1. =Anterior= or =long digital extensor= (M. extensor digitalis longus;
extensor pedis; anterior extensor of the phalanges).—This muscle is
situated superficially on the antero-external aspect of the leg, and is
provided with a long tendon which passes down over the front of the
tarsus, metatarsus, and digit.
_Origin._—The small fossa (Fossa extensoria) between the external
condyle and the trochlea of the femur.
_Insertion._—(1) The extensor process of the third phalanx; (2) the
anterior surface of the proximal extremities of the first and second
phalanges.
_Action._—To extend the digit and flex the hock. It also assists in
fixing the stifle joint.
_Structure._—The origin is by means of a strong tendon in common with
the peroneus tertius, on which also many fibers arise. The common tendon
passes downward in the groove between the outer condyle and the
tuberosity of the tibia, where a pouch from the femoro-tibial capsule
descends four or five inches (ca. 10 to 12 cm.) beneath the tendon. The
belly is fusiform and somewhat flattened. The long tendon of insertion
begins in the belly about its middle, and is clear of the fleshy part
near the tarsus. It passes downward over the front of the hock, bound
down by the three annular ligaments already described (see crural
fascia), and enveloped by a =synovial sheath= which begins a little
above the level of the external malleolus, and extends nearly to the
junction with the lateral extensor tendon. This union occurs usually
about a hand’s breadth below the tarsus. In the angle of union the
extensor brevis also joins the principal tendon. Beyond this point the
arrangement is the same as in the fore limb.
_Relations._—Superficially, the skin and fascia; deeply, the
femoro-tibial capsule, peroneus tertius, and tibialis anterior; behind,
the lateral extensor and the superficial and deep peroneal nerves. In
front of the tarsus the anterior tibial artery crosses the deep face of
the tendon (Fig. 460).
_Blood-supply._—Anterior tibial artery.
_Nerve-supply._—Peroneal nerve.
2. =Lateral digital extensor= (M. extensor digitalis lateralis;
peroneus; lateral extensor of the phalanges).—This muscle lies on the
outer surface of the leg, behind the preceding one.
_Origin._—The external lateral ligament of the stifle joint, the fibula,
the external border of the tibia, and the interosseus ligament.
_Insertion._—The tendon of the anterior extensor, about a third of the
way down the metatarsus.
_Action._—To assist the anterior extensor.
_Structure._—The belly is fusiform, flattened, and pennate. The tendon
runs through the entire length of the belly and becomes free from it at
the lower fourth of the tibia. It passes downward through the groove on
the external malleolus, bound down by an annular ligament, and,
inclining forward, blends (usually) with the tendon of the anterior
extensor. It is provided with a =synovial sheath=, which begins about
one inch (ca. 2 to 3 cm.) above the external malleolus and ends about
one and one-half inches (ca. 3 to 4 cm.) above the junction. Sometimes
the fusion does not occur, and the tendon then passes down the
metatarsus, alongside of that of the long extensor, to lie inserted into
the first phalanx like the corresponding muscle of the thoracic limb.
_Relations._—Externally, the skin and fascia and the superficial
peroneal nerve; internally, the tibia and fibula; anteriorly, the
intermuscular septum, the long extensor, and the tibialis anterior;
posteriorly, the deep flexor and the soleus.
_Blood-supply._—Anterior tibial artery.
_Nerve-supply._—Peroneal nerve.
3. =Peroneus tertius= (Tendinous part of the flexor metatarsi; tendo
femorotarseus [Schmaltz]).—This consists in the horse of a strong tendon
which lies between the anterior extensor and the tibialis anterior.[72]
[Illustration:
FIG. 202.—MUSCLES OF PELVIC LIMB OF HORSE, ANTERIOR VIEW. _y_, Rectus
femoris; _20_, patella. (After Ellenberger-Baum, Anat. für
Künstler.)
]
_Origin._—The fossa extensoria (between the external condyle and the
trochlea of the femur), in common with the anterior extensor.
_Insertion._—(1) The proximal extremities of the large (third) and
external small (fourth) metatarsal bones, and the third tarsal bone; (2)
the fibular and fourth tarsal bones.
_Action._—Mechanically to flex the hock when the stifle joint is flexed.
_Structure._—This is entirely tendinous. The proximal end and the
underlying prolongation of the synovial membrane of the femoro-tibial
joint have been mentioned in the description of the anterior extensor.
Fibers of the latter muscle and of the tibialis anterior arise on the
tendon as it passes downward to the front of the hock. At the distal end
of the tibia the tendon divides into two branches, between which the
tendon of the tibialis anterior emerges. The anterior branch is attached
to the third tarsal and third and fourth metatarsal bones, while the
outer one curves outward, bifurcates, and is inserted into the fibular
and fourth tarsal bones (Fig. 460).
_Relations._—Superficially, the anterior extensor; deeply, the tibialis
anterior. The anterior tibial vessels cross the deep face of the outer
branch.
4. =Tibialis anterior= (Muscular portion of the flexor metatarsi).—This
lies on the antero-external face of the tibia; it is wide and flattened
above, pointed below.
_Origin._—The external condyle and border of the tibia and a small area
on the external surface of the tuberosity.
_Insertion._—(1) The metatarsal tuberosity; (2) the first and second
tarsal bones.
_Action._—To flex the hock joint.
_Structure._—The origin is fleshy, and is divided by the groove in which
lie the common tendon of the long extensor and peroneus tertius and a
synovial pouch. Passing downward on the tibia, the belly is united by
tendinous and fleshy fibers with the peroneus tertius, and terminates
close to the tarsus in a point on the tendon of insertion. The latter
emerges between the branches of the peroneus tertius and bifurcates, the
anterior branch being inserted into the metatarsal tuberosity, the inner
one (cunean tendon) into the fused first and second tarsal bones
(cuneiform parvum). The tendon is provided with a =synovial sheath= at
its emergence, and a =bursa= (cunean bursa) is interposed between the
inner branch and the internal lateral ligament.
_Relations._—Superficially, the anterior and lateral extensors, the
peroneus tertius, and the deep peroneal nerve; deeply, the tibia, the
deep flexor, and the anterior tibial vessels.
_Blood-supply._—Anterior tibial artery.
_Nerve-supply._—Peroneal nerve.