The =tibia= is a long bone which extends obliquely downward and backward
from the stifle to the hock. It articulates above with the femur, below
with the tarsus, and externally with the fibula. It possesses a shaft
and two extremities.
The =shaft= or =body= (Corpus tibiæ), large and three-sided above,
becomes smaller and flattened in the sagittal direction below, but
widens a little at the distal end. It presents for notice three surfaces
and three borders. The =internal surface= (Facies medialis) is broad
above, where it furnishes insertion to the internal lateral ligament and
the sartorius and gracilis muscles; below this it is convex and
subcutaneous. The =external surface= (Facies lateralis) is smooth and
somewhat spiral. It is wide and concave in its upper part, below which
it becomes narrower and slightly convex, and winds gradually to the
front of the bone; near the distal end it widens a little, becomes flat,
and faces forward. The =posterior surface= (Facies posterior) is
flattened, and is divided into two parts by the rough =popliteal line=,
which runs obliquely from the upper part of the external border to the
middle of the internal border. The triangular area above the line is
occupied by the popliteus muscle, while the area below is marked by
rough lines (Lineæ musculares) to which the deep flexor muscle of the
digit is attached; the lines fade out below, where the surface is smooth
and flat. The =nutrient foramen= is situated on or near the popliteal
line. The =anterior border= is very prominent in its upper third,
forming the =tibial crest= (Crista tibiæ); below it is reduced to a
rough line, which ends at a small elevation near the distal end of the
bone. The =internal border= (Margo medialis) is rounded in its upper
half, to which the popliteus muscle is attached, and a tubercle is found
on this part. The lower part is a rough line on well-marked bones. The
=external border= (Crista interossea) is concave in its upper part and
concurs with the fibula in the formation of the interosseous space of
the leg; a smooth impression indicates the course of the anterior tibial
vessels through the space to the front of the leg. Lower down the border
divides and incloses a narrow triangular surface.
[Illustration:
FIG. 68.—RIGHT TIBIA AND FIBULA OF HORSE, EXTERNAL VIEW.
]
The =proximal extremity= (Extremitas proximalis) is large and
three-sided. It bears two lateral eminences, the =internal= and
=external condyles= (Condylus medialis, lateralis). Each presents a
somewhat saddle-shaped surface for articulation with the condyle of the
femur and the semilunar cartilage. The =spine= or =intercondyloid
eminence= (Eminentia intercondyloidea) is the central prominence, upon
which the articular surfaces are continued; it consists of a high inner
part and a lower outer part (Tuberculum intercondyloideum mediale,
laterale). On, before, and behind the spine are the =anterior= and
=posterior intercondyloid fossæ=, in which the anterior crucial ligament
and the semilunar cartilages are attached. The condyles are separated
behind by the deep =popliteal notch= (Incisura poplitea), on the inner
side of which is a tubercle for the attachment of the posterior crucial
ligament. The external condyle has an overhanging outer margin, below
which is a =facet= for articulation with the fibula. The =large
anterior= eminence is the =tuberosity= of the tibia (Tuberositas tibiæ).
It is marked in front by a =groove=, the lower part of which gives
attachment to the middle patellar ligament, and the groove is flanked by
rough areas for the attachment of the internal and external patellar
ligaments. A semicircular smooth =notch= (Sulcus muscularis) separates
the tuberosity from the external condyle, and gives passage to the
tendon of origin of the anterior extensor and the peroneus tertius.
[Illustration:
FIG. 69.—RIGHT TIBIA AND FIBULA OF HORSE, POSTERIOR VIEW.
]
The =distal extremity= (Extremitas distalis) is much smaller than the
proximal; it is quadrangular in form and larger internally than
externally. It presents an =articular surface= (Cochlea tibiæ), which is
adapted to the trochlea of the tibial tarsal bone (astragalus), and
consists of two grooves separated by a ridge. The ridge and grooves are
directed obliquely forward and outward, and are bounded laterally by the
=malleoli=, to which the lateral ligaments of the hock joint are
attached. The =internal malleolus= (Malleolus tibialis) is the more
prominent of the two, and forms the anterior boundary of a groove for
the tendon of the inner head of the flexor perforans. The =external
malleolus= (Malleolus fibularis) is broader, and is marked by a vertical
groove for the passage of the lateral extensor tendon.
[Illustration:
FIG. 70.—RIGHT TIBIA AND FIBULA OF HORSE, ANTERIOR VIEW.
]
=Development.=—The tibia has the usual three chief centers of
ossification and supplementary ones for the tuberosity and the external
malleolus. The latter is really the distal end of the fibula; it is a
separate piece at birth, and the line of union is commonly quite evident
in the adult in the external articular groove. The proximal end unites
with the shaft at about three and a half years, and the distal end at
about two years of age.