The =central tarsal= bone (Os tarsi centrale, scaphoid, or navicular) is irregularly quadrilateral, and is situated between the tibial tarsal above and the third tarsal below. It is flattened from above downward, and may be described as having two surfaces and four borders. The =superior surface= is concave from before backward, and almost all of it articulates with the tibial tarsal; a non-articular depression cuts into its outer part, and sometimes there is a facet for the fibular tarsal bone on the posterior angle. The =inferior surface= is convex, and is crossed by a non-articular groove, which separates facets for articulation with the third and the first and second (fused) tarsals. The =anterior= or =dorsal border= and the =internal border= are continuous, convex, and rough. The =posterior= or =plantar border= bears two prominences, separated by a notch. The =external border= is oblique, and bears anterior and posterior facets for articulation with the fourth tarsal, between which it is excavated and rough. [Illustration: FIG. 84.—RIGHT TARSAL AND METATARSAL BONES OF HORSE. EXTERNAL VIEW. (After Schmaltz, Atlas d. d. Anat. d. Pferdes.) ] FIRST AND SECOND TARSAL BONES The =first= and =second tarsal= bones (Os tarsale primum et secundum, cuneiform parvum) are usually fused in the horse, forming a bone of very irregular shape, situated in the inner and posterior part of the lower row, below the central and behind the third tarsal. It is the smallest of the tarsal bones, and may be described as having two surfaces, two borders, and two extremities. The =internal surface= faces backward and inward, and is convex. Its anterior part is ridged, and gives attachment to the internal lateral ligament, and its posterior part bears an imprint where the inner tendon of the tibialis anterior is inserted. The =external surface= is marked by a deep notch which indicates the division between the first and second tarsal elements; it bears on its anterior part a large concave facet for the central tarsal. The =superior border= is convex. The =inferior border= is broad in front, where it articulates with the large and inner small metatarsal bones. The =anterior extremity= has a small facet for articulation with the third tarsal, and bears internally a ridge or tubercle. The =posterior extremity= is a blunt point. In some cases the first and second tarsal bones remain separate—a remarkable reversion to the condition in the early ancestors of the horse. In such specimens the first tarsal is a discoid bone, articulating above with the central, below with the small metacarpal bone. The second tarsal is quadrangular, equivalent to the thick anterior part of the bone as described above, and overlapped by the anterior part of the first tarsal. THE THIRD TARSAL BONE