Part 52

The three following deep muscles on the ventral surface of the shank are covered by a deep fascia (the deep crural fascia) which separates them from the overlying muscles. This fascia stretches from the dorsal border of the tibia about the ventral side of the shank to the medial border of the tibia. It sends a slip beneath the border of the fibula.

=M. flexor longus digitorum= (Fig. 91, m and n).--Owing to the reduction of the first digit in the cat the tendon of the flexor longus hallucis (m) has become united to that of the flexor longus digitorum (n), so that the two might be considered separate heads of a single muscle. The head corresponding to the flexor longus hallucis (m) is much larger than that corresponding to the flexor longus digitorum (n). The two heads will be described separately.

(1) =M. flexor longus hallucis= (Fig. 91, m).--This lies against the ventral surface of the tibia and fibula beneath the popliteus (Fig. 92, k) and soleus (Fig. 90, o).

Origin by fleshy fibres, (1) from the sheet of deep crural fascia which dips between it and the tibialis posterior (Fig. 91, o) and the flexor longus digitorum (n). (2) From the ventral surface of the tibia distad of the lateral oblique line to within one to three centimeters of the distal end of the shaft. (3) From the medial surface of the shaft and head of the fibula and from the whole interosseous ligament. On the fibular side the origin may extend onto the tendon of the popliteus and the fascia of the peroneus longus. The fibres form a flat fusiform mass ending one centimeter from the heel in a strong flat tendon (m′) which passes over the groove on the distal surface of the astragalus and over the groove on the sustentaculum tali. The two grooves are converted into a continuous canal by strong transverse ligaments, and the canal is lined by an extension of the synovial bursa of the ankle-joint. Emerging from the canal the tendon broadens and receives on its medial side the insertion of the tendon of the flexor longus digitorum (n). The tendon gives origin on its outer surface to the lumbricales and to the common plantar ligament. The tendon continues to broaden until it reaches the middle of the length of the metacarpals; here it divides into four tendons which pass to the terminal phalanges of the digits.

Insertion.--The relations of the four tendons and their insertions are identical with those of the flexor profundus digitorum of the hand.

Relations.--Outer surface with the soleus (Fig. 90, o) and plantaris (Fig. 91, l). Lateral border with the soleus, the peroneus tertius (Fig. 90, r), and peroneus brevis (Fig. 90, s). Medial border with the tibialis posterior (Fig. 91, o) and flexor longus digitorum (Fig. 91, n).

Action.--Flexor of the phalanges.

(2) =M. flexor longus digitorum= (Fig. 91, n).

Origin by muscle- and tendon-fibres from the ventral surface of the tibia over its proximal half between the oblique ridges; by fleshy fibres from an aponeurosis between it and the tibialis posterior (Fig. 91, o), and by tendon from the medial surface of the head of the fibula. At the junction of the middle and distal thirds of the bone the muscle ends in a slender tendon which passes through the ventral groove on the medial surface of the tibia and, curving onto the sole of the foot, becomes attached to the medial border of the common tendon described under the last.

Relations.--Outer surface with the medial head of the gastrocnemius (Fig. 91, k) and with the superficial fascia. Inner surface with the tibialis posterior (Fig. 91, o). Medial border at the proximal end with the popliteus (Fig. 92, k), lateral border with the tibialis posterior (Fig. 91, o), and flexor longus hallucis (Fig. 91, m).

Action.--Flexor of the phalanges.

=M. tibialis posterior= (Fig. 91, o).--A slender flat fusiform muscle beneath the flexor longus digitorum and between it and the flexor longus hallucis.

Origin by fleshy fibres from nearly the whole medial surface of the head of the fibula, from the inner surface of the aponeurosis between it and the flexor longus digitorum (n), and by a few fleshy fibres from the ventral surface of the tibia between the oblique ridges. Some fibres may also arise from the outer surface of the aponeurosis covering the flexor longus hallucis (m). The muscle ends at about the middle of the tibia in a slender flat tendon which passes parallel to the tendon of the flexor longus digitorum (n) through the dorsal groove on the medial surface of the distal end of the tibia. This groove is converted into a canal by a transverse ligament. Beyond the canal the tendon turns onto the plantar surface of the foot and passes through a groove on the ventral surface of the scaphoid bone. It then divides.

Insertion into the outer tuberosity on the surface of the scaphoid, and onto the proximal end of the ventral surface of the medial cuneiform.

Relations.--Outer and medial surface with the flexor longus digitorum (n). Lateral surface with the flexor longus hallucis (m).

Action.--Extensor of the foot.

B. MUSCLES ON THE DORSAL AND LATERAL SURFACES OF THE LOWER LEG.--=M. peroneus longus= (Fig. 90, q).--A slender, fusiform muscle lying superficially on the lateral side of the leg, dorsad of the soleus.

Origin by tendon-fibres from the lateral surface of the head of the fibula and from the proximal half of the lateral surface of its shaft.

At the junction of the middle and distal thirds of the lower leg the muscle ends in a slender tendon which passes through the groove on the lateral surface of the lateral malleolus. This groove is converted into a canal by a transverse ligament. The tendon passes through the groove on the peroneal tubercle of the calcaneus, then turns onto the sole of the foot and passes through the peroneal groove on the cuboid bone. It then turns mediad and passes through the groove between the ventral processes of the lateral cuneiform and the metatarsal until it reaches the first metatarsal. The entire groove is converted into a canal by the overlying ligaments.

Insertion into (1) the base of the fifth metatarsal, and (2) the outer side of the base of the first metatarsal, and (3) by slender branches into the bases of the other metatarsals.

Relations.--Outer (lateral) surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the peroneus tertius (r) and peroneus brevis (s), and with the extensor longus digitorum (p). At the proximal end the dorsal border touches the tibialis anterior (n).

Action.--Flexor of the foot.

=M. peroneus tertius= (Fig. 90, r).--A slender fusiform muscle beneath the peroneus longus (q).

Origin by fleshy fibres from about the second quarter of the lateral surface of the fibula. The muscle ends in a slender tendon (r′), which passes with that of the peroneus brevis (s) through the groove on the ventral border of the lateral malleolus. The groove is converted into a canal by a transverse ligament. Emerging from it, the tendon turns and passes along the outer margin of the foot. In passing the sesamoid at the base of the first phalanx of the fifth digit it is united to it by a band which passes from the sesamoid to the tendon.

The insertion is finally into the lateral border of the extensor tendon of the fifth digit as it passes from the first to the second phalanx.

Relations.--Outer surface with the peroneus longus (q), the soleus (o), and the superficial fascia. Ventral border with the soleus (o) and flexor longus hallucis (Fig. 91, m). Inner surface with the peroneus brevis (Fig. 90, s).

Action.--Extensor and abductor of the fifth digit and flexor of the foot.

=M. peroneus brevis= (Fig. 90, s) lies beneath the other peronei.