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.