolved in excess of water. Albumen, on the contrary, is not precipitated by organic acids, and its precipitate with alcohol is flocculent, and not re-dissolved by water. Mucin is detectable by a colour test, the dye Used being toluidin blue. This substance, which is nearly allied to methylblue, produces a blue nuclear staining, but gives a red colour with mucin. The physical characters of mucin are notable in that, even in small amounts, it gives fluids a sticky, tenacious character. Thus a fluid containing 5 per cent, of mucin is tenacious, while the blood serum which contains 9 per cent, of albumen is quite liquid. Paralbumin is closely allied to mucin, if not the same substance.

Mucin is present pathologically either in cells or in the intercellular substance. In cells it has its physiological type in the secretion of mucus. This takes place by a transformation of epithelial cells, which may be either in proper mucous glands or else on the surface of mucous membranes. The cells show in their protoplasm a clear substance which gradually distends them, and they become goblet cells. The mucin is discharged, the cell being either destroyed or returning to the normal condition. An exaggeration of this process occurs in catarrhs of mucous membranes, but this can scarcely be called mucous degeneration. There may also be an accumulation of mucus in a cavity or cyst, but this also is to be distinguished from degeneration.

A definite mucous degeneration occurs in tumours, notably in ovarian tumours, where the result is rather paralbumin than mucin, and in certain cancers. In the colloid ovarian cystoma, the mucous (or colloid) matter is produced by a process of secretion in glandular structures, goblet cells being characteristically present. (See Fig. 52.) In colloid or mucous cancers the epithelial cells of the tumours undergo a mucous transformation. The tumours which present this change have mostly their seat where cylindrical epithelium is a normal constituent.

From internal surface of a colloid ovarian cyst.

Fig. 52. - From internal surface of a colloid ovarian cyst. The lining epithelium is mostly in the form of goblet cells, the superficial parts having clear transparent contents, x 350.

Mucin differs from albumen in respect that it has no tendency to become absorbed, so that a cavity containing a mucous fluid is not likely to have its contents reduced by absorption.

Mucin is present in the intercellular substance in the tissue of the umbilical cord, which may be taken as the type of Mucous tissue. In this tissue the intercellular substance is composed of a soft jelly. Tumours occur which are composed of a similar tissue. (See Myxoma.) A true mucous degeneration occurs when the dense matrix of cartilage or bone becomes transformed into a jelly containing mucin, or when the adipose tissue becomes like mucous tissue.

Myxoedema is a condition in which mucin is present in the skin, subcutaneous tissue, etc. It is associated with changes in the thyroid gland. (See further on).