Part 98

Origin.—(1) The posterior edge of the external condyle of the tibia; (2) the external border of the external condyle of the tibia, just behind the facet for the fibula; (3) the middle third of the posterior surface and the upper part of the external border of the tibia, the posterior border of the fibula, and the interosseous ligament.

Insertion.—The semilunar crest of the third phalanx and the adjacent surface of the lateral cartilage.

Action.—To flex the digit and to extend the hock joint.

Structure.—(1) The inner head (M. flexor digitalis longus s. flexor accessorius) is easily isolated (Figs. 203, 459). It has a fusiform belly, which crosses the leg obliquely and lies in a groove formed by the other heads and the popliteus. This terminates near the lower third of the tibia on a round tendon which passes downward, partly embedded in the internal lateral ligament of the hock, and joins the common tendon about a third of the way down the metatarsus. In its course over the inner surface of the hock the tendon lies in a canal formed by the strong tarsal fascia, the tibia, and the lateral ligament, and is provided with a =synovial sheath= which extends from the distal fourth of the tibia to the junction with the principal tendon. (2) The superficial head (M. tibialis posterior) is only partially separable. It has a flattened belly, terminating near the lower third of the tibia on a flat tendon which soon fuses with the principal tendon. (3) The deep head (M. flexor hallucis longus) is much the largest. It lies on the posterior surface of the tibia, from the popliteal line outward and downward. The belly contains much tendinous tissue, and terminates behind the distal end of the tibia on a strong round tendon. The latter receives the tendon of the tibialis posterior, passes downward in the tarsal groove, bound down by the strong tarsal fascia (Ligamentum laciniatum) and enveloped in a synovial sheath, receives the tendon of the inner head below the hock, and, a little further down, the so-called check ligament (Caput tendineum [Schmaltz]). The =tarsal sheath= (Vagina tarsea) begins about two to three inches (ca. 5 to 7.5 cm.) above the level of the internal malleolus, and extends about one-fourth of the way down the metatarsus. (Distention of the sheath, as in “thoroughpin,” affects chiefly its proximal end.) The check ligament resembles that of the fore limb, except that it is longer and very much weaker; it may be absent. The remainder of the tendon is arranged like that of the thoracic limb.

Relations.—Anteriorly, the tibia and fibula, the popliteus, lateral extensor, tibialis anterior, and the tibial vessels; posteriorly, the gastrocnemius, superficial flexor, and the tibial nerve; externally, the fascia, skin, and the soleus; internally, the fascia and skin.

Blood-supply.—Posterior tibial artery.

Nerve-supply.—Tibial nerve.

5. =Popliteus= (Fig. 203).—This thick and triangular muscle lies on the posterior surface of the femoro-tibial articulation and the posterior surface of the tibia above the popliteal line.

Origin.—A small depression on the external epicondyle of the femur, close to the articular surface and under the lateral ligament.

Insertion.—A triangular area on the posterior surface of the tibia, above and internal to the popliteal line; also the proximal half of the inner border and a narrow adjacent part of the internal surface of the tibia.

Action.—To flex the femoro-tibial joint and to rotate the leg inward.

Structure.—The strong tendon of origin lies at first under the lateral ligament, and curves backward and inward over the external condyle of the tibia and its semilunar cartilage, being invested by a reflection of the synovial capsule of the joint (Fig. 459). The tendon is succeeded by a thick triangular belly, the fibers of which are directed obliquely downward and inward.

Relations.—Superficially, the fascia and skin, semitendinosus, gastrocnemius, superficial flexor; deeply, the femoro-tibial joint, the tibia, the popliteal vessels and their divisions. The saphenous vessels and nerve lie along the inner border of the muscle, separated from it, however, by the deep fascia.

Blood-supply.—Popliteal and posterior tibial arteries.

Nerve-supply.—Tibial nerve.

MUSCLES OF THE METATARSUS AND DIGIT

=Extensor pedis brevis= (M. extensor digitalis brevis).—This small muscle lies in the angle of union of the long and lateral extensors of the digit. (Shown in Fig. 201, but not marked.)

Origin.—The outer tendon of the peroneus tertius, the middle annular ligament, and the outer lateral ligament of the hock.

Insertion.—The tendon of the anterior or long extensor.

Action.—To assist the anterior or long extensor.

Structure.—It is principally fleshy, having a superficial origin from the annular ligament, and a deep one (by a thin tendon) from the outer tendon of the peroneus tertius. The insertion is by a thin tendon.

Relations.—Superficially, the skin and fascia and the tendons of the anterior and lateral extensors; deeply, the joint capsule, the great metatarsal artery, and the deep peroneal nerve.

Blood-supply.—Great metatarsal artery.

Nerve-supply.—Deep peroneal nerve.

The =interossei= and =lumbricales= are arranged like those of the thoracic limb, the only noticeable difference being the greater development of the lumbricales in the pelvic limb.

THE MUSCLES OF THE OX

MUSCLES OF THE FACE

The =panniculus= is much more developed than in the horse, presenting as a special feature the extensive =frontalis= muscle, which covers the frontal and nasal regions.