The =third tarsal= bone (Os tarsale tertium, third or great cuneiform) resembles the central, but is smaller and triangular in outline. It is situated between the central above and the large metatarsal bone below. It possesses two surfaces and three borders. The =superior surface= is concave, and is crossed by a non-articular depression which divides it into two unequal facets; it articulates with the central tarsal. The =inferior surface= is slightly convex, and rests on the large metatarsal bone; it has an extensive central rough excavation. The =anterior= or =dorsal border= is convex and bears a rounded ridge on its inner part. The =internal border= is deeply notched and has a small facet for the second tarsal on its anterior part. The =external border= is also divided by a notch into two parts, and bears two diagonally opposite facets for articulation with the fourth tarsal. In some cases there is a facet for the inner small metatarsal bone. [Illustration: FIG. 85.—CROSS-SECTIONS OF METACARPAL AND METATARSAL BONES OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] THE FOURTH TARSAL BONE The =fourth tarsal= bone (Os tarsale quartum, cuboid) is the outer bone of the lower row, and is equal in height to the central and third together. It is cuboid in shape and presents six surfaces. The =superior surface= is convex from side to side, and articulates chiefly with the fibular tarsal, but to a small extent with the tibial tarsal also. The =inferior surface= rests on the large and external small metatarsal bones. The =internal surface= bears four facets for articulation with the central and third tarsal bones. It is crossed from before backward by a smooth groove, which by apposition with the adjacent bones forms the =canal of the tarsus= (Canalis tarsi) for the passage of the perforating tarsal vessels. The =anterior= or =dorsal=, =external=, and =posterior= or =plantar surfaces= are continuous and rough. A tuberosity behind gives attachment to the plantar ligament. =Development.=—The fibular tarsal bone has two centers of ossification, one for the main mass and the other for the tuber calcis; the latter fuses with the rest of the bone at about three years of age. The first and second tarsals have separate centers, but fusion usually occurs before birth. Each of the other bones ossifies from a single center. [Illustration: FIG. 86.—DIGITAL BONES OF FORE LIMB OF HORSE, VOLAR ASPECT. ] [Illustration: FIG. 87.—DIGITAL BONES OF HIND LIMB OF HORSE, VOLAR ASPECT. ] THE METATARSUS The =metatarsal= bones (Ossa metatarsalia), three in number, have the same general arrangement as the metacarpal bones, but present some important differences. Their direction is slightly oblique, downward and a little forward.