Femur bones. 18th-century illustration of human femurs (thigh bones) and patellae (kneecaps). The right femur is shown at the front and the left behind. The balls at the top of the femurs fit into the hip sockets. At the bottom are the upper halves of the hinge joints that form the knees. The patellae (centre right) protect the front of the knee joints. This illustration is from 'Osteographia, or the Anatomy of the bones' (London, 1733) by English surgeon and anatomist William Cheselden (1688-1752). It was the first full and accurate description of the anatomy of the human skeletal system.