288 FASCIA AND MUSCLES OF THE HORSE 



A. Anterior Group 



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 l)readth 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 super- 

 ficial 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 ex- 

 ternal 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 forw^ard, blends (usually) with 

 the tendon of the anterior extensor. It is ]:)rovided with a synovial sheath, which 

 begins al^out 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 be 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. 



