The question that next suggests itself is, " What relations do these fluids bear to the serous membranes ?" From a comparison of the analyses quoted above, it is sufficiently obvious that amid multiform phases of com position all these fluids preserve a close re semblance to the serum of the blood ; a feature which sufficiently testifies to their origin and import, and which refers their production to the conditions of the blood, and their consi deration to the pathology of this fluid, rather than to the serdus membranes in contact with which they are found. And the bearing of this evidence is corroborated by several other facts. In a vast majority of cases, as above mentioned, their occurrence may be directly traced to blood disorders ; either a qualitative affection of this fluid, or a me chanical distention of its containing vessels,— a mutual dependence which tends still more to allot them to the blood rather than to the serous membranes. Again, instead. of their presenting the cellular form, in which the ele ments of secretion, morbid as well as healthy, are usually involved, and which they might be expected to assume were they essentially the product of the cell-growing membrane, they are devoid of all appearances of such organ ization. While in place of being peculiar to these membranes, it is found that an iden tical effusion obtains in the areolar tissue ; a structure which is alike destitute of their membranous form and epithelial covering.
Inflanzmatory or fibrinous effusions. — A large ntunber of the fluids which are found effused in the interior of the serous mem branes offer characters which essentially dis tinguish them from the dropsical effusions above described. The first and most pro minent differences are those presented by their appearance and chemical composition. In addition to the albumen and salts which form the main constituents of the serous effusions, they also offer a greater or lesser quantity of fibrine ; and as this substance retains its or dinary power of spontaneous coagulation, its presence is readily recognized by the eye.
To these physical differences accede equally important pathological grounds of discrimina tion. The effusion of the fibrinous fluid is usually attended by more or fewer of those symptoms, the aggregate of which is known by the name of inflammation ; and in the few instances where these external indications are absent, the presence of the fluid is itself con sidered sufficient evidence of the previous oc currence of the inflammatory process ; while the mechanical causes, which often appear mediately or immediately to determine the occurrence of the shnply albuminous effusion, seem to have no influence in the production of these phenomena. Lastly, the fibrinous effusion is distinguished by this important quality, that it is susceptible of organization, or capable of an apparent conversion into tissues, the structure of' which closely approxi mates to that of some one or other of the normal and permanent textures of the human body.
The class of effusions characterised by the possession of the common properties of fibri nous composition, inflammatory origin, and susceptibility. of organization, is. a very large one, and includes a great variety of fluids, The extremes of these numerous gradations offer some contrast ; in one the symptoms of inflammation are well marked, and the effusion chiefly consists of matters which are plastic : i.e. which pass rapidly from a fluid state, through that of an uniform pasty mass, into a solid form ; and which for the most part ex perience a rapid and complete organization, being converted either into pus or into some inure permanent structure. In the other sub division, the symptoms of inflammation are usually less marked, the fluid contains less fibrine, is less susceptible of organization, and not only remains chiefly fluid, but, in a large number of instances, does not deposit any part of its contents in a solid form until sub sequently to its removal from the living body, or after the death of the patient, In the earliest stage of inflammation, and before effimion has yet occurred, the morbid appearances of the serous membranes are limited to au injection, or active congestion of their vessels, Most of these, it will be recol lected, are arranged as a flattened plexus in the areolar tissue which forms so large a part of the membrane ; and the injection of this plexus, at first in isolated points, and after wards in larger patches, gives to these parts of the free surface a heightened red colour, which is clearly visible through the thin and almost transparent layer of cells, alone inter vening between the capillaries and the interior of the membrane. But although a superficial, patchy, and well-marked redness, dependent on congestion of the minutest vessels, con stint tes a tolerable presumption of the presence of inflammation, yet such a state can be so closely imitated by conditions which are not inflammatory,— such as a merely passive ve nous congestion, due to position of the body, mode of death, and a variety of other causes, —as to be, in a majority of cases, of little value as evidence of this process. And even in instances where the symptoms during life have rendered the existence of inflammation probable, an examination after death has often detected no such appearance ; whence it would seem that this vascularity is capable of dis turbance or removal, either during the phe nomena of death, or after that event has hap. pened. And it is also to be noted, that the different serous structures seem liable to this appearance in a very different degree : some, as the arachnoid, scarcely ever presenting any trace of such a suffusion ; while in others, as the pleura, it is much more frequent. So that, on the whole, it may be stated that neither does its presence affirm, nor its ab sence deny, the occurrence of inflammation ; still less, where present, is its amount tcy he considered any measure of the intensity of the process.