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. [Illustration: 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.]