MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 141 



ankle where it changes into a round strong tendon. This enters the 

 superior peroneal retinaculum superficial to that of the peroneus 

 brevis, then passes over the lateral surface of the calcaneus, goes 

 through the inferior peroneal retinaculum and leaves the brevis by 

 approaching the plantar surface of the foot. The tendon here runs 

 transversely across the cuboid and the plantar surface of the distal 

 tarsal row where it is contained within an osteofibrous canal formed 

 by the bones, the plantar ligaments of the foot and, particularly, on 

 the tibial half, by the laterally diverging fibers of the tibialis posterior. 

 Insertion takes place upon the lateroplantar side of the base of the 

 first metatarsal. I found no difference between this muscle in the 

 northern red howler and that of Alouatta jusca (Sirena, 1871) or 

 Mycetes niger {= Alouatta palliata) studied by Frets (1911). The 

 latter author's description is not complete. 



Nerve supply: The common peroneal nerve sends a long branch 

 to the lateral crural compartment which supplies all three muscles 

 here included. 



Function: Plantar flexion and eversion of the foot, opposition 

 and flexion of the hallux. 



M. peroneus brevis: It is the more anterior of the muscles in the 

 lateral crural compartment. Origin is by fleshy fibers from the inter- 

 mediate three-fifths of the lateral fibular surface (fig. 38), the upper 

 fifth of the anterior intermuscular septum, and the fascia between this 

 muscle and the longus. Near the superior peroneal retinaculum the 

 elongated muscular belly gives rise to a round tendon which is now 

 anterior to that of m. peroneus longus and grooves the rear aspect of 

 the fibular malleolus. It curves around the distal end of this bone 

 above and medial to the tendon of longus (fig. 44), runs along the 

 lateral calcaneal surface between that tendon superficially and the 

 one of the peroneus digiti V medially and enters its own compartment 

 in the inferior peroneal retinaculum. The brevis tendon becomes 

 superficial as the longus aims toward the plantar surface. The former 

 is split by the tendon of m. peroneus digiti quinti into a fine deep and 

 a larger superficial subtendon, both of which are inserted on the per- 

 oneal process of metatarsale V. The detaUs of this muscle, as ex- 

 plained by Sirena (1871), accord with those of my study. The account 

 of Frets (1911) is again incomplete. 



Nerve supply: A branch of the common peroneal nerve. 



Function: Plantar flexion and eversion of the foot. 



M. peroneus digiti quinti: The third muscle of the lateral com- 

 partment of the leg lies between the other two peronei. Origin is 

 from the distal fifth of the fibular shaft where the lateral surface 

 of the bone twists backward to form the so-called peroneal canal 

 (fig. 38) (see Testut and Latarjet, 1959) and from the septa which 



