leg, one of the paired limbs of an animal used for support of the body and for locomotion. Properly, the human leg is that portion of the extremity between the foot and the thigh. This section of the human leg contains two long bones, the tibia and the fibula. The upper end of the tibia joins with the lower end of the thighbone (femur) and forms a hinged joint. The kneecap (patella), a flat triangular-shaped bone, surrounds and protects this joint. The lower end of both tibia and fibula join with the talus, a bone in the foot, to form the ankle joint. The upper end of the femur, which is the longest bone in the body, forms a ball and socket joint where it meets the hipbone. In quadrupeds, both the hind and fore limbs are referred to as legs.