MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 165 



dorsal ex})ansion that reach the capsule of the corresponding meta- 

 tarsophalangeal joint. The second plantar interosseous arises from 

 the tibial half of the shaft and base of metatarsale IV. Its fibers end 

 by a tendon in the same side of the base of the first phalanx of toe IV. 

 The same relation exists here with the dorsal extensor expansion and 

 contrahens IV. The third plantar interosseous originates from the 

 tibial shaft and base of metatarsale V and is inserted by a tendon on 

 the tibioplantar base of the first phalanx of toe V. There are equivalent 

 relations to m. contrahens IV and the dorsal extensor expansion. The 

 same muscles in AlouaUa fusca (Sirena, 1871) present no significant 

 difference. 



Nerve supply: All of them are supplied by branches of the deep 

 ramus of the lateral plantar nerve. 



Function: Dorsal interossei abduct the medial and lateral toes 

 from the third digit. In view of their respective relations with the 

 dorsal extensor expansions, the dorsals are also extensors of the 

 metatarsophalangeal joints, whereas the plantars are flexors. The 

 latter adduct toes II, IV and V toward the third digit. 



M. Uexor digiti quinti (—flexor digiti minimi brevis of N.A.): It 

 lies on the fibular and plantar side of the fifth metatarsal and is 

 not as well developed as m. abductor hallucis, but it is still a strong 

 muscle (fig. 48). Origin is by a mixture of fleshy and tendinous fibers 

 from the plantar surface of the base of metatarsale V, cuboideo- 

 metatarsal joint, and the cuboid bone. Its fibers appear to be a con- 

 tinuation of those of the calcaneocuboidal ligament. As it passes over 

 the lateral aspect of the metatarsal it picks up additional fibers from 

 the shaft of this bone. The muscle continues its course to reach the 

 peroneal base of the first phalanx in the fifth toe. It ends here after 

 fusing with the capsule of this metatarsophalangeal joint. Many of 

 its fascicles are inserted on the tendon of m. abductor digiti V which 

 passes through the center of the flexor belly. The muscle in Alouatta 

 fusca (Sirena, 1871) presents no difference. 



Nerve supply: The deep branch of the lateral plantar nerve 

 sends a branch to the muscle before passing medially under the layer 

 of the contrahentes muscles. 



Function: It flexes the fifth toe and, because of its insertion on 

 the peroneal base of its first phalanx, simultaneously contributes to 

 its abduction. 



M. opponens hallucis: This is AveU differentiated and well 

 developed, lying between both parts of m. flexor hallucis brevis and 

 the first metatarsal (fig. 48). Origin is by long tendinous fibers in com- 

 mon with those of the short flexor from the second cuneiform and the 

 peroneal sheath. The fleshy fibers spread fanwise over the metatarsal 

 to insertion along the tibial margin of its shaft. Sirena (1871) does 

 not mention this muscle at all. 



