290 FASCIA AND MUSCLES OF THE HORSE 



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 men- 

 tioned 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 l^ranch. 



4. Tibialis anterior (Aluscular portion of the flexor metatarsi). — This lies 

 on the antero-external face of the tibia; it is wide and flattened al:)0ve, pointed 

 below. 



Origin. — The external condyle and border of the tiliia 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 supracondy- 

 loid 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 



