This muscle, fusiform in shape, arises at its upper part from the external surface of the inferior extremity of the femur, then its tendon passes into a groove hollowed on the external tuberosity of the tibia. The fleshy body which succeeds is directed towards the tarsus, but before reaching it is replaced by a tendon. This tendon, at the level of the metatarsal bones, divides into four slips, which pass towards the toes, and are inserted into the second and third phalanges of the latter.
In the horse it covers, to a great extent, the tibialis anticus, so that it is the latter which forms the large fusiform prominence especially noticeable in the middle region, to which the contour of the anterior surface of the leg is due.
Fig. 86.—Myology of the Horse: Left Hind-limb, External Aspect.
1, Semi-tendinosus; 2, biceps cruris; 3, patella; 4, extensor longus digitorum (anterior extensor of the phalanges); 5, reinforcing band arising from the ligament of the fetlock; 6, peroneus brevis (lateral extensor of the phalanges); 7, external head of gastrocnemius; 8, soleus; 9, tendo-Achillis; 10, calcaneum; 11, tendon of the superficial flexor of the toes (superficial flexor of the phalanges); 12, flexor longus pollicis and tibialis posticus (deep flexor of the phalanges); 13, 13, tendon of the superficial flexor of the phalanges; 14, 14, tendon of the deep flexor of the phalanges; 15, suspensory ligament of the fetlock; 16, principal metatarsal: 17, external rudimentary metatarsal.
It arises above from the inferior extremity of the femur, from the fossa situated between the trochlea and the external condyle; therefore, it has a common origin with the tendinous portion of the tibialis anticus, or flexor of the metatarsus.
The tendon, which at the level of the inferior part of the leg succeeds to the fleshy body, passes in front of the tarsus, the principal metatarsal, and receives the tendon of the peroneus brevis which we will describe [later on]. It then reaches the anterior surface of the fetlock. There it presents an arrangement analogous to that which we have pointed out in connection with the anterior extensor of the phalanges—a muscle which, in the fore-limbs, corresponds to the common extensor of the digits; that is to say, it is inserted, in form of an expansion, into the pyramidal prominence of the third phalanx, after having formed attachments to the first and second, and having received on each side a strengthening band from the suspensory ligament of the fetlock.
In the ox the long extensor of the toes is united above, and for a great part of its length, with the portion of the tibialis anticus, which represents, albeit in the fleshy state, the tendinous cord of the latter in the horse.