In the Western world horses are now mainly bred for use in sport and recreation, pursuits that often make heavy demands on their speed and endurance and expose their limbs to continual strain and repeated risk of injury.
Even relatively minor incapacity may unfit a horse for this work, and the importance of soundness of limb is crisply stated by the old adage “no foot, no horse.” Since lameness accounts for much of the work of equine practitioners, it follows that they have need of a more detailed knowledge of the anatomy of the limbs than is necessary for those who deal with other species.
The limbs of the horse display extreme adaptations for fast running with a concomitant loss of versatility. Although both forelimbs and hindlimbs find their main indeed almost exclusive employment in supporting the body when at rest and in driving it forward when in motion, they do manifest significant division of labor. It is the forelimbs that carry the greater part (some 55% to 60%) of the body weight at rest; they also supply the principal shock absorbers that are necessary in the faster gaits and especially when landing from a jump. The hindlimbs are less committed to these tasks and furnish the main propulsive thrust. However, this distribution of duties is not invariable; in particular, the share of the load that is supported by each limb may be altered by varying the posture to shift the center of gravity. The most obvious maneuver is to raise the head, thus shortening the lever arm of the neck and displacing the center caudally; the reciprocal movement brings the center of gravity cranially. These alterations in the carriage of the head may be pronounced in a lame animal, which lifts the head when a painful forelimb is placed on the ground and lowers it when the sound limb bears weight. Since it is the latter movement that usually strikes an observer with more force, a horse with forelimb lameness is said to “nod on the sound foot.” When there is a painful condition of a hindlimb, the head is lowered as the affected limb assumes support.
A forelimb with good conformation is straight when viewed from the front.
A line dropped from the point of the shoulder bisects the limb and passes through the center of the hoof; the digit continues the cannon (metacarpus) in a straight line, neither “toeing-in” nor “toeing-out” (Figure 23-1). Much of the limb should also be straight when viewed from the side; a line dropped from the tuberosity of the scapular spine should bisect it to the fetlock and then pass just behind the hoof, whose slope should parallel that of the digit. Deviations from the normal conformation can result in abnormal movements, which in turn may cause interference between the feet, unequal and abnormal hoof wear, and development of lameness.The more common deviations seen when viewing from the front are categorized as “base-wide,” in which the limbs slope laterally, and “base-narrow,” in which they slope medially. Deviations seen from the side include “standing under,” in which the limbs slope cau- dally, and “camped,” in which they slope cranially. Cranial, caudal, medial, and lateral deviations of the carpus are also recognized; the last two faults are “knock-knees” and “bowlegs.”
Retention of the full length of the shaft of the ulna is a congenital anomaly that is fairly common in Shetland ponies. It is associated with a valgus deformity— sometimes very severe—of the limb.
The distinctive “leggy” appearance of the young foal must be familiar to every reader (Figure 23-2). The acquisition of the adult shape involves changes in the ratios of the lengths of the limbs (taken as a whole) to that of the trunk and in the ratios between the lengths of successive segments of the limbs—arm (thigh), forearm (leg), and metacarpus (metatarsus). According to one source, in the newborn Thoroughbred the ratio of the humerus (femur) to the metacarpus (metatarsus) is approximately 4 : 5 (4 : 5); in the adult the ratio is approximately 6 : 5 (6.5 : 5). These changes are achieved through a postnatal growth in length of the metacarpal (metatarsal) bones of about 20% and growth of the humerus and femur of about 100%.
The cutaneous features known as chestnuts and ergots are described on page 362.