A horse’s skeletal system is held together with ligaments, tendons and muscles. Each bone works in relation to its neighbouring bones and must be proportionate in size so that the joints work correctly. If a bone is too long or too short then the joint will be unstable, and if the muscles that connect the bones are not sufficiently strong then it will be difficult to move the limb.
The horse is a cursorial animal and its musculo-skeletal structure has been adapted to minimize the load that it carries over long distances in order to avoid predation. This has been achieved by reducing the weight of the limbs, and by reducing the number and size of bones in each limb. The limbs are bipedal, with two complete side digits, but with the lower row of the hoof (also called the frog) being reduced in depth.
These changes have made the limbs relatively lighter and more flexible, which helps the horse to travel faster and over greater distances than other animals. It is important to be able to travel at speed in order to escape from predators and competitors, so the flexibility of the limbs is very valuable.
Horses have a very flexible forelimb and a relatively flexible hind limb, but they also have a very stiff and powerful back. These features are useful for the horse in its role as a pack animal and in transporting loads over long distances.
The hind limb of the horse is supported by a pelvic girdle, which connects to the axial skeleton. This is composed of the femur, tibia and fibula. The tibia and fibula are covered by a tough membrane called the periosteum. This protects the bones from excessive pressure, but does not protect them from twisting. The periosteum is lined by a synovial membrane which contains fluid that lubricates the joints.
Horse’s forelimbs consist of the humerus, radius and ulna. The humerus is one of the strongest bones in the body, and articulates with the scapula at the shoulder and elbow joints. The radius extends downward from the elbow to the forelimb, and is joined to the ulna by a piece of cartilage which forms the withers.
The lateral and medial carpal phalanges of the hind leg are joined to the metacarpal phalanges of the front leg by a deep groove that houses a pair of wing-shaped projections, called the angles. The angles, along with the middle and distal phalanges of the distal forelimb, form the carpal articulation, which is part of the carpo-metacarpal joint. This joint is relatively stable, but may become unstable under extreme loading, especially in the case of a fall. The metacarpals and phalanges are connected to each other by strong ligaments. In the front limb, there are additional ligaments and tendons connecting the metacarpals to each other, and to the distal phalanx of the foot.