MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 155 



ending on the deep aspect of a long tendon which develops on the 

 dorsum of the muscle at about the middle of the leg. The tendon 

 gUdes over a groove of the posterior end of the talus transformed 

 into an osteofibrous canal by the tendinous fibers which join its two 

 edges. It is the deepest structure in the so-called medial calcaneal 

 canal (see Testut and Latarjet, 1959) and reaches the central plantar 

 compartment, where it is covered by that of the flexor tibiaUs (fig. 47). 

 Its fibers form the powerful tendon to the hallux and the perforating 

 tendons to toes II to IV in combination with contributions from the 

 tibial flexor. The distal part of each of these tendons is inserted on 

 the plantar base of the respective distal phalanx. Sirena (1871) refers 

 to this muscle as flexor hallucis and says that it contributes only to 

 the tendons of the hallux, third, and fourth toes. 



Nerve supply: A muscular ramus of the tibial nerve. 



Function: It contributes to the flexion of the distal interpha- 

 langeal joint in toes I to IV, its influence decreasing from the first 

 to the fourth. 



M. flexor digitorum tibialis: Origin (fig. 38) is by fleshy fibers 

 from (1) the lower hp of the long pophteal Une, (2) the intermediate 

 three-fifths of the dorsal tibial surface along a narrow area bound 

 medially by the pophteal fine and the attachment of the transverse 

 septum on the medial margin of the tibia and laterally by the origin 

 of m. tibiahs posterior, and (3) the septum between the two tibial 

 flexor muscles. A long muscular belly is formed, the fibers of which 

 extend through the ankle into the foot where they seem to be con- 

 tinued by those of the deep head m. flexor brevis without any inter- 

 ruption (fig. 7). Their insertion is on a long tendon which develops 

 in the muscle midway between the knee and the foot. In the tibial 

 retinaculum it is superficial to that of m. tibiahs posterior (fig. 47) 

 and is accompanied by the tibial nerve and the posterior tibial vessels. 

 In the medial calcaneal canal (see Testut and Latarjet, 1959) it 

 passes directly under and against the sustentaculum taU until in 

 the central compartment it joins the tendon of the deeper lying 

 flexor fibularis. Its fibers form the long perforating tendon to the 

 fifth and contribute to the tendon of the other toes (fig. 47). Only 

 a few fascicles join the tendon of the flexor fibularis to the hallux. 

 The description of this muscle by Sirena (1871) is not very clear, 

 but it appears as if he found the same arrangement as I did. 



Nerve supply: A muscular ramus of the tibial nerve. 



Function: It flexes the distal phalanx of toe V and contributes to 

 the same function in the remaining pedal digits. 



M. tibialis posterior: It lies completely covered by the flexor 

 tibiahs (fig. 46) and arises (fig. 38) by fleshy fibers from (1) the 

 dorsal tibial surface laterally to the origin of the former muscle, 



275-199 O - 68 - 11 



