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.
]