22. FEMUR OR THIGH BONE.--The os femoris, femur or thigh bone, the largest, thickest, and strongest bone in the body, belongs to the class of long bones, and is placed in a direction obliquely downwards and forwards, articulating with the cup-shaped cavity in the pelvic or hip bones superiorly (or upper), and with the tibia and patella inferiorly (or below). This bone is roughened for the attachment of the powerful muscles of the hip. 23. PATELLA, OR STIFLE BONE.--This, the kneepan or stifle bone, is placed in front of the pully-shaped groove of the femur. It is very compact, its front surface being irregular, round and very much roughened, for ligamentous attachment, and its posterior (or back) surface very smooth to articulate with the groove in the femur, presenting two depressions divided by a ridge, the inner being the larger, and in the fresh state enlarged still more by projecting cartilaginous lip, or elevation. The patella increases the power of the hind leg, and it is this bone that causes stifle joint lameness when it slips out of the groove in which it glides normally. 24. TIBIA, OR LEGBONE.--The tibia or leg bone, is a long bone, larger at its upper than its lower end, situated between the femur and the astragalus, slanting downwards and backwards. This bone is three-sided, possessing outer, inner and back surfaces, all of which are wider above than below. This bone gives attachment to the flexor muscles of the hip. 25. FIBULA.--This is a long slender bone, little developed in the horse, and is an appendage to the tibia, being attached to the outer side of that bone, and extending from its head to its lower third, to which it is affixed by a ligament; the space between the two bones is called the tibial arch. The fibula gives attachment to the peroneus muscle, the muscle that is supposed to be severely contracted when an animal is affected with string-halt.