Part 53

Origin by fleshy fibres from the distal half of the surface of the fibula, which it embraces. At the lateral malleolus the muscle ends in a thick tendon which passes through the canal on the ventral surface of the malleolus. In the canal it is enlarged and surrounded by a synovial bursa. It then turns onto the foot and passes over the dorsal surface of the calcaneus on its lateral side and has its

Insertion into the tubercle on the lateral side of the base of the fifth metatarsal.

Relations.--Outer surface with the peronei longus (q) and tertius (r). Inner surface with the bone.

Action.--Extensor of the foot.

=M. extensor digitorum longus= (Fig. 90, p).--A fusiform muscle which lies beneath the tibialis anterior (n), against the interosseous membrane, and between the tibialis anterior and the peronei.

Origin by a thin flat tendon from the lateral surface of the lateral epicondyle of the femur just dorsad of the origin of the popliteus. The tendon becomes narrower and thicker and passes through the capsule of the knee-joint and over a slight groove on the tibia just dorsad of the head of the fibula. The belly of the muscle extends the entire length of the tibia, passing beneath the transverse ligament (5) along with the tendon of the tibialis anterior (n). At the ankle it runs through a fibrous loop which is attached to the dorsal surface of the calcaneus (the annular ligament of the calcaneus); it then turns and passes onto the dorsum of the foot. The muscle ends in four slender tendons, the two middle of which begin at the transverse ligament, while the others begin at the fibrous loop. The tendons diverge to the four digits. Each tendon is connected with the fibrous sheath surrounding the base of the first phalanx, and each has a synovial bursa beneath it at this point. Near the distal end of the first phalanx the tendon is united on the lateral side to the conjoined tendon of the extensor brevis digitorum and an interosseus muscle, and on the medial side to the tendon of the interosseus. The lateral side of the most lateral tendon is not thus united, but is joined on the outer side by the tendon of the peroneus tertius.

Insertion.--The tendon thus formed on the dorsum of each phalanx by the junction of the two or three tendons above mentioned is firmly attached to the base of the second phalanx at its proximal end in the middle of its dorsal surface. In crossing the joint between the first and second phalanges the tendon forms a fibrous pad which protects the dorsum of the joint and glides over it. It then continues to be inserted into the base of the terminal phalanx.

Relations.--Outer surface with the tibialis anterior (n) and the superficial fascia. Inner surface with a part of the tibialis anterior, with the bones, the interosseous membrane, and the peroneus brevis (s).

Action.--Extensor of the phalanges.

=M. tibialis anterior= (Fig. 90, n).--This is the superficial muscle covering the lateral side of the tibia.

Origin by fleshy fibres from the proximal one-sixth of the lateral surface of the shaft of the tibia, from the proximal third of the medial border of the shaft and head of the fibula, and from the intervening interosseous ligament. The triangular body of the muscle covers the lateral surface of the tibia and the outer surface of the extensor longus digitorum (p). Near the malleolus it ends in a strong tendon which passes beneath the transverse ligament along with the tendon of the extensor longus (p). The tendon crosses the dorsal surface of the foot obliquely toward its medial side.

Insertion into the outer surface of the first metatarsal.

Relations.--Outer surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the extensor longus (p), the tibia, and the interosseous membrane.

Action.--Flexor of the foot.

4. Muscles of the Foot.

A. MUSCLES ON THE DORSUM OF THE FOOT.--=M. extensor brevis digitorum= (Fig. 90, u).--A broad thin muscle covering the tarsus and part of the metatarsus on the lateral side of the dorsum of the foot. It may be divided into three slips.

Origin from the distal border of nearly the whole calcaneal annular ligament (5), and from the proximal end of the dorsal surface of the three lateral metatarsals. At the middle of the metatarsals the muscle ends in three flat tendons which pass into the three interspaces between the four tendons of the extensor longus. Each tendon is divided into two branches.

Insertion.--The lateral branch of each tendon is inserted into the cartilaginous plate which lies in the metatarsophalangeal articulation of the digit on the outer side. The medial branch joins the lateral side of the extensor longus tendon on the dorsum of the first phalanx.

Action.--Extensor of the digits.

B. MUSCLES OF THE SOLE OF THE FOOT.--=M. flexor brevis digitorum= (=pedis perforatus=).--This is the direct continuation of the tendon of the plantaris. It lies immediately beneath the superficial plantar fascia. The flat belly of the muscle has its medial border attached by an oblique tendinous band to the medial surface of the scaphoid and the medial cuneiform. It divides into four slips which diverge to the four toes, each ending in a flat tendon. The slips decrease in size from the lateral to the medial side. The three lateral ones overlap one another proximally.

Insertion.--Each tendon expands at the distal end of the metatarsal and is wrapped about the tendon of the deep flexor which perforates it. The two halves unite beneath the perforating tendon and pass together with the perforating tendon through fibrous rings, the annular ligaments. One of these is attached to the sesamoids at the base of the phalanx. The other is attached to the head of the first phalanx. The tendons are finally inserted into the bases of the second phalanges.

The two annular ligaments are connected by fascia so as to form a continuous canal for the tendons. This canal is lined by a synovial membrane. Covering this canal is a more superficial layer of fascia. Before passing into the proximal annular ligament the two middle tendons unite each with the tendon of the corresponding lumbrical muscle. Each also gives off a branch, the lateral one on its lateral side and the medial one on its medial side. These branches, like those of the two middle ones, are inserted into the common phalangeal fascia.

Action.--Flexor of the second phalanges of the digits.

=M. quadratus plantæ= (called also the plantar head of the flexor longus digitorum) is a thin flat muscle which takes origin from the dorsal part of the lateral surface of the calcaneus and the cuboid. Its fibres converge to a flat tendon which passes transversely across the flexor longus digitorum and beneath the flexor brevis digitorum and calcaneometatarsal.

Insertion.--Into the medial part of the outer surface of the tendon of the flexor longus digitorum.

Action.--It holds the flexor longus tendon in place.

=Mm. lumbricales.=--These are six.

Origin.--The three larger ones have origin from the outer surface of the expanded portion of the flexor longus digitorum on its distal half. Each ends in a slender tendon.