Inflammations of varying intensity are of frequent occurrence; they maybe due to rheumatism or gout, to traumatism, to the action of microorganisms, or, to disturbances of innervation. They may be slight or severe, acute or chronic. They may terminate in resolution, in permanent new formations, more or less deforming and disabling, or in the destruction of the articulation. Inflammations may arise in the joint structures proper or may extend to it from contiguous structures, such as the cancellous bone ends, the overlying tendons or the periarticular connective tissue. They may be largely confined to a single structure, the synovial membrane being ordinarily affected, or they may involve the whole joint. +Acute synovitis.+ Synovitis may occur as a result of a simple injury, such as a subcutaneous wound, a contusion, or a sprain. Exposure to cold and the presence of a movable cartilage are also common causes. Aseptic conditions in the synovial membrane seldom extend to the other joint structures (see “Arthritis”) and heal with or without impairment of the joint, depending on the degree of inflammation. +Symptoms.+ The joint is painful, especially upon motion, and particularly so at night. It is swollen and tense and may be fluctuating. At the knee, the patella is floated up from the condyles and can be depressed upon slight pressure. The joint is held in a position of partial flexion which permits of the greatest ease, because of the diminished tension in this position. Local heat and tenderness are not necessarily great, and constitutional symptoms, if present, are moderate in degree. In the suppurative affections of joints, all of the above symptoms are intense and there is a general arthritis. After a few hours or days the intensity of the symptoms subsides, the pain lessens, the swelling diminishes, as the effusion and extravasated blood are absorbed, the limb takes its natural position, and recovery promptly takes place. If there has been much hemorrhage into the joint, adhesions due to the organization of the clot may cause some restriction of motion. +Treatment.+ The joint must be placed at rest and an ice bag kept in constant contact. Even pressure with cotton and broad bandages often hastens absorption, but cannot at first be borne with comfort. In rare instances aspiration of the effusion must be resorted to, but the certainty should exist that absorption is impossible, before a joint is punctured. The greatest care must be exercised in introducing a needle into a joint to avoid infection.