294 FASCIA AND MUSCLES OF THE HORSE 



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

 of the lateral cartilaj2;e. 



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



Structure. — (1) The inner head (M. fiexor digitalis longus s. flexor accesso- 

 rius) 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 tlie 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-suppiy.— Tibial nerve. 



