May 18, 2012
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Anterior Shin (crural) Muscles

III. THE MUSCLES OF THE LEG (crura)

The muscles of the leg may be divided into three groups : anterior, posterior, and lateral.

THE ANTERIOR CRURAL MUSCLES (fig. 655)

Tibialis anterior	Extensor digitorum longus
Extensor hallucis longus	Peroneus tertius

The fascia cruris or deep fascia of the leg is continuous above with the fascia lata, and is attached around the knee to the patella, the ligamentum patella, the tubercle and condyles of the tibia and the head of the fibula. Behind, it forms the popliteal fascia, which covers the popliteal fossa; here it is strengthened by transverse fibers, and perforated by the short saphenous vein. It receives an expansion from the tendon of the Biceps femoris laterally, and expansions from the tendons of the Sartorius, Gracilis, Semitendinosus and Semimembranosus medially; it blends with the periosteum covering the subcutaneous surface of the tibia, and with that covering the head and malleolus of the fibula; below, it is continuous with the extensor and flexor retinacula (transverse crural and laciniate ligaments). It is thick and dense in the upper and anterior part of the leg, and gives origin, by its deep surface, to some fibers of the Tibialis anterior and Extensor digitorum longus; it is thinner behind, where it covers the Gastrocnemius and Soleus. On the lateral side of the leg it gives off the anterior and posterior crural intermuscular septa, which are attached respectively to the anterior and posterior borders of the fibula; in the anterior and posterior crural regions the fascia also gives off several slender processes which enclose the individual muscles. A broad, transverse, intermuscular septum, called the deep transverse fascia of the leg, intervenes between the superficial and deep posterior crural muscles.

The Tibialis anterior (figs. 654, 655) is situated on the lateral side of the tibia; it is thick and fleshy above, tendinous below. It arises from the lateral condyle and upper one-half or two-thirds of the lateral surface of the shaft of the tibia; from the adjoining part of the anterior surface of the interosseous membrane; from the deep surface of the fascia cruris; and from the intermuscular septum between it and the Extensor digitorum longus. The fibers runt vertically downwards, and end in a tendon which is apparent on the anterior surface of the muscle at the lower one-third of the leg; it passes through the medial compartments of the superior and inferior extensor retinacula (transverse and cruciate crural ligaments), inclines towards the medial side of the foot, and is inserted into the medial and under surfaces of the medial cuneiform bone, and the base of the first metatarsal bone. This muscle overlaps the anterior tibia-1 vessels and nerve in the upper part of the leg.

Nerve-supply.---The Tibialis anterior is supplied by the anterior tibial (deep peroneal) nerve (L. 4 and 5 and S. 1).

Actions.-The Tibialis anterior is a dorsiflexor of the ankle-joint; it also raises the medial border of the foot, i.e. inverts the foot.

The Extensor hallucis longus (figs. 655, 660) lies between, and is partly hidden by the Tibialis anterior and the Extensor digitorum longus. It arises from the anterior surface of the fibula, for about the. middle two-fourths of its extent, medial to the origin of the Extensor digitorum longus; it also arises from the anterior surface of the interosseous membrane to a similar extent. The anterior tibial vessels and nerve lie between it and the Tibialis anterior. The fibers pass downwards, and end in a tendon which occupies the anterior border of the muscle, passes deep to the superior and through the inferior extensor retinaculum, crosses to the medial side of the anterior tibial vessels near the ankle-joint, and is inserted into the dorsal aspect of the base of the distal phalanx of the great toe. Opposite the metatarsophalangeal articulation a thin prolongation is given off from each side of the tendon and covers the dorsal surface of the joint. An expansion from the medial side of the tendon is usually inserted into the base of the proximal phalanx.

Figure 654
Leg muscles, arteries, nerves, veins transverse section - Figure 654

Nerve-supply.-The Extensor hallucis longus is supplied by the anterior tibial (deep peroneal) nerve (L. 4 and 5 and S. 1).

Actions.---The Extensor hallucis longus extends the phalanges of the great toe; in continued action it dorsiflexes the ankle-joint.

The Extensor digitorum longus (figs. 654, 655, 660) is a pennate muscle, situated at the lateral part of the front of the leg. It arises from the lateral condyle of the tibia, the upper three-fourths of the anterior surface of the shaft of the fibula, the upper part of the anterior surface of the interosseous membrane, the deep surface of the fascia cruris, the anterior crural intermuscular septum and the septum between it and the Tibialis anterior. In the upper part of the leg the anterior tibial vessels and nerve intervene between the muscle and the Tibialis anterior and at a lower level, the Extensor hallucis longus also intervenes between them. The tendon of the Extensor digitorum longus passes behind the superior extensor retinaculum and within the loop of the inferior extensor retinaculum in company with the Peroneus tertius (fig. 661). It divides into four slips, which run forward on the dorsum of the foot, and are inserted into the middle and distal phalanges of the four lesser toes. Opposite the metatarsophalangeal joints the tendons to the second, third, and fourth toes are each joined on the lateral side by a tendon of the Extensor digitorum brevis. The tendons are inserted as follows : each receives a fibrous expansion from the corresponding Lumbrical and Interosseous muscles; and then spreads out into a broad aponeurosis, which covers the dorsal surface of the proximal phalanx; at the joint of the proximal with the middle phalanx this aponeurosis divides into three slips-an intermediate; which is inserted into the base of the middle phalanx; and two collateral slips, which, after uniting with each other on the dorsal surface of the middle phalanx, are inserted into the base of the distal phalanx.

Figure 655
Superficial anterior leg muscles anterior view - Figure 655

Nerve-supply.-The Extensor digitorum longus is supplied by the anterior tibial (deep peroneal) verve (L. 4 and 5 and S. 1).

Actions.--The Extensor digitorum longus extends the toes at the metatarsophalangeal joints, and when its action is continued dorsiflexes the ankle joint.

The Peroneus tertius (figs. 655, 659) is a part of the Extensor digitorum longus, and might be described as its fifth tendon. The fibers belonging to this tendon arise from the lower one-third or more of the anterior surface of the fibula, the lower part of the anterior surface of the interosseous membrane and the anterior crural intermuscular septum. The tendon passes behind the superior and within the loop of the inferior extensor retinaculum in company with the Extensor digitorum longus (fig. 661), and is inserted into the medial part of the dorsal surface of the base of the fifth metatarsal bone, but often spreads into a thin sheet which extends forwards along the medial border of the shaft of the bone. This muscle is sometimes wanting.