PATELLA (Lat., a kind of dish), the knee-pan, a chestnut-shaped bone placed in front of the knee joint, the cavity of which it protects from external violence. It is attached above to the tendon of the extensor muscles on the front of the thigh, and below, by means of a strong and broad ligament, to the tuberosity on the upper and front part of the tibia or leg bone. Its anterior surface is convex and rough, while its back part presents a double articular surface, invested with a thin layer of cartilage, corresponding with the articular surface of the front part of the lower extremity of the femur. The patella thus takes part in the formation of the knee joint. It is liable to fracture and to lateral dislocation; but neither of these accidents is very frequent.