This is seen most typically in acute inflammations of serous cavities such as the pericardium or pleura. (Figs. 57 and 58.) In such cases there is, in addition to the serous exudation occupying the cavity, a deposition on the surface of a soft yellow layer of coagulated fibrine. There is frequently a similar deposition on the surface of a freshly-inflicted wound, the fibrine forming a glaze on the surface, while the serous fluid passes off as a discharge. Fibrine is seldom deposited in the meshes of the tissues, unless there be an actual necrosis, as in the case of a boil or a carbuncle, where the slough which forms in the skin is composed partly of dead tissue and partly of fibrine. In acute pneumonia, however, the exudation in the lung alveoli is fibrinous.
Fig. 58. - Acute Pleurisy. Fibrinous exudation on surface of greatly thickened pleura.
The term Lymph, or coagulable lymph, is often applied to the fibrinous exudation as seen on serous surfaces, but the use of this term is not to be commended. From its use by John Hunter the term has interesting historical relations, yet, as it implies a theory which is not now held, namely, that the so-called lymph has the power of developing into organized tissue, its use is apt to lead to confusion. This is all the more true, because the term lymph is frequently used in a very loose way to designate connective tissue formed as a result of inflammation. According to Hunter's view, the connective tissue develops directly from the fibrine, and so it was legitimately called lymph; but as his views are now departed from, this use of the term is quite unwarranted.
The fibrine of the exudation has all the characters of that in an ordinary blood-clot. This is shown in Fig. 59, where a network of fibres is seen filling a lung alveolus. It has also a similar origin to that in the blood-clot. We have seen (under Thrombosis, p. 92) that coagulation of blood occurs when the necessary constituents are present and that in order to this there must be a disintegration of the leucocytes so as to yield the ferment. Leucocytes are always present in the serous exudation, and to bring about their disintegration they must be sufficiently removed from .the influence of the living tissue. This can scarcely occur except in the case of an extended surface, as of a membrane. Just as thrombosis does not occur in the capillaries, because the blood in these narrow vessels is in intimate contact with the living cells forming the capillary walls, so in inflammation the fibrinous exudation scarcely occurs in the serous spaces unless there be actual necrosis of the tissue. In some very acute inflammations of connective tissue (acute phlegmon) there may be a deposition of fibrine, forming a kind of fibrinous oedema, such as sometimes occurs in the skin in erysipelas, but in these cases necrosis is a frequent if not a constant concomitant. It is where the exudation is on a surface, and is to a great extent removed from contact with the living endothelium, that the fibrinous exudation is most constantly found.
Fig. 59. - Fibrine in a lung alveolus.
Epithelium, like endothelium, has the power of preventing the disintegration of the leucocytes, and hence a fibrinous exudation seldom occurs on a mucous, membrane or the skin. Its occurrence implies that the epithelium has undergone necrosis or has been shed.