This is one of the most frequent of all diseases: it befalls chiefly the larger sections of the system of serous and synovial membranes of large joints, like the knee and hip. It is sometimes a primary disease, and arises from disordered function of the skin, from mechanical injury, concussion, irritation, or contusion, or from contact with heterogeneous effusions, whether gaseous or fluid. Sometimes it is secondary: and then it is either sympathetic with, and induced by, disease of organs which are invested by the membrane, or contained in it, and is in fact merely an extension of the diseased process to the membrane; or it comes on from the metastasis of anomalous exanthe-matous processes, typhus, gout, and rheumatism; from absorption of pus and sanies into the blood, etc.; in short, as a consequence of any (secondary) morbid affections of the mass of the blood which are distinguished by a tendency to exudations.

It is very frequently an acute disease, but tolerably often, too, its course is lingering and chronic. In its acute form, and particularly when excited by metastasis, it often assumes an exudative character, which is very remarkable, considering the internal cause of the inflammation, viz., the general disease, or the local process itself.

A. Acute Inflammation

The anatomical characters of this disease are:

A. Redness And Injection

These appearances commence with injection of the subserous tissue, which is seen through the serous membrane; fine hair-like streaks of vessels soon extend here and there to the membrane, and when clustered together, give its internal surface an appearance similar to the pile of red velvet. At the same time, small quantities of blood (suffusions) escape from the vessels both of the membrane itself and of the cellular tissue beneath it, so that the surface looks speckled as well as red. The hue of the redness depends chiefly upon the duration of the congestion and the constitution of the blood, and varies from a bright to quite a dark color. The extent and the intensity of the redness and injection also differ widely in different cases; sometimes they are partial and scarcely perceptible, sometimes they are universal and stain the membrane through. In many serous membranes, particularly those which, like synovial membranes, or still more the arachnoid, have a very delicate structure, the reddening which takes place is usually very slight, or there may be none whatever: these membranes admit of injection with blood only with much difficulty, and the degree of redness and injection is no criterion of the intensity of the process, so far as regards the quantity of product which the inflammation will supply; for the exudation, especially in inflammations of a croupy character, is remarkably disproportioned to the redness and injection of the membrane.