The =tibial tarsal= bone (Os tarsi tibiale, astragalus, or talus) is the
inner bone of the proximal row. It is extremely irregular in form, but
may be considered as offering six surfaces for description.
[Illustration:
FIG. 73.—RIGHT PATELLA OF HORSE, ANTERIOR VIEW.
]
[Illustration:
FIG. 74.—RIGHT PATELLA OF HORSE, POSTERIOR VIEW.
]
The =superior= and the =anterior= or =dorsal surface= are continuous,
and form a =trochlea= for articulation with the distal end of the tibia.
The trochlea consists of two oblique ridges with a deep groove between
them, which curve spirally forward, downward, and outward. There is
usually a shallow =synovial fossa= in the groove. The =inferior surface=
is convex from before backward, and most of it articulates with the
central tarsal; externally it has an oblique facet for the fourth
tarsal, and a non-articular groove cuts into the surface to its middle.
The =posterior= or =plantar surface= is oblique and extremely irregular;
it presents four facets for articulation with the fibular tarsal bone;
the facets are separated by rough excavated areas, and the largest fossa
(Sulcus tali) forms with a corresponding one on the fibular tarsal a
cavity termed the =sinus tarsi=. The =internal surface= bears on its
lower part a large tuberosity, and on its upper part a small one for the
attachment of the internal lateral ligament. The =external surface= is
smaller than the internal, and is marked by a wide rough fossa in which
the external ligament is attached.