Relations.—Outer surface with the gastrocnemius ([Fig. 92], j) and plantaris ([Fig. 91], l), and with the tendon of the semitendinosus ([Fig. 92], i). Distal border with the origins of the soleus and flexor longus digitorum. Inner surface with the capsule of the knee-joint and the tibia.
Action.—Rotates the thigh so as to turn the toes inward.
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).