We will divide the leg into three regions: anterior, external, and posterior. With regard to the internal region, there are no muscles which belong exclusively to it; for it is in great measure formed by the internal surface of the tibia, which, as in man, is subcutaneous. Muscles of the Anterior Region We first note that in the human species the tibialis anticus, extensor proprius pollicis, extensor longus digitorum and the peroneous tertius or anticus, form the subcutaneous layer of this region. We now proceed to study these muscles in quadrupeds. [Illustration: FIG. 83.--MYOLOGY OF THE HORSE: THE ANTERIOR TIBIAL MUSCLE (FLEXOR OF THE METATARSUS), LEFT LEG, ANTERIOR VIEW. 1, Femoral trochlea; 2, tibia; 3, tendinous portion of the tibialis anticus; 4, cuboid branch of same; 5, 5, its metatarsal branch; 6, fleshy portion; 7, cuneiform branch of its tendon; 8, metatarsal branch of the same tendon; 9, extensor longus digitorum (anterior extensor of the phalanges turned outwards); 10, peroneus brevis (lateral extensor of the phalanges).] =Tibialis Anticus= (Fig. 83; Fig. 84, 6; Fig. 85, 4; Fig. 87, 10; Fig. 88, 10, 11).--It is further named by veterinarians the _flexor of the metatarsus_. In the dog and the cat this muscle, which is rather large, arises from the external tuberosity of the tibia and from the crest of this bone. In its superior part it is flat, but lower down it is thick and produces a prominence in front of the tibia. Finally, it becomes tendinous, and passes towards the tarsus; thence it is directed towards the inner side of the metatarsus, and is inserted into the great-toe, this latter being sometimes well developed, but also often merely represented by a small bony nodule on which the muscle is then fixed. In the other animals with which we here occupy ourselves, the tibialis anticus presents a complexity which would be incomprehensible unless this muscle be first studied in the horse. In this latter the tibialis anticus consists of two distinct portions, placed one in front of the other: a fleshy portion, and a tendinous portion running parallel to it. The muscle is covered, except on its internal part and inferiorly, by a muscle with which we will occupy ourselves later on--that is, the common extensor of the toes. The tendinous portion of the tibialis anticus (Fig. 83), especially covered by the extensor of the toes, arises from the inferior extremity of the femur, from the fossa situated between the trochlea and the external condyle; thence it descends towards a groove which is hollowed out on the external tuberosity of the tibia, and is directed towards the tarsus, where it divides into two branches, which are inserted into the cuboid bone and the superior extremity of the principal metatarsal. These two parts form a ring through which the terminal tendon of the fleshy portion of the same muscle passes.