The equine forelimb is unique among the mammalian limbs. Its proximal segments (the stylopod and zeugopod) have undergone significant morphological variation throughout equid evolution while its distal autopod segments have remained relatively conservative. It is not known if this difference reflects the development of the forelimb as an independent clade or if it reflects a change in functional requirements.
The front leg is held in place by a “girdle” of muscles, and not by the bone structure of the hip or spine. This means the bones of the humerus and radius are not firmly attached to the vertebrae. As the humerus rotates inward or outward, this affects the rotation of the limb at its base, which then effects the rotation of the entire forelimb. This is what causes a horse to have a toed-in or toed-out stance.
The radius is the primary weight bearing bone in the antebrachium. It presents grooves at its distal end for several extensor tendons including the extensor carpi radialis, the extensor digitorum superficialis and the common digital extensor tendon. It also presents a distinct area of incomplete fusion between the radius and ulna, which is called an interosseous space. Both the common interosseous nerve (EQUINE) and the cranial interosseous nerve (OX) pass through this area to reach the antebrachial muscles.
Both the lateral and medial metacarpals are full proximally but become gradually smaller distally. Metacarpals II and III are more curved than those of the thumb, with a distinctive median claw shape. Metacarpal IV is more rounded than those of the other metacarpals, with a ridge at its ventral surface that narrows and extends to a long curved point.
All these splint bones are isometrically smaller than the metacarpals of the hindlimb. This is a result of their early evolutionary reduction to a monodactyl forelimb. In the ancestor of the horse, Hyracotherium, the metacarpals were tridactyl, and the most recent incarnation of this group, Mesohippus, was monodactyl as well.
The articulation between the shaft of the radius and the scapula is a ball and socket joint. The scapula is attached to the humorous bone with a strong suction-type ligament. This is what enables the horse to stand and walk on two feet. It is not, however, a hinge joint and it is not capable of flexing the hip or extending the back, which requires a different set of muscle chains. This muscle chain is the one that makes possible a powerful extended trot and a beautiful piaffe. It is also important for the control of movement in the fetlock and coffin joints.