Processes of exudation are frequently met with in particular portions of the system of mucous membranes; but their nature is very various, as their products, and the condition of the mucous membrane in connection with them, manifest.

The best known exudative processes upon mucous membranes are those named croupy inflammations, especially those that occur in the pharynx and air-passages. They are characterized by their plastic product, which varies in consistence from that of cream to the toughness of leather, and is grayish-white, or yellowish and fibrinous; it sometimes covers the membrane at a few insulated spots, and sometimes forms a more extensive film over it like hoar-frost; occasionally it invests the membrane like a layer of gauze, while in some cases it constitutes a membranous, and very often a tubular, lining for the mucous surface.

From all analogy it is probable that, at the commencement of the process, a serous fluid is effused by which the epithelium of the diseased mucous membrane is destroyed, and that the exudation of the plastic matter takes place afterwards. This matter, the general characters of which have just been depicted, forms in a severe case a membranous coagulum, the thickness of which may vary, but not unfrequently equals, or even exceeds, a line: towards its margin it is thinner and less tough, and it is at length lost in a layer of muco-purulent substance. At first it adheres to the mucous membrane, and on that surface which adheres to the membrane some incipient vascularity is sometimes seen in the form of small bloody points; some of these points are single, others divide into fine twigs towards their peripheral extremity. At a later period, a viscid, muco-purulent fluid is effused beneath the plastic exudation, so that it becomes loose, and is at length set free.

The mucous membrane underneath the exudation is variously tinted, but for the most part is of a very pale rosy color: it looks sore and excoriated, and is more or less swollen, and its papillae especially are distinctly swollen; its surface is covered with numerous red, soft, bleeding spots like granulations, which correspond to the vascular points on the adherent surface of the exudation. The submucous tissues, especially the cellular tissue, appear infiltrated.

Neither during nor after the croupy process does the mucous membrane suffer any material injury to its texture; the speedy production of mucus and epithelium prevents any further organization of the plastic exudation beyond the rudimentary condition just described, and it never enters into an organic connection with the mucous membrane.

The croupy process occurs on all mucous membranes, and sometimes extends over a very wide tract. The mucous membrane of the respiratory organs shows an especial tendency to it, and we meet accordingly with laryngeal croup, tracheal croup, bronchial croup, and croupy pneumonia. In those parts, and on the inner surface of the uterus after childbirth, it is very often a primary process; while on most other mucous membranes it is only secondary, and occurs principally as a consequence and an expression of the degeneration of an exanthematous, or typhous, or some other process attended with exudation, such, for instance, as the cholera process; it arises also from pyaemia, etc.

Other exudative processes give rise, either from the first and exclusively, or else after furnishing, or whilst furnishing, a plastic product, to a loose, pulpy, puriform or sanious exudation, of a variously shaded brown and green color, and a very offensive smell. The mucous membrane, under such an exudation, softens to a pulpy, or a shreddy and crumbling mass, which has an offensive smell and the same color, or may be also dark brown, chocolate-colored, or like coffee-grounds from hemorrhage. These processes are named putrefactive, and may be primary, but they are much more frequently secondary.

A special form in which these exudations appear, is that of the benign and malignant aphthae, - exudations, that is, which, at first, at least, are confined to rounded or oval spots. They are most common by far on the mucous membrane of the mouth and pharynx; they do, however, occur on all other membranes, but are then generally secondary. The process of softening that goes on beneath the exudation, occasions a loss of substance in the mucous membrane, that may be called an aphthous ulcer.

Other exudative processes, which for the most part extend over large portions, or the whole tract, of a mucous membrane, furnish products that are either albuminous, jelly-like, and pellucid; or milky, mixed with delicate fibrinous flakes, and pasty; or thin fluid, mostly serous, and of a very pale grayish-white, yellowish, or reddish color, or quite colorless. They run their course, sometimes with moderate redness and injection, sometimes with remarkable paleness of the mucous membrane, with tumefaction, infiltration, and at length softening and removal of the epithelium, with softening of the tissue of the membrane itself, and degeneration of it to a pale-grayish, yellowish, rosy, or dark-red stratum that is apt to bleed, and may be wiped off like pap, and with similar softening of the follicles. Such processes are, for the most part, secondary, and their chief seat is the mucous membrane of the intestinal tract. The most remarkable of them for extent, for quantity of product, and for the rapidity of its course, has been learnt in modern times, - the Asiatic cholera.

In very severe cases of the exudative process, the submucous muscular tissues become paralyzed: they are blanched, relaxed, and infiltrated.