Fig. 34.—Left femur of Wolf, front side. h., head. gt.tr., great trochanter. tr. 2, second trochanter. int. con., internal condyle. r.g., rotular groove, ext. con., external condyle.
Fig. 35.—Left femur of Horse. tr. 3, third trochanter. Other letters as in [Fig. 34], than which this drawing is very much more reduced.
The hind leg is constituted in very much the same manner as the fore, but with certain well-marked and constant differences. The thigh-bone, or femur, is usually the longest and stoutest of the limb-bones and in very large animals may be extremely massive. At the upper end is the hemispherical head, which is set upon a distinct neck and projects inward and upward, fitting into the acetabulum of the hip-bone. Nearly all land mammals have a small pit on the head of the femur, in which is inserted one end of the round ligament, while the other end is attached in a corresponding depression in the floor of the acetabulum. This ligament helps to hold the thigh-bone firmly in place and yet allows the necessary freedom of movement. On the outer side of the upper end of the femur is a large, roughened protuberance, which often rises higher than the head and is called the great trochanter; another, the second or lesser trochanter, is a small, more or less conical prominence on the inner side of the shaft, below the head. These two processes are well-nigh universal among land mammals; and in a few of the orders occurs the third trochanter, which arises from the outer side of the shaft, usually at or above the middle of its length. Though comparatively rare in the modern world, the third trochanter is an important feature, and the early members of most, if not all, of the mammalian orders possessed it. The shaft of the femur is elongate and, except in certain very bulky mammals, of nearly cylindrical shape. The lower end of the bone is thick and heavy and bears on the posterior side two large, rounded prominences, the condyles, which articulate with the shin-bone to form the knee-joint. On the anterior side is a broad, shallow groove, the rotular groove, in which glides the patella, or knee-cap. The patella is a large ossification, of varying shape, in the tendon common to the four great extensor muscles of the thigh, the action of which is to straighten the leg.
Fig. 36.—Left femur of Wolf, inside of lower end. ext. con., external condyle. int. con., internal condyle. r.g., rotular groove. Above, are two views of the left patella, anterior and internal sides.
The lower leg, like the fore-arm, has two bones, which, however, are always parallel, never crossed, and have no power of rotation. Of these, the inner one is the shin-bone, or tibia, which is always the larger and alone enters into the knee-joint. The external bone is the fibula, which is almost entirely suppressed in certain highly specialized forms, such as the horses and ruminants, the tibia carrying the whole weight. The upper end of the tibia is enlarged and extends over that of the fibula; it has two slightly concave surfaces for articulation with the condyles of the femur, the approximate edges of which are raised into a bifid spine. The upper part of the shaft is triangular, with one edge directed forward, and the superior end of this edge is roughened and thickened to form the cnemial crest, to which is attached the patellar ligament. The middle portion of the shaft is rounded and the lower end broad and usually divided by a ridge into two grooves or concavities for the ankle-bone; from the inner side of this end projects downward a tongue-like process, the internal malleolus, which prevents inward dislocation of the ankle.
The fibula is relatively stoutest in the less advanced mammals and is usually straight and slender, with enlarged ends, the lower one forming the external malleolus, which serves to prevent outward dislocation of the ankle. In many forms the fibula is coössified with the tibia at both ends, and in the most highly specialized hoofed animals, such as the horses, camels and true ruminants, the bone has apparently disappeared. The young animal, however, shows that the ends of the fibula have been retained and the shaft completely lost; the upper end is in the adult firmly fused with the tibia and, in the horses, the lower end is also, but this remains separate in the ruminants and camels, forming the malleolar bone, which is wedged in between the tibia and the heel-bone. Because of its importance in holding the ankle-bone in place, this lower end of the fibula is never lost in any land mammal.
