1. The lines of fracture confirm the suggestions of earlier authors on the lines of strength in cartilage, with the additional feature of a transverse plane of weakness at the apex of the calcified zone.

2. The normal nutrition of cartilage is synovial, and access of a free blood supply is followed by destruction of hyaline articular cartilage.

3. Minor traumatic events in the articular lamella are common, particularly in osteoarthritic joints; the results of these on the cartilage are like the changes of osteoarthritis.

4. The removal of uncalcified cartilage can be described in two stages of a physico-chemical kind; the removal of calcified cartilage is a single cellular process.

5. There is evidence that the carbohydrate moiety of cartilage is present in two separable phases, one fixed to collagen, the other free.

6. The repair mechanisms after fracture are those available to restore the damage of osteoarthritis, and reasons can be shown why in fact they are ineffective.

1. The bone cysts of osteoarthritis are found in relation to defects in the watertight layer between the joint space and the cancellous bone; these defects are sometimes obvious openings, but more often they are openings closed by fibrous tissue, fibrocartilage or new bone. The small cysts lie close to the surface. Their rounded outlines suggest the agency of fluid pressure in their formation, and the fluid and cells found in the cysts are compatible with origin in the joint space and articular cartilage.

2. The hypothesis is advanced that the cysts are formed by the intrusion of synovial fluid under pressure into the substance of the bone, and the sclerosis around by displacement of the trabeculae and their reinforcement by new bone.

3. The objections to the hypothesis that the cysts represent a primary degeneration in the bone are discussed.