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LEG The leg is that part of the lower limb between the knee joint and ankle joint (Fig. 6.79): proximally, most major structures pass between the thigh and leg through or in relation to the popliteal fossa behind the knee; distally, structures pass between the leg and foot mainly through the tarsal tunnel on the posteromedial side of the ankle, the exceptions being the anterior tibial artery and the ends of the deep and superficial fibular nerves, which enter the foot anterior to the ankle. page 585 page 586 Figure 6.79 Posterior view of leg; cross-section through the left leg (inset). The bony framework of the leg consists of two bones, the tibia and fibula, arranged in parallel. The fibula is much smaller than the tibia and is on the lateral side of the leg. It articulates superiorly with the inferior aspect of the lateral condyle of the proximal tibia, but does not take part in formation of the knee joint. The distal end of the fibula is firmly anchored to the tibia by a fibrous joint and forms the lateral malleolus of the ankle joint. The tibia is the weightbearing bone of the leg and is therefore much larger than the fibula. Above, it takes part in the formation of the knee joint and below it forms the medial malleolus and most of the bony surface for articulation of the leg with the foot at the ankle joint. The leg is divided into anterior (extensor), posterior (flexor), and lateral (fibular) compartments by: an interosseous membrane, which links adjacent borders of the tibia and fibula along most of their length; two intermuscular septa, which pass between the fibula and deep fascia surrounding the limb; and by direct attachment of the deep fascia to the periosteum of the anterior and medial borders of the tibia (Fig. 6.79). Muscles in the anterior compartment of leg dorsiflex the ankle, extend the toes, and invert the foot. Muscles in the posterior compartment plantarflex the ankle, flex the toes, and invert the foot. Muscles in the lateral compartment evert the foot. Major nerves and vessels supply or pass through each compartment. Bones Shaft and distal end of tibia The shaft of tibia is triangular in cross-section and has anterior, interosseous, and medial borders and medial, lateral, and posterior surfaces (Fig. 6.80): the anterior and medial borders, and the entire medial surface are subcutaneous and easily palpable; the interosseous border of the tibia is connected, by the interosseous membrane, along its length to the interosseous border of the fibula; the posterior surface is marked by an oblique line (the soleal line). The soleal line descends across the bone from the lateral side to the medial side where it merges with the medial border. In addition, a vertical line descends down the upper part of the posterior surface from the midpoint of the soleal line. It disappears in the lower one-third of the tibia. The shaft of the tibia expands at both the upper and lower ends to support the body's weight at the knee and ankle joints. The distal end of the tibia is shaped like a rectangular box with a bony protuberance on the medial side (the medial malleolus; Fig. 6.80). The upper part of the box is continuous with the shaft of the tibia while the lower surface and the medial malleolus articulate with one of the tarsal bones (talus) to form a large part of the ankle joint. The posterior surface of the box-like distal end of the tibia is marked by a vertical groove, which continues inferiorly and medially onto the posterior surface of the medial malleolus. The groove is for the tendon of the tibialis posterior muscle. The lateral surface of the distal end of the tibia is occupied by a deep triangular notch (the fibular notch), to which the distal head of the fibula is anchored by a thickened part of the interosseous membrane. page 586 page 587 Figure 6.80 Tibia and fibula. A. Anterior view. B. Posterior view. C. Cross-section through shafts. D. Posteromedial view of distal ends. Shaft and distal end of fibula The fibula is not involved in weightbearing. The fibular shaft is therefore much narrower than the shaft of the tibia. Also, and except for the ends, the fibula is enclosed by muscles. Like the tibia, the shaft of the fibula is triangular in cross-section and has three borders and three surfaces for the attachment of muscles, intermuscular septa, and ligaments (Fig. 6.80). The interosseous border of the fibula faces and is attached to the interosseous border of the tibia by the interosseous membrane. Intermuscular septa attach to the anterior and posterior borders. Muscles attach to the three surfaces. The narrow medial surface faces the anterior compartment of leg, the lateral surface faces the lateral compartment of leg, and the posterior surface faces the posterior compartment of leg. The posterior surface is marked by a vertical crest (medial crest), which divides the posterior surface into two parts each attached to a different deep flexor muscle. The distal end of the fibula expands to form the spade-shaped lateral malleolus (Fig. 6.80). The medial surface of the lateral malleolus bears a facet for articulation with the lateral surface of talus, thereby forming the lateral part of the ankle joint. Just superior to this articular facet is a triangular area, which fits into the fibular notch on the distal end of the tibia. Here the tibia and fibula are joined together by the distal end of the interosseous membrane. Posteroinferior to the facet for articulation with the talus is a pit or fossa (the malleolar fossa) for the attachment of the posterior talofibular ligament associated with the ankle joint. The posterior surface of the lateral malleolus is marked by a shallow groove for the tendons of the fibularis longus and fibularis brevis muscles. page 587 page 588 Joints Interosseous membrane of leg The interosseous membrane of leg is a tough fibrous sheet of connective tissue that spans the distance between facing interosseous borders of the tibial and fibular shafts (Fig. 6.81). The collagen fibers descend obliquely from the interosseous border of the tibia to the interosseous border of the fibula, except superiorly where there is a ligamentous band, which ascends from the tibia to fibula. There are two apertures in the interosseous membrane, one at the top and the other at the bottom, for vessels to pass between the anterior and posterior compartments of leg. The interosseous membrane not only links the tibia and fibula together, but also provides an increased surface area for muscle attachment. Figure 6.81 Interosseous membrane. A. Anterior view. B. Posteromedial view. The distal ends of the fibula and tibia are held together by the inferior aspect of the interosseous membrane, which spans the narrow space between the fibular notch on the lateral surface of the distal end of the tibia and the corresponding surface on the distal end of the fibula. This expanded end of the interosseous membrane is reinforced by anterior and posterior tibiofibular ligaments. This firm linking together of the distal ends of the tibia and fibula is essential to produce the skeletal framework for articulation with the foot at the ankle joint. Posterior compartment of leg Muscles Muscles in the posterior (flexor) compartment of leg are organized into two groups, superficial and deep, separated by a layer of deep fascia. Generally, the muscles mainly plantarflex and invert the foot and flex the toes. All are innervated by the tibial nerve. Superficial group The superficial group of muscles in the posterior compartment of leg comprises three muscles-the gastrocnemius, plantaris, and soleus (Table 6.6)-all of which insert onto the heel (calcaneus) of the foot and plantarflex the foot at the ankle joint (Fig. 6.82). As a unit, these muscles are large and powerful because they propel the body forward off the planted foot during walking and can elevate the body upward onto the toes when standing. Two of the muscles (gastrocnemius and plantaris) originate on the distal end of the femur and can also flex the knee. page 588 page 589 Gastrocnemius The gastrocnemius muscle is the most superficial of the muscles in the posterior compartment and is one of the largest muscles in the leg (Fig. 6.82). It originates from two heads, one lateral and one medial: the medial head is attached to an elongate roughening on the posterior aspect of the distal femur just behind the adductor tubercle and above the articular surface of the medial condyle; the lateral head originates from a distinct facet on the upper lateral surface of the lateral femoral condyle where it joins the lateral supracondylar line. At the knee, the facing margins of the two heads of the gastrocnemius form the lateral and medial borders of the lower end of the popliteal fossa. In the upper leg, the heads of the gastrocnemius combine to form a single elongate muscle belly, which forms much of the soft tissue bulge identified as the calf. In the lower leg, the muscle fibers of the gastrocnemius converge with those of the deeper soleus muscle to form the calcaneal tendon, which attaches to the calcaneus (heel) of the foot. The gastrocnemius plantarflexes the foot at the ankle joint and can also flex the leg at the knee joint. It is innervated by the tibial nerve. Plantaris The plantaris has a small muscle belly proximally and a long thin tendon, which descends through the leg and joins the calcaneal tendon (Fig. 6.82). The muscle takes origin superiorly from the lower part of the lateral supracondylar ridge of the femur and from the oblique popliteal ligament associated with the knee joint. Table 6-6. Superficial group of muscles in the posterior compartment of leg (spinal segments in bold are the major segments innervating the muscle) Muscle Origin Insertion Innervation Function Gastrocnemius Medial head-posterior surface of distal femur just superior to medial condyle; lateral head-upper posterolateral surface of lateral femoral condyle Via calcaneal tendon, to posterior surface of calcaneus Tibial nerve [S1, S2] Plantarflexes foot and flexes knee Plantaris Inferior part of lateral supracondylar line of femur and oblique popliteal ligament of knee Via calcaneal tendon, to posterior surface of calcaneus Tibial nerve [S1, S2] Plantarflexes foot and flexes knee Soleus Soleal line and medial border of tibia; posterior aspect of fibular head and adjacent surfaces of neck and proximal shaft; tendinous arch between tibial and fibular attachments Via calcaneal tendon, to posterior surface of calcaneus Tibial nerve [S1, S2] Plantarflexes the foot The short spindle-shaped muscle body of the plantaris descends medially, deep to the lateral head of the gastrocnemius, and forms a thin tendon, which passes between the gastrocnemius and soleus muscles and eventually fuses with the medial side of the calcaneal tendon near its attachment to the calcaneus. The plantaris contributes to plantarflexion of the foot at the ankle joint and flexion of the leg at the knee joint, and is innervated by the tibial nerve. Soleus The soleus is a large flat muscle under the gastrocnemius muscle (Fig. 6.82). It is attached to the proximal ends of the fibula and tibia, and to a tendinous ligament, which spans the distance between the two heads of attachment to the fibula and tibia: on the proximal end of the fibula, the soleus originates from the posterior aspect of the head and adjacent surface of the neck and upper shaft of the fibula; on the tibia, the soleus originates from the soleal line and adjacent medial border; the ligament, which spans the distance between the attachments to the tibia and fibula, arches over the popliteal vessels and tibial nerve as they pass from the popliteal fossa into the deep region of the posterior compartment of leg. In the lower leg, the soleus muscle narrows to join the calcaneal tendon that attaches to the calcaneus. The soleus muscle, together with the gastrocnemius and plantaris, plantarflexes the foot at the ankle joint. It is innervated by the tibial nerve. page 589 page 590 Figure 6.82 Superficial group of muscles in the posterior compartment of leg. A. Posterior view. B. Lateral view. page 590 page 591 Deep group There are four muscles in the deep posterior compartment of leg (Fig. 6.83)-the popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior (Table 6.7). The popliteus muscle acts on the knee, whereas the other three muscles act mainly on the foot. Popliteus The popliteus is the smallest and most superior of the deep muscles in the posterior compartment of the leg. It unlocks the extended knee at the initiation of flexion and stabilizes the knee by resisting lateral (external) rotation of the tibia on the femur. It is flat and triangular in shape, forms part of the floor of the popliteal fossa (Fig. 6.83), and is inserted into a broad triangular region above the soleal line on the posterior surface of the tibia. The popliteus muscle ascends laterally across the lower aspect of the knee and originates from a tendon, which penetrates the fibrous membrane of the joint capsule of the knee. The tendon ascends laterally around the joint where it passes between the lateral meniscus and the fibrous membrane and then into a groove on the inferolateral aspect of the lateral femoral condyle. The tendon attaches to and originates from a depression at the anterior end of the groove. When initiating gait from a standing position, contraction of the popliteus laterally rotates the femur on the fixed tibia, unlocking the knee joint. The popliteus muscle is innervated by the tibial nerve. Flexor hallucis longus The flexor hallucis longus muscle originates on the lateral side of the posterior compartment of leg and inserts into the plantar surface of the great toe on the medial side of the foot (Fig. 6.83). It arises mainly from the lower two-thirds of the posterior surface of the fibula and adjacent interosseous membrane. The muscle fibers of the flexor hallucis longus converge inferiorly to form a large cord-like tendon, which passes behind the distal head of the tibia and then slips into a distinct groove on the posterior surface of the adjacent tarsal bone (talus) of the foot. The tendon curves anteriorly first under the talus and then under a shelf of bone (the sustentaculum tali), which projects medially from the calcaneus, and then continues anteriorly through the sole of the foot to insert on the inferior surface of the base of the distal phalanx of the great toe. The flexor hallucis longus flexes the great toe. It is particularly active during the toe-off phase of walking when the body is propelled forward off the stance leg and the great toe is the last part of the foot to leave the ground. It can also contribute to plantarflexion of the foot at the ankle joint and is innervated by the tibial nerve. Figure 6.83 Deep group of muscles in the posterior compartment of leg. page 591 page 592 Flexor digitorum longus The flexor digitorum longus muscle originates on the medial side of the posterior compartment of leg and inserts into the lateral four digits of the foot (Fig. 6.83). It arises mainly from the medial side of the posterior surface of the tibia inferior to the soleal line. The flexor digitorum longus descends in the leg and forms a tendon, which crosses posterior to the tendon of the tibialis posterior muscle near the ankle joint. The tendon continues inferiorly in a shallow groove behind the medial malleolus and then swings forward to enter the sole of the foot. It crosses inferior to the tendon of the flexor hallucis longus muscle to reach the medial side of the foot and then divides into four tendons, which insert on the plantar surfaces of the bases of the distal phalanges of digits II to V. The flexor digitorum longus flexes the lateral four toes. It is involved with gripping the ground during walking and propelling the body forward off the toes at the end of the stance phase of gait. It is innervated by the tibial nerve. Tibialis posterior The tibialis posterior muscle originates from the interosseous membrane and the adjacent posterior surfaces of the tibia and fibula (Fig. 6.83). It lies between and is overlapped by the flexor digitorum longus and the flexor hallucis longus muscles. Near the ankle, the tendon of the tibialis posterior is crossed superficially by the tendon of the flexor digitorum longus muscle and lies medial to this tendon in the groove on the posterior surface of the medial malleolus. The tendon curves forward under the medial malleolus and enters the medial side of the foot. It wraps around the medial margin of the foot to attach to the plantar surfaces of the medial tarsal bones, mainly to the tuberosity of the navicular and to the adjacent region of the medial cuneiform. The tibialis posterior inverts and plantarflexes the foot, and supports the medial arch of the foot during walking. It is innervated by the tibial nerve. Table 6-7. Deep group of muscles in the posterior compartment of leg (spinal segments in bold are the major segments innervating the muscle) Muscle Origin Insertion Innervation Function Popliteus Lateral femoral condyle Posterior surface of proximal tibia Tibial nerve [L4 to S1] Stabilizes knee joint (resists lateral rotation of tibia on femur) Unlocks knee joint (laterally rotates femur on fixed tibia) Flexor hallucis longus Posterior surface of fibula and adjacent interosseous membrane Plantar surface of distal phalanx of great toe Tibial nerve [S2, S3] Flexes great toe Flexor digitorum longus Medial side of posterior surface of the tibia Plantar surfaces of bases of distal phalanges of the lateral four toes Tibial nerve [S2, S3] Flexes lateral four toes Tibialis posterior Posterior surfaces of interosseous membrane and adjacent regions of tibia and fibula Mainly to tuberosity of navicular and adjacent region of medial cuneiform Tibial nerve [L4, L5] Inversion and plantarflexion of foot; support of medial arch of foot during walking In the clinic Neurological examination of the legs Some of the commonest conditions that affect the legs are peripheral neuropathy (particularly associated with diabetes mellitus), lumbar nerve root lesions (associated with pathology of the intervertebral discs), fibular nerve palsy, and spastic paraparesis. Look for muscle wasting-loss of muscle mass may indicate loss of or reduced innervation. Test the power in muscle groups-hip flexion (L1, L2-iliopsoas-straight leg raise); knee flexion (L5 to S2-hamstrings-the patient tries to bend the knee while the examiner applies force to the leg to hold the knee in extension); knee extension (L3, L4-quadriceps femoris-the patient attempts to keep the leg straight while the examiner applies a force to the leg to flex the knee joint); ankle plantarflexion (S1, S2-the patient pushes the foot down while the examiner applies a force to the plantar surface of the foot to dorsiflex the ankle joint); ankle dorsiflexion (L4, L5-the patient pulls the foot upward while the examiner applies a force to the dorsal aspect of the foot to plantarflex the ankle joint). Examine knee and ankle reflexes-a tap with a tendon hammer on the patella ligament (tendon) tests reflexes at the L3-L4 spinal levels and tapping the calcaneal tendon tests reflexes at the S1-S2 spinal levels. Assess status of general sensory input to lumbar and upper sacral spinal cord levels-test light touch, pin prick, and vibration sense at dermatomes in the lower limb. page 592 page 593 Arteries Popliteal artery The popliteal artery is the major blood supply to the leg and foot and enters the posterior compartment of leg from the popliteal fossa behind the knee (Fig. 6.84). The popliteal artery passes into the posterior compartment of leg between the gastrocnemius and popliteus muscles. As it continues inferiorly it passes under the tendinous arch formed between the fibular and tibial heads of the soleus muscle and enters the deep region of the posterior compartment of leg where it immediately divides into an anterior tibial artery and a posterior tibial artery. Two large sural arteries, one on each side, branch from the popliteal artery to supply the gastrocnemius, soleus and plantaris muscles (Fig. 6.84). In addition, the popliteal artery gives rise to branches that contribute to a collateral network of vessels around the knee joint (see Fig. 6.76). Anterior tibial artery The anterior tibial artery passes forward through the aperture in the upper part of the interosseous membrane and enters and supplies the anterior compartment of leg. It continues inferiorly onto the dorsal aspect of the foot. Posterior tibial artery The posterior tibial artery supplies the posterior and lateral compartments of the leg and continues into the sole of the foot (Fig. 6.84). The posterior tibial artery descends through the deep region of the posterior compartment of leg on the superficial surfaces of the tibialis posterior and flexor digitorum longus muscles. It passes through the tarsal tunnel behind the medial malleolus and into the sole of the foot. In the leg, the posterior tibial artery supplies adjacent muscles and bone and has two major branches, the circumflex fibular artery and the fibular artery: the circumflex fibular artery passes laterally through the soleus muscle and around the neck of the fibula to connect with the anastomotic network of vessels surrounding the knee (Fig. 6.84; see also Fig. 6.76); the fibular artery parallels the course of the tibial artery, but descends along the lateral side of the posterior compartment adjacent to the medial crest on the posterior surface of the fibula, which separates the attachments of the tibialis posterior and flexor hallucis longus muscles; Figure 6.84 Arteries in the posterior compartment of leg. page 593 page 594 The fibular artery supplies adjacent muscles and bone in the posterior compartment of leg and also has branches that pass laterally through the intermuscular septum to supply the fibularis muscles in the lateral compartment of leg. A perforating branch that originates from the fibular artery distally in the leg passes anteriorly through the inferior aperture in the interosseous membrane to anastomose with a branch of the anterior tibial artery. The fibular artery passes behind the attachment between the distal ends of the tibia and fibula and terminates in a network of vessels over the lateral surface of the calcaneus. Veins Deep veins in the posterior compartment generally follow the arteries. Nerves Tibial nerve The nerve associated with the posterior compartment of leg is the tibial nerve (Fig. 6.85), a major branch of the sciatic nerve that descends into the posterior compartment from the popliteal fossa. The tibial nerve passes under the tendinous arch formed between the fibular and tibial heads of the soleus muscle and passes vertically through the deep region of the posterior compartment of leg on the surface of the tibialis posterior muscle with the posterior tibial vessels. The tibial nerve leaves the posterior compartment of leg at the ankle by passing through the tarsal tunnel behind the medial malleolus. It enters the foot to supply most intrinsic muscles and skin. In the leg, the tibial nerve gives rise to: branches that supply all the muscles in the posterior compartment of leg; and two cutaneous branches, the sural nerve and medial calcaneal nerve. Branches of the tibial nerve that innervate the superficial group of muscles of the posterior compartment and popliteus muscle of the deep group originate high in the leg between the two heads of the gastrocnemius muscle in the distal region of the popliteal fossa (Fig. 6.86). Branches innervate the gastrocnemius, plantaris, and soleus muscles, and pass more deeply into the popliteus muscle. Figure 6.85 Tibial nerve. A. Posterior view. B. Sural nerve. page 594 page 595 Branches to the deep muscles of the posterior compartment originate from the tibial nerve deep to the soleus muscle in the upper half of the leg and innervate the tibialis posterior, flexor hallucis longus, and flexor digitorum longus muscles. Sural nerve The sural nerve originates high in the leg between the two heads of the gastrocnemius muscle (Fig. 6.85). It descends superficial to the belly of the gastrocnemius muscle and penetrates through the deep fascia approximately in the middle of the leg where it is joined by a sural communicating branch from the common fibular nerve. It passes down the leg, around the lateral malleolus, and into the foot. The sural nerve supplies skin on the lower posterolateral surface of the leg and the lateral side of the foot and little toe. Medial calcaneal nerve The medial calcaneal nerve is often multiple and originates from the tibial nerve low in the leg near the ankle and descends onto the medial side of the heel. The medial calcaneal nerve innervates skin on the medial surface and sole of the heel (Fig. 6.85). Lateral compartment of leg Muscles There are two muscles in the lateral compartment of leg-the fibularis longus and fibularis brevis (Fig. 6.86 and Table 6.8). Both evert the foot (turn the sole outward) and are innervated by the superficial fibular nerve, which is a branch of the common fibular nerve. Fibularis longus The fibularis longus muscle arises in the lateral compartment of leg, but its tendon crosses under the foot to attach to bones on the medial side (Fig. 6.86). It originates from both the upper lateral surface of the fibula and from the anterior aspect of the fibular head and occasionally up onto the adjacent region of the lateral tibial condyle. The common fibular nerve passes anteriorly around the fibular neck between the attachments of the fibularis longus to the fibular head and shaft. Figure 6.86 Muscles in the lateral compartment of leg. A. Lateral view. B. Inferior view of the right foot, with the foot plantarflexed at the ankle. Table 6-8. Muscles of the lateral compartment of leg (spinal segments in bold are the major segments innervating the muscle) Muscle Origin Insertion Innervation Function Fibularis longus Upper lateral surface of fibula, head of fibula, and occasionally the lateral tibial condyle Undersurface of lateral sides of distal end of medial cuneiform and base of metatarsal I Superficial fibular nerve [L5, S1, S2] Eversion and plantarflexion of foot; supports arches of foot Fibularis brevis Lower two-thirds of lateral surface of shaft of fibula Lateral tubercle at base of metatarsal V Superficial fibular nerve [L5, S1, S2] Eversion of foot page 595 page 596 Distally, the fibularis longus descends in the leg to form a tendon, which, in order: passes posterior to the lateral malleolus in a shallow bony groove; swings forward to enter the lateral side of the foot; descends obliquely down the lateral side of the foot where it curves forward under a bony tubercle (fibular trochlea) of the calcaneus; enters a deep groove on the inferior surface of one of the other tarsal bones (the cuboid); and swings under the foot to cross the sole and attach to the inferior surfaces of bones on the medial side of the foot (lateral sides of the base of metatarsal I and the distal end of the medial cuneiform). The fibularis longus everts and plantarflexes the foot. In addition, the fibularis longus, tibialis anterior, and tibialis posterior muscles, which all insert on the undersurfaces of bones on the medial side of the foot, together act as a stirrup to support the arches of the foot. The fibularis longus supports mainly the lateral and transverse arches. The fibularis longus is innervated by the superficial fibular nerve. Fibularis brevis The fibularis brevis muscle is deep to the fibularis longus muscle in the leg and originates from the lower two-thirds of the lateral surface of the shaft of the fibula (Fig. 6.86). The tendon of fibularis brevis passes behind the lateral malleolus with the tendon of the fibularis longus muscle and then curves forward across the lateral surface of the calcaneus to attach to a tubercle on the lateral surface of the base of metatarsal V (the metatarsal associated with the little toe). The fibularis brevis assists in eversion of the foot and is innervated by the superficial fibular nerve. Arteries No major artery passes vertically through the lateral compartment of leg. It is supplied by branches (mainly from the fibular artery in the posterior compartment of leg) that penetrate into the lateral compartment (Fig. 6.87). Veins Deep veins generally follow the arteries. Nerves Superficial fibular nerve The nerve associated with the lateral compartment of leg is the superficial fibular nerve. This nerve originates as one of the two major branches of the common fibular nerve, which enters the lateral compartment of leg from the popliteal fossa (Fig. 6.87B). The common fibular nerve originates from the sciatic nerve in the posterior compartment of thigh or in the popliteal fossa (Fig. 6.87A), and follows the medial margin of the biceps femoris tendon over the lateral head of the gastrocnemius muscle and toward the fibula. Here it gives origin to two cutaneous branches, which descend in the leg: the sural communicating nerve, which joins the sural branch of the tibial nerve and contributes to innervation of skin over the lower posterolateral side of the leg; the lateral sural cutaneous nerve, which innervates skin over the upper lateral leg. The common fibular nerve continues around the neck of the fibula and enters the lateral compartment by passing between the attachments of the fibularis longus muscle to the head and shaft of fibula. Here the common fibular nerve divides into its two terminal branches: the superficial fibular nerve; the deep fibular nerve. The superficial fibular nerve descends in the lateral compartment deep to the fibularis longus and innervates the fibularis longus and fibularis brevis (Fig. 6.87B). It then penetrates deep fascia in the lower leg and enters the foot where it divides into medial and lateral branches, which supply dorsal areas of the foot and toes except for: the web space between the great and second toes, which is supplied by the deep fibular nerve; the lateral side of the little toe, which is supplied by the sural branch of the tibial nerve. The deep fibular nerve passes anteromedially through the intermuscular septum into the anterior compartment of leg, which it supplies. Anterior compartment of leg Muscles There are four muscles in the anterior compartment of the leg-the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius (Fig. 6.88 and Table 6.9). Collectively they dorsiflex the foot at the ankle joint, extend the toes, and invert the foot. All are innervated by the deep fibular nerve, which is a branch of the common fibular nerve. Tibialis anterior page 596 page 597 Figure 6.87 Common fibular nerve, and nerves and arteries of the lateral compartment of leg. A. Posterior view, right leg. B. Lateral view, right leg. Figure 6.88 Muscles of the anterior compartment of leg. The tibialis anterior muscle is the most anterior and medial of the muscles in the anterior compartment of leg (Fig. 6.88). It originates mainly from the upper two-thirds of the lateral surface of the shaft of the tibia and adjacent surface of the interosseous membrane. It also originates from deep fascia. page 597 page 598 Table 6-9. Muscles of the anterior compartment of leg (spinal segments in bold are the major segments innervating the muscle) Muscle Origin Insertion Innervation Function Tibialis anterior Lateral surface of tibia and adjacent interosseous membrane Medial and inferior surfaces of medial cuneiform and adjacent surfaces on base of metatarsal I Deep fibular nerve [L4, L5] Dorsiflexion of foot at ankle joint; inversion of foot; dynamic support of medial arch of foot Extensor hallucis longus Middle one-half of medial surface of fibula and adjacent surface of interosseous membrane Dorsal surface of base of distal phalanx of great toe Deep fibular nerve [L5, S1] Extension of great toe and dorsiflexion of foot Extensor digitorum longus Proximal one-half of medial surface of fibula and related surface of lateral tibial condyle Via dorsal digital expansions into bases of distal and middle phalanges of lateral four toes Deep fibular nerve [L5, S1] Extension of lateral four toes and dorsiflexion of foot Fibularis tertius Distal part of medial surface of fibula Dorsomedial surface of base of metatarsal V Deep fibular nerve [L5, S1] Dorsiflexion and eversion of foot The muscle fibers of the tibialis anterior converge in the lower one-third of the leg to form a tendon, which descends into the medial side of the foot, where it attaches to the medial and inferior surfaces of one of the tarsal bones (medial cuneiform) and adjacent parts of metatarsal I associated with the great toe. The tibialis anterior dorsiflexes the foot at the ankle joint and inverts the foot at the intertarsal joints. During walking, it provides dynamic support for the medial arch of the foot. The tibialis anterior is innervated by the deep fibular nerve. Extensor hallucis longus The extensor hallucis longus muscle lies next to and is partly overlapped by the tibialis anterior muscle (Fig. 6.88). It originates from the middle one-half of the medial surface of the fibula and adjacent interosseous membrane. The tendon of the extensor hallucis longus appears between the tendons of the tibialis anterior and extensor digitorum longus in the lower one-half of the leg and descends into the foot. It continues anteriorly on the medial side of the dorsal surface of the foot to near the end of the great toe where it inserts on the upper surface of the base of the distal phalanx. The extensor hallucis longus extends the great toe. Because it crosses anterior to the ankle joint, it also dorsiflexes the foot at the ankle joint. Like all muscles in the anterior compartment of leg, the extensor hallucis longus muscle is innervated by the deep fibular nerve. Extensor digitorum longus The extensor digitorum longus muscle is the most posterior and lateral of the muscles in the anterior compartment of leg (Fig. 6.88). It originates mainly from the upper one-half of the medial surface of the fibula lateral to and above the origin of the extensor hallucis longus muscle, and extends superiorly onto the lateral condyle of the tibia. Like the tibialis anterior muscle, it also originates from deep fascia. The extensor digitorum longus muscle descends to form a tendon, which continues into the dorsal aspect of the foot, where it divides into four tendons, which insert, via dorsal digital expansions, into the dorsal surfaces of the bases of the middle and distal phalanges of the lateral four toes. The extensor digitorum longus extends the toes and dorsiflexes the foot at the ankle joint, and is innervated by the deep fibular nerve. Fibularis tertius The fibularis tertius muscle is normally considered part of the extensor digitorum longus (Fig. 6.88). The fibularis tertius originates from the medial surface of the fibula immediately below the origin of the extensor digitorum longus muscle and the two muscles are normally connected. The tendon of the fibularis tertius descends into the foot with the tendon of the extensor digitorum longus. On the dorsal aspect of the foot, it deviates laterally to insert into the dorsomedial surface of the base of metatarsal V (the metatarsal associated with the little toe). The fibularis tertius assists in dorsiflexion and possibly eversion of the foot, and is innervated by the deep fibular nerve. Arteries Anterior tibial artery The artery associated with the anterior compartment of leg is the anterior tibial artery, which originates from the popliteal artery in the posterior compartment of leg and passes forward into the anterior compartment of leg through an aperture in the interosseous membrane. page 598 page 599 Figure 6.89 Anterior tibial artery and deep fibular nerve. The anterior tibial artery descends through the anterior compartment on the interosseous membrane (Fig. 6.89). In the distal leg, it lies between the tendons of the tibialis anterior and extensor hallucis longus muscles. It leaves the leg by passing anterior to the distal end of the tibia and ankle joint and continues onto the dorsal aspect of the foot as the dorsalis pedis artery. In the proximal leg, the anterior tibial artery has a recurrent branch, which connects with the anastomotic network of vessels around the knee joint. Along its course, the anterior tibial artery supplies numerous branches to adjacent muscles and is joined by the perforating branch of the fibular artery, which passes forward through the lower aspect of the interosseous membrane from the posterior compartment of leg. Distally, the anterior tibial artery gives rise to an anterior medial malleolar artery and an anterior lateral malleolar artery, which pass posteriorly around the distal ends of the tibia and fibula, respectively, and connect with vessels from the posterior tibial and fibular arteries to form an anastomotic network around the ankle. Veins Deep veins follow the arteries and have similar names. Nerves Deep fibular nerve The nerve associated with the anterior compartment of the leg is the deep fibular nerve (Fig. 6.89). This nerve originates in the lateral compartment of leg as one of the two divisions of the common fibular nerve. The deep fibular nerve passes anteromedially through the intermuscular septum that separates the lateral from the anterior compartments of leg and then passes deep to the extensor digitorum longus. It reaches the anterior interosseous membrane where it meets and descends with the anterior tibial artery. The deep fibular nerve: innervates all muscles in the anterior compartment; then continues into the dorsal aspect of the foot where it innervates the extensor digitorum brevis, contributes to the innervation of the first two dorsal interossei muscles, and supplies the skin between the great and second toes.