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Posterior Crural Muscles

2. THE POSTERIOR CRURAL MUSCLES

The muscles of the back of the leg are subdivided into two groups-superficial and deep. Those of the superficial group constitute a powerful muscular mass, forming the calf of the leg. Their large size is one of the most characteristic features of the muscular apparatus in man and bears a direct relation to his erect attitude and his mode of progression (walking & running).

Figure 656

Superficial Group (fig. 656)

The Gastrocnemius (figs. 6:54, 656); which is the most superficial muscle of the group, forms the greater part of the calf. It arises by two heads, which are connected to the condyles of the femur by strong, flat tendons. The medial and larger head tales its origin from a depression at the upper and posterior part of the medial condyle behind the adductor tubercle, and from a rounded tubercle on the popliteal surface of the femur just above the medial condyle. The lateral bead arises from an impression on the lateral surface of the lateral condyle and from the lower part of the corresponding supracondylar line. Both heads also arise from the subjacent part of the capsular ligament of the knee-joint. Each head spreads out into a tendinous expansion which covers the posterior surface of the corresponding part of the muscle. From the anterior surfaces of these tendinous expansions, muscular fibers are given off; those of the medial head extending lower than those of the lateral. The two heads remain separate and their fibers are inserted into a broad aponeurosis which is developed on the anterior surface of the muscle. The aponeurosis, gradually contracting, unites with the tendon of the Soleus, and forms with it the tendo calcaneus.

Relations.-The fascia cruris separates the superficial surface of the muscle from the short saphenous vein, and the sural communicating and sural nerves; the lateral popliteal nerve crosses the lateral head of the muscle, lying partly under cover of Biceps femoris. The deep surface is in relation with the oblique posterior ligament of the knee-joint, the Popliteus, Soleus, Plantaris, popliteal vessels and medial popliteal nerve. In front of the tendon of the medial head is a bursa which, in some cases, communicates with the cavity of the knee-joint. The tendon of the lateral head sometimes contain a sesamoid fibrocartilage or bone, where it plays over the corresponding condyle; and one is occasionally found in the tendon of the medial head.

Nerve-supply.--The Gastrocnemius is supplied by the medial popliteal (tibial) nerve (S. 1 and 2).

Actions.-Acting from above the Gastrocnemius plantar-flexes the ankle-joint; acting from below it flexes the knee-joint.

The Soleus (figs. 654, 656) is a broad flat muscle situated immediately in front of the Gastrocnemius. It arises by tendinous fibers from the back of the head, and from the upper one-fourth of the posterior surface of the shaft of the fibula: from the soleal line (popliteal line) and the middle one-third of the medial border of the tibia; and from a fibrous band which stretches between the tibia and fibula, and arches over the popliteal vessels and medial popliteal (tibial) nerve. The muscular fibers end in a flat tendon which covers the posterior surface of the muscle, and, gradually becoming thicker and narrower, joins with the tendon of the Gastrocnemius and forms with it the tendo calcaneus .

Relations.--Its superficial surface is in relation with the Gastrocnemius and Plantaris; its deep surface, with the Flexor digitorum longus, Flexor hallucis longus, Tibialis posterior, and posterior tibial vessels and nerve, from all of which it is separated by the deep transverse fascia of the leg.

Nerve-supply.-The Soleus is supplied by the medial popliteal (tibial) nerve (S. 1 and 2).

Actions.-The Soleus is a plantar-flexor of the ankle-joint; in standing, the Soleus, taking its fixed point from below, steadies the leg on the foot.

The Gastrocnemius and Soleus together form a muscular mass which is occasionally described as the Triceps surae; its tendon of insertion is the tendo calcaneus.

The tendo calcaneus (fig. 656), the common tendon of the Gastrocnemius and Soleus, is the thickest and strongest in the body. It is about 15 cm, long, and begins near the middle of the leg, but its anterior surface receives fleshy fibers from the Soleus, almost to its lower end. It gradually narrows and thickens until it reaches a level about 4 cm. above the calcaneum; below this level it expands and is inserted into the middle part of the posterior surface of the calcaneum, a bursa being interposed between the tendon and the upper part of this surface.

Actions.--The muscles of the calf Gastrocnemius and Soleus-are the chief plantar-flexors of the ankle-joint. They possess considerable power, and are called into use in standing, walking, dancing and leaping; hence they are usually of large size. In walking, these muscles raise the heel from the ground; the body being thus supported on the raised foot, the opposite limb can be carried forwards.

The Plantaris (fig. 656) arises from the lower part of the lateral supracoudylax line, and from the oblique posterior ligament of the knee-joint. It forms a small, fusiform belly, from 7cm to 10 cm. long; this ends in a long slender tendon, which crosses obliquely between the Gastrocnemius and Soleus and runs along the medial border of the tendo calcaneus to be inserted with it into the posterior part of the calcaneum. This muscle is sometimes double, and at other times wanting. Occasionally, its tendon is lost in the flexor retinaculum (laciniate ligament), or in the fascia of the leg.

Nerve-supply.-The Plantaris is supplied by the medial popliteal (tibial) nerve (L. 4 and 5 and S. 1).

Actions.-The Plantaris is the rudiment of a large muscle, the tendon of which in some of the lower animals is inserted into the plantar aponeurosis in man it is an accessory to the Gastrocnemius, plantar-flexing the ankle-joint if the foot be free, or flexing the knee-joint if the foot be fixed.

Deep Group (fig. 657).

The deep transverse fascia of the leg is a septum between the superficial and deep muscles of the back of the leg. At the sides it is connected to the medial margin of the tibia and the posterior border of the fibula. Above, where it covers the Popliteus, it is thick and dense, and receives an expansion from the tendon of the Semimembranosus; it is thin in the middle of the leg: but below, where it covers the tendons passing behind the malleoli, it is thick and continuous with the flexor retinaculum (laciniate ligament) and the superior peroneal retinaculum. The Popliteus (fig. 657) is a flat, triangular muscle, which forms the floor of the lower part of the popliteal fossa. It arises by a strong tendon about 2.5 cm. long, from a depression at the anterior part of the groove on the lateral condyle of the femur, and to a small extent from the oblique posterior ligament of the knee-joint. It is inserted into the medial two thirds of the triangular area above the soleal line (popliteal line) on the posterior surface of the shaft, of the tibia, and into the tendinous expansion covering the muscle.

Figure 657

Relations.-Its tendon of origin is intracapsular and is covered by the lateral ligament of the knee, and the tendon of the Biceps femoris (fig. 566). Invested on its deep surface by the synovial membrane, it grooves the posterior border of the lateral semilunar cartilage and the adjoining part of the tibia, and emerges from under cover of the posterior band of the arcuate ligament (fig. 561). On the floor of the popliteal fossa it is covered by a strong layer of fascia which is derived for the most part from the tendon of the Semimembranosus.

Nerve-supply.-The Popliteus is supplied by the medial popliteal (tibial) nerve (L. 4 and 5 and

Actions.-The Popliteus flexes the knee-joint; when the joint is flexed, it rotates the tibia medially. It is specially called into action at the beginning of flexion of the fully extended knee-joint inasmuch as it produces the slight medial rotation of the tibia which is essential in the early stage of this movement.

The Flexor hallucis longus (figs. 657, 660) is situated on the fibular side of the leg. It arises from the inferior two-thirds of the posterior surface of the shaft of the fibula, with the exception of about 2.5 cm, at its lowest part; from the lower part of the posterior surface of the interosseous membrane; from the posterior crural intermuscular septum, and from the fascia covering the Tibialis posterior, which it overlaps to a considerable extent. The fibers pass obliquely downwards and backwards, and end in a tendon which occupies nearly the whole length of the posterior surface of the muscle. This tendon lies in a groove which crosses the posterior surface of the lower end of the tibia, the posterior surface of the talus. and the under surface of the sustentaculum tali of the calcaneus (figs. 658, 661.). In the sole of the foot it runs forwards between the two heads of the Flexor hallucis brevis, and is inserted into the plantar surface of the base of the distal phalanx of the great toe. The grooves on the talus and calcaneum which contain the tendon of the muscle are converted by tendinous fibers into a canal, which is lined by a synovial sheath. As the tendon passes forwards in the sole of the foot, it is situated above, and crosses from the lateral to the medial side of the tendon of the Flexor digitorum longus, to which it is connected by a fibrous slip. This slip varies considerably in size; it is usually distributed to the tendon for the second and third toes, but is sometimes restricted to that for the second, and occasionally is distributed to the tendon for the fourth toe, in addition.

Relations.-Its superficial surface is in relation with the Soleus and tendo calcaneus, from which it is separated by the deep transverse fascia; its deep surface, with the fibula, Tibialis posterior; the peroneal vessels, the lower part of the interosseous membrane; end the ankle-joint; its lateral border, with the Peronei; its medial border, with the Tibialis posterior, posterior tibial vessels and nerve.

Nerve-supply.-The Flexor hallucis longus is supplied by the posterior tibial nerve (L. .5 and S. 1 and 2).

Actions.-The Flexor hallucis longus flexes the great toe, and, continuing its action, plantar-flexes the ankle-joint.

The Flexor digitorum longus (fig. 657) is situated on the tibial side of the leg. Its upper part is thin and pointed, but the muscle gradually increases in size as it descends. It arises from the posterior surface of the shaft of the tibia, medial to the tibial origin of the Tibialis posterior; this origin extends from just below the soleal line (popliteal line) to within 7 cm or 8 cm, of the lower extremity of the bone; it also arises from the fascia covering the Tibialis posterior. The fibers end in a tendon which runs nearly the whole length of the posterior surface of the muscle. This tendon gradually crosses the Tibialis posterior and passes behind the medial malleolus, in a groove common to it and the Tibialis posterior, but separated from the latter by a fibrous septum; each tendon being contained in a special compartment lined by a separate synovial sheath. It passes obliquely forwards and laterally, in contact with the medial side of the sustentaculun tali (fig. 658) and deep to the flexor retinaculum (laciniate ligament) and enters the sole of the foot (fig. 667), where it crosses below (i.e. superficial to) the tendon of the Flexor hallucis longus, and receives from it a strong slip. It then expands and is joined by the Flexor digitorum accessorius (Quadratus plantae), and finally divides into four tendons, which are inserted into the plantar surfaces of the bases of the distal phalanges of the second, third, fourth, and fifth toes, each tendon passing through an opening in the corresponding tendon of the Flexor digitorum brevis opposite the base of the proximal phalanx.

Relations.-In the leg its superficial surface is in relation with the deep transverse fascia, which separates it from the Soleus, and, distally, with the posterior tibial vessels and nerve; its deep surface, with the tibia and Tibialis posterior. In the foot, it is covered by the Abductor hallucis and Flexor digitorum brevis, and crosses superficial to the Flexor hallucis longus.

Nerve-supply-The Flexor digitorum longus is supplied by the posterior tibial nerve (L. 5 and S. 1.).

Actions.-The Flexor digitorum longus flexes the phalanges of the toes, and in continued action plantar-flexes the ankle-joint. In consequence of the oblique direction of its tendons it draws the toes medially, but this is counteracted by the Quadratus plantae (Flexor digitorum accessorus), which is inserted into the lateral side of the tendon.

The Tibialis posterior (figs. 654, 657) lies between the Flexor hallucis longus and Flexor digitorum longus, and is the deepest muscle on the back of the leg. It begins above by two pointed processes, separated by an angular interval through which the anterior tibial vessels pass to the front of the leg. It arises from the posterior surface of the crural interosseous membrane, with the exception of its lowest part; from the lateral portion of the posterior surface of the shaft of the tibia, between the commencement of the soleal line above and the junction of the middle with the lower one-third of the shaft below; and from the upper two-thirds of the medial part of the posterior surface of the fibula; some fibers also arise, from the sleep transverse fascia, and from the intermuscular septa separating it from the adjacent muscles. In the lower one-fourth of the leg its tendon passes in front of (i.e. deep to) that of the Flexor digitorum longus and lies with it in a groove behind the medial malleolus, but enclosed in a separate sheath: it next passes deep to the flexor retinaculum (laciniate ligament) and superficial to the deltoid ligament (fig. 658) into the foot, and then below the plantar calcaneonavicular ligament, where it contains a sesamoid fibrocartilage. It is inserted into the tuberosity of the navicular bone, and gives off fibrous slips, one of which passes backwards and is attached to the sustentaculum tali of the calcaneum, while others pass forwards and laterally and are fixed to the three cuneiform bones, the cuboid bone, and the bases of the second, third and fourth metatarsal bones (fig. 574).

Figure 658

Relations.--Its superficial surface is in relation with the Soleus, from which it is separated by the deep transverse fascia., the Flexor digitorum longus, the Flexor hallucis longus, the posterior tibia] vessels, nerve, and the peroneal vessels; its deep surface with the interosseous membrane, the tibia, fibula and ankle-joint.

Nerve-supply-The Tibialis posterior is supplied by the posterior tibial nerve. (L. 5 and S. 1).

Actions.---The Tibialis posterior plantar-flexes the ankle-joint; it also pulls up the medial border of the foot, i.e. inverts the foot. In the sole of the foot its tendon lies inferomedial to the plantar calcaneonavicular ligament, and, together with its additional slips of insertion, is an important factor in maintaining the longitudinal arch of the foot.

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