» Conformation of the Croup
Although the hindlimbs support little more than 40% of the body weight, they supply by far the greater part of the forward impetus in locomotion. This thrust is delivered through the hip and sacroiliac joints, which are intrinsically more stable than the shoulder and scapulothoracic synsarcosis, the corresponding "joints" of the forelimb.
The sacroiliac joint is strengthened by tight ligaments, and both it and the hip joint are well supported by the muscles of the croup and thigh. These muscles are particularly massive in the horse, in which they round the contours in a distinctive fashion. In consequence, it is more difficult to appreciate the features and orientation of the pelvis of the horse than those of the pelves of other domestic species.The coxal tuber is a conspicuous landmark that is palpable in its whole extent and visible in its upper part (Fig. 24.1/2). The sacral tuber (Fig. 24.1/2'), difficult to palpate in most animals, rises a little above the level of the adjacent spinous processes. The ischial tuber (Fig. 24.1/3) is also not always easy to appreciate, although its location and a general impression of its form may be obtained on deep palpation over the muscles that form the caudal contour of the croup and thigh. The slope of the pelvis may be estimated by visualizing the line joining the coxal and ischial projections. In the standard, generally approved conformation, this line forms an angle of about 30 degrees with the horizon, and the sacrum more or less horizontal. When the angle is significantly smaller—and the two tubers come close to sharing the same horizontal plane—the tail appears to be set high. When the angle is significantly greater, the animal is said to be goose rumped. The croup is short in such animals, and leverage and length of the hamstring muscles are reduced. Although this is clearly disadvantageous, some compensation is obtained from the more stable support the limbs afford the trunk, and many horsemen and horsewomen find a gently sloping croup acceptable in a saddle horse. Undue prominence of the sacral tubers ("hunter's bumps") sometimes develops, especially in show jumpers and other horses subjected to similar repeated stress. The deformity is commonly ascribed to subluxation of the sacroiliac joints.
The position of the hip joint may be deduced from its relationship to the greater trochanter of the femur. This protuberance is divided into low cranial and high caudal parts, separately identifiable on palpation (Fig. 24.1/5 and 5'). At more distal levels, the third trochanter (prominent only in this species) and the lateral epicondyle are easily distinguished and may be used to reveal the orientation of the femur. This bone is more nearly vertical than is often supposed (see Fig. 19.1).