158 U.S. NATIONAL MUSEUM BULLETIN 2 73 



(2) the entire dorsal aspect of the interosseous membrane in the upper 

 third of the leg, the area of its membranous origin diminishing distally 

 to a narrow strip along the tibia, and (3) the septum between the 

 two tibial flexors. The belly descends to the proximities of the ankle 

 and its fibers are attached to the anterior aspect of a long tendon 

 which forms Avithin the muscle very high up in the leg. As it approaches 

 the distal end of the tibia, the tendon changes from deep to medial 

 in relation to the flexor tibialis (fig. 47). It passes around the lower 

 end of the bone in a groove on the dorsal border of the tibial malle- 

 olus. After running through the tibial retinaculum, this tendon enters 

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

 ending on the plantar surface of the navicular (fig. 48), where it 

 expands in taking firm insertion. Some of its fibers pass distolaterally 

 underneath the calcaneonavicular Ugament before ending in the 

 middle of the tarsus upon the third cuneiform. Sirena (1871) adds 

 a peroneal origin. 



Nerve supply: N. tibiaUs. 



Function: Plantar flexion and some inversion of the foot. 



M. guadratus plantae: A rectangular flattened muscle arising deep 

 to the origin of the flexor digitorum brevis, caput superficiale, from 

 the medial aspect of the calcaneal tubercle. Its fleshy belly runs 

 distally along the fibular border of the flexor tibiahs and over the 

 plantar aspect of the flexor fibularis. Insertion is in the distal part 

 of the tarsus on the fibular border of the flexor tibiahs. The lateral 

 plantar nerve crosses the plantar surface of the muscle after passing 

 between the two parts of the flexor digitorum brevis. Sirena (1871) 

 found the muscle in all his specimens and notes that it is better 

 developed than in man. Strode (1937) also reports it in his howler. 



Nerve supply: A small branch of the lateral plantar nerve reaches 

 the muscle near its origin and also sends a twig to the ankle joint. 



Function: By pulling on the still undivided tendon of the flexor 

 tibiahs it enhances the action of the long flexor upon the toes. 



Comparative anatomy of the cruropedal group. — The dif- 

 ferences between Alouatta and Ateles (Glaesmer, 1910; Hill, 1962), 

 Brachyteles (Hill, 1962), and Lagothtix (HiU, 1962) in the cruro- 

 l)edal group of the ventral musculature appear to be of no major 

 significance. Frey (1913) studied the triceps surae of the Primates 

 in great detail and found that in Cehus the gastrocnemius tends 

 to arise more from the knee ca])sule than from the back of the femur. 

 Contrariwise, in Ateles more fibers come directly from tlie l)()ne, 

 as was the case Avith my howlers. Frey (1913) describes sesamoids 

 at the origin of each head of the gastrocnemius in both the capuchin 

 and the spider monkey, Glaesmer (1910) does not say anything 

 about their ])resence or absence in either genus, and Hill (1962) 



