#Pyogenic diseases# result from infection through the blood stream, from one of the adjacent bones, or from a penetrating wound of the joint. The commoner types include the _synovitis_ associated with disease in the adjacent bone, _acute arthritis of infants_, joint suppuration in _pyæmia_, _pyogenic arthritis_ following upon penetrating wounds, and the affections which result from _gonorrhœal_ or _pneumococcal_ infection. _Treatment._--The limb is immobilised on a posterior splint so padded as to allow slight flexion at the knee, and extension applied with sufficient weight to relieve the pain; it is also of benefit to induce hyperæmia by one or other of the methods devised by Bier. To tap the joint, the needle is introduced obliquely into the supra-patellar pouch, and if it is necessary to open the joint, the incision is made on one or on both sides of the patella, and Murphy's plan of inserting formalin-glycerine may be employed. If the infection progresses and threatens the life of the patient, it may be necessary to lay the joint freely open from side to side, sawing across the patella, and, the limb being flexed, the whole wound is left open and packed with gauze. As the infection subsides, the limb is gradually straightened. If these methods fail, amputation through the thigh may be the only means of saving life. #Arthritis deformans# affects the knee more frequently than any of the other large joints. The changes related to the synovial membrane here attain their maximum development, and may assume the form of hydrops with or without fibrinous bodies, or of overgrowth of the synovial fringes and the formation of pedunculated loose bodies. It is suggested that these synovial changes follow upon repeated sprains or upon a previous pyogenic infection of the joint. The effusion and stretching of the ligaments that follow upon a sprain are incompletely recovered from; the synovial membrane becomes puckered, the quadriceps atrophies and no longer puts the ligamentum mucosum on the stretch; and the infra-patellar pad of fat, not undergoing the normal compression during extension, is readily nipped between the femur and tibia. Each nipping implies a fresh sprain, with return of the effusion, and so a vicious circle is set up which terminates in what has been called a _villous arthritis_, with fringes and loose bodies; in time, the articular cartilage at the line of the synovial reflection undergoes fibrillation and conversion into connective tissue, and the process spreading to the articular surfaces, the picture of a rheumatoid arthritis is complete. Fibrillation of the cartilage imparts a feeling of roughness when the joint is grasped during flexion and extension, and lipping of the margins of the trochlear surface of the femur may be felt when the joint is flexed; it is also readily seen in skiagrams. When a portion of the "lipping" is broken off, it may give rise to a loose body. In advanced cases