Recklinghausen regards a hyaline substance, hyalin, as a normal constituent of cell-protoplasm which escapes in drops when the cell dies. Its presence indicates a diminution in the vitality of the cell from various causes. Under the microscope it appears as a sharply defined, highly refractive meshwork, enclosing spaces of irregular shape and size, in {82} which are frequently found nuclei, more rarely cells or granules. Langhans has described this appearance as channelled fibrin. It has been met with in the placenta, diphtheritic membranes, blood-vessels, tubercles, and gummata. The latest contribution to the history and nature of this form of degeneration has been furnished by Vallat,[33] from whose article many of the above data have been obtained. [Footnote 33: _Virchow's Archiv_, 1882, lxxxix. 193.] _Mucous Degeneration, Mucous Metamorphosis, Mucous Softening._ Of the various degenerations presenting a colloid--_i.e._ gelatinous--condition, the mucous variety is one of the most striking. Its gross appearances may not differ materially from those to be described under the head of colloid degeneration, but the diagnostic characteristic of the change is to be found in the presence of mucin. The presence of this substance is readily detected by the addition of acetic acid to mucus, the effect being a fibrillated appearance of the latter, the fibres presenting a more or less parallel distribution. This fibrillation of mucus is regarded as the result of a coagulation of its mucin, previously held in solution by an alkali. Mucin is thus present in the body as a normal constituent, and, in the secretions from mucous membranes, owes its origin to the existence of epithelial cells, whether these represent gland-cells, as in the case of the muciparous glands of the bronchial mucous membranes, or whether they are superficial cells, as those of the gastric and intestinal mucous membranes. In the origin of mucus as a secretion from glands Heidenhain[34] claims that a destruction of gland-cells accompanies the continuance of the secretion. At the outset, however, the mucin escapes from the cells, the latter remaining relatively intact. With the persistence of the secretion there results a destruction and a new formation of the muciparous cells. In the pathological production of mucus from mucous membranes, as in catarrh, there is no reason to doubt that the persistence of an irritation is the cause of abundant mucus, and that the latter is dependent upon the rapid formation and destruction of epithelial cells. [Footnote 34: _Hermann's Handbuch der Physiologie_, 1880, v. 64.]