B. Opacity And Thickening

In those parts where the membrane is reddened and injected, and still more evidently in the interspaces between them, it becomes dull, loses its lustre, transparency, and smoothness, and acquires opacity and a velvety internal surface. Serous membranes of very delicate structure, such as the arachnoid, become at once opaque, dull, and turbid, like whey or milk. This change, and to some extent also the thickening, are due to infiltration of the membrane; but the thickening is produced rather by the simultaneous affection of the subserous cellular tissue; for that tissue is always injected, is early filled with an opaque serous fluid, and is consequently tumid. The infiltration of the subserous tissue extends to the membrane itself, and the two become so blended as to lose all trace of separation from one another. From the expansion, or loosening, of the tissues which takes place, the serous membrane is rendered not only very fragile in itself, but also easily separable from the structures beneath it.

y. The effusion or inflammatory product upon the free surface of the serous membrane is sometimes an exudation, of plastic nature, but at other times, and especially in inflammations which arise from metastasis, it is diffluent and puriform, or actually purulent, or sanious. In quantity it is usually not very considerable, at least when compared with the amount of effusion attending chronic inflammation, though there are exceptions to this rule, especially in the case of exudations of a croupy character.

The plastic exudation is mostly accompanied by an effusion of serous fluid, so that the whole product may be distinguished into a plastic or coagulable portion, and another which is not coagulable. The relative quantity of the two portions varies considerably: there are some exudations which have no serous part whatever, while others contain no more of the plastic matter than suffices to render the serous effusion slightly opaque. An effusion of perfectly clear serous fluid, unaccompanied by any deposit of lymph upon the inner surface of the serous membrane, is scarcely such as can be attributed to actual inflammation.

The plastic portion of the exudation is deposited upon the inner surface of the serous membrane, and forms there a peripheral fibrinous layer which encloses the serous effusion, if any exist: it is of a grayish-red, a yellow, or a grayish color, and may vary in thickness from that of a scarcely perceptible film, like hoar frost, to that of several lines. Its inner free surface is sometimes tolerably smooth, sometimes villous, sometimes shreddy, sometimes areolar; occasionally it resembles waves of sand, or the back of a bullock's tongue. When the plastic matter is very abundant, it forms other large masses also of loose texture, and soaked through with more or less of the serum: it may also render the serum opaque, or may lie in it in flakes, which soon become arranged in plates and cords, and form a network or honeycombed cellular structure, the large interspaces of which enclose part of the serum. It may also fall upon the inner surface of the original peripheral coagulum, and form a soft shreddy covering for it; and in that case, the exudation lining the serous membrane consists of two layers: - the original, more consistent, plastic exudation; and the secondary, looser, shreddy precipitate upon it.

Inflammations of an eminently exudative character are particularly remarkable.

For the inconsiderable reddening and injection of the serous membrane:

For the disproportion which subsists between the reddening and injection, and the great quantity of exudation deposited at one time:

For the marked loosening of tissue, and infiltration observable both in the membrane and in the subserous structure:

For their frequently coexisting with exudative processes in mucous membranes, the plastic character of which may differ from that of the inflammation in the serous membrane:

And as a general rule also for the homogeneous nature of the whole product, and for the absence or mere indication of a separation of it into one part which is plastic, and another incapable of coagulation: it is a uniform exudation, which coagulates more or less, or degenerates into pus, or is sanious.

There is sufficient general connection between the exudations under consideration and another found upon large serous membranes, like the peritoneum and pleurae, to allow of the latter being mentioned here. It is a viscid coating upon these membranes which gives them a dull lustreless aspect: it is best marked and most constantly seen in cases of Asiatic cholera, but it occurs also in the course of other exudative processes which are attended by exhaustion, such, for instance, as the diarrhoea of children.

Plastic exudations very frequently produce adhesions between the walls of a serous cavity and the viscera it contains, as well as of the viscera with one another: but a large quantity of serous effusion holds the lamellae of the plastic exudation apart, and no adhesion can take place unless the fluid be absorbed before the lymph is completely organized.

Absorption commences as soon as the lymph is deposited, and the intensity of the inflammation subsides: it depends, therefore, upon the cessation of the inflammatory process; but it is also influenced by the thickness and density, that is to say, by the permeability, as well as by the stage of organization, of the lymph.

In the organization of plastic exudations, new vessels are spontaneously formed in more or less abundance, and a tissue is produced, which is either cellular, loose, and cellulo-serous; or of closer texture, strong and, as it is called, cellulo-fibrous. If the serous surfaces have been agglutinated together by the exudation, their complete vital adhesion is effected by the formation of a loose filaceous, or of a dense and more compact tissue in the exudation: but in the opposite case, the old serous membrane is either covered with isolated delicate cellular flakes, or.with larger shreddy masses of cellular tissue, or with a (second) delicate serous membrane, which can be moved over the original one, or lastly with a thicker, fibro-serous, and firmly attached layer. The layer last mentioned may be uniformly thick, or thinner here and there, and thus acquire a knitted areolar, or cribriform appearance: it may, with a little care, be stripped off the serous membrane, and when confined to small spots, it constitutes the tendinous or white spots (maculae lactese) which are found on some serous membranes, especially on the pericardium.

In some cases, the solid exudation, as it becomes organized, encloses within it a part of the serous fluid, and thus forms delicate transparent vesicles, which are mostly found afterwards attached by a pedicle.

These new structures again may themselves be attacked by inflammation: this remark, however, is opposed by an observation made by Laennec upon the pleura; for he found that inflammations of the portion of a serous membrane which had been left unaffected by a previous inflammation, were usually circumscribed by the new structures, and at the adhesions produced by them.

The puriform, the really purulent, and the sanious, exudations, are either deposited as such, or are formed out of the plastic exudation, which degenerates thus in consequence of some peculiar quality inherent in it. Such exudations are rather thick, cream-like, and yellow or greenish; or of a thin fluid consistence, and a greenish, brownish, or reddish color: the serous membrane itself is discolored; both it and the tissue beneath it are opaque and much infiltrated; and its inner surface is very dull, and appears, particularly when the exudation is purulent, like velvet or spongio-piline. If the inflammation does not destroy life, either of itself, or through the general constitutional disorder which accompanies it, it usually becomes chronic.