The =tarsus= comprises seven bones. The tibial tarsal consists of a body, neck, and head, like the bone in man. The body presents a trochlea above for articulation with the tibia and fibula. The posterior surface has three facets for articulation with the fibular tarsal bone. The head is directed a little inward and articulates with the central. The fibular tarsal has a long anterior process or “beak,” but the inner process (sustentaculum) is short. The tuber calcis is grooved from before backward. The central has a concave proximal surface adapted to the head of the tibial tarsal. Its distal surface articulates with the first, second, and third tarsals. It bears two tubercles posteriorly. The first tarsal is flattened and irregularly quadrangular; it articulates above with the central and below with the first metatarsal. The second tarsal is the smallest and is wedge-shaped; it articulates below with the second metatarsal bone. The third tarsal is also wedge-shaped, the base being in front; it articulates with the third metatarsal below. The fourth tarsal is remarkably high, and resembles a quadrangular prism; it articulates with the fibular tarsal above, the fourth and fifth metatarsal below, and the central and third tarsal bones internally. A groove for the tendon of the peroneus longus crosses its outer and posterior surface, and above it are one or two tubercles. Five =metatarsal bones= are present. The first is commonly very small and has the form of a blunt cone, somewhat compressed laterally. It articulates with the first tarsal and furnishes insertion to the tibialis anterior muscle. In some cases it fuses with the first tarsal; when the first digit is well developed, its metatarsal may resemble the others (except in size) or be reduced in its proximal part to a fibrous band. The other metatarsals are a little longer than the corresponding metacarpals. Their proximal ends are elongated from before backward and have posterior projections, which in the case of the third and fourth usually bear facets for articulation with two small rounded sesamoid bones. In other respects they resemble the metacarpals. The first digit is often absent. When present, its development varies and it contains one or two phalanges. In other cases the digit is double. The phalanges of the other digits resemble those of the fore limb. ARTHROLOGY THE ARTICULATIONS OR JOINTS An =articulation= or =joint= is formed by the union of two or more bones or cartilages by other tissue. Bone is the fundamental part of most joints; in some cases a bone and a cartilage, or two cartilages, form a joint. The uniting medium is chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the skeleton by muscles (Synsarcosis), as in the attachment of the thoracic limb in the horse, will not be considered in this section.