Part 96

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.

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.

B. POSTERIOR GROUP

1. =Gastrocnemius= (Figs. 179, 201, 203).—This muscle extends from the lower third of the femur to the point of the hock. It arises by two heads.

Origin.—(1) Outer head, from the external rough margin of the supracondyloid fossa (Fossa plantaris); (2) inner head, from the supracondyloid crest.

Insertion.—The posterior part of the tuber calcis.

Action.—To extend the hock.

Structure.—The two bellies are thick, fusiform, and somewhat flattened. They are covered by a strong aponeurosis and contain tendinous intersections. They terminate toward the middle of the leg on a common tendon (Tendo calcaneus s. Achillis), which at first lies posterior to that of the superficial flexor, but, by a twist in both, comes to lie in front of the latter. The deep fascia blends with the tendon throughout its length, and the soleus muscle is inserted into its anterior edge. A small =bursa= (Bursa tendinis calcanei) lies in front of the insertion on the tuber calcis, and a large =bursa= is interposed between the two tendons from the twist downward. The superficial flexor lies between the two heads and is adherent to the outer one (Fig. 459).