MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 137 



defined iliotibial tract. Waterman (1929) indicates that this muscle 

 is present in the howler but reduced in size. Alouatta fusca (Sirena, 

 1871) does not differ from the red howler in this respect. 



Nerve supply: Branches of the superior gluteal nerves reach the 

 muscle after emerging from below the ventrocranial margin of m. 

 gluteus medius and traveling below the scansorius. 



Function: That which is implied by its name. 



M. scansorius: It is a large, fusiform muscle found deep to the 

 tensor fasciae latae between the gluteus medius dorsally and the 

 origin of sartorius ventrally (fig. 41). Its size is about one-third that 

 of the medial gluteus and larger than the minimus. The scansorius 

 arises by strong fleshy fibers from (1) the lower third of the acetabular 

 border together with the tensor fasciae latae and the sartorius, and 

 (2) the antacetabular tubercle (fig. 36). Some of its fibers slightly 

 invade the gluteal plane. The muscle passes distally across the ilio- 

 femoral articulation and is inserted by a strong and short tendon to 

 a facet on the lateral surface of the trochanter behind its ventral 

 border (fig. 40). A similar scansorius is present in Alouatta fusca 

 (Sirena, 1871). Hill (1962) correctly points out that what Waterman 

 (1929) describes as an origin of gluteus minimus from the acetabular 

 border is actually the scansorius, but he goes on to say that the 

 insertion is on the digital fossa together with the piriformis. 



Nerve supply: A branch from the superior gluteal nerve supplies 

 the muscle on its deep surface. 



Function : It is probably the same as that described for the gluteus 

 medius when the animal squats; otherwise the muscle is a medial 

 rotator of the thigh. 



M. gluteus maximus: It is a flat, extensive, and triangular sheet 

 formed by thick muscular bundles which from their origin on the 

 proximal caudal vertebrae converge to the lateral side of the upper 

 third of the femur (fig. 43). Its fleshy fibers arise from the costal 

 processes of Cd 1 to 4 (fig. 36) and from the gluteal aponeurosis 

 which covers the whole region. As in the case of the howlers of Sirena 

 (1871), the fibers from Cd 3 and 4 (distocaudal fibers) have a cranio- 

 lateral direction and those from Cd 1 and 2 (proximocaudal fibers) 

 are oriented caudolaterally. The insertion of the first is on the proxi- 

 mal half of the fascia lata along the lateral margin of the thigh. 

 The proximocaudal bundles are attached directly to the bone by 

 means of a tendon in whose formation all gluteal fibers participate. 

 The muscle thus has a bipennate arrangement. The cranial border 

 of the belly adjoins the tensor fasciae latae, and the caudal is inti- 

 mately connected by fibrous tissue to the long head of m. biceps 

 femoris. The two muscles are easy to differentiate by their nerve 

 supply, and careful dissection allows their separation. A gluteal 



