_Pathology_.--Uncomplicated acute synovitis never causes death. The pathological changes in connection with it have therefore been studied in cases purposely induced, and the animal afterwards slaughtered. It is then found that, as in inflammation elsewhere, the synovial membrane is showing the usual inflammatory phenomena--that it is thick and swollen as a result of the inflammatory hyperæmia and commencing exudation. Later, the synovial fluid becomes increased in quantity, is thin and serous, and after a time is seen to be mixed with the inflammatory exudation poured into it. We then find that it has lost its clear appearance, has become thick and muddy, and has floating in it flakes of fibrin. If the case progresses favourably these materials are soon absorbed and resolution occurs. In rarer cases the thickening and congestion of the membrane increases, and the articular capsule becomes so distended with the increased synovia and accumulated inflammatory discharges that a kind of chemosis occurs. In other words, there oozes through, without actual rupture of the membrane, a thin, blood-stained, and purulent-looking discharge. It is an important point to note that in cases of synovitis the fringes of the synovial membrane become swollen and blood-injected, forming noticeable red elevations at the margins of the cartilages. It is then that the diseased condition soon spreads and runs into arthritis. Further, it is important, especially with regard to the question of the degree of pain and lameness likely to be caused, to note that often granulations are thrown out upon the looser folds of the membrane. As these increase in size they come to form fringed and villous membranous projections inserting themselves between the bones forming the articulation. In such cases there is no doubt that the intense pain sometimes observed in these cases is due to pinching of these prolongations of the synovial membrane by the opposing bones of the joint. _Symptoms and Diagnosis_.--Acute synovitis of a joint leads to heat of the parts, pain, distension of the capsule, and, where the joint may be easily felt, fluctuation. In the articulation with which we are dealing, however, these last two symptoms are not easily detected, for the surrounding structures--namely, the lateral and other ligaments of the joint, the extensor pedis tendon in front, and the perforans behind, together with the dense and comparatively unyielding nature of the skin of the parts--are such as to prevent distension and fluctuation becoming marked to a visible extent. We are able to diagnose the case as one of foot lameness, and, with a history of a severe blow or other injury, are able to assume that this condition, perhaps attended with periostitis, is in existence.