Bones of the leg. Artwork of human femurs (thigh bones) and patellae (kneecaps), taken from anatomist William Cheselden's textbook Osteographia, or the Anatomy of the Bones, published in 1733. 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 protect the front of the knee joints.