138 U.S. NATIONAL MUSEUM BULLETIN 2 73 



rugosity in the upper fourth of the lateral side of the femoral shaft 

 corresponds to the femoral insertion of gluteus maximus (fig. 40). 

 It is continuous caudally with the lateral line of bifurcation of the 

 linea aspera. 



Waterman (1929) points out that the muscle has no origin from 

 the iliac blade. My description does not differ from that of Sirena 

 (1871). 



Nerve supply: The inferior gluteal nerve reaches the deep sur- 

 face of the muscle closer to the origin than to the insertion. 



Function: It abducts the thigh from the tail and vice versa. 

 It probably has no significant effect on extending the limb, this 

 action being dependent on the strong hamstring muscles. 



M. piriformis: This muscle has no intrapelvic part and is found 

 entirely in the gluteal region deep to the gluteus maximus and just 

 caudal to the medius with which it covers the minimus (fig. 41). 

 Origin is from the tip of the costal process of the first caudal vertebra 

 and the free lateral margin of the sacrum. The fleshy fibers converge 

 toward the posterior slope of the greater trochanter as a round belly. 

 The sciatic trunk with the associated nerve and vessels passes deep 

 to the muscle. Insertion is on the dorsal margin of the femoral tro- 

 chanter (fig. 40) and by some fibers on the tendon of the gluteus 

 medius. Sirena (1871) did not recognize the sacral and caudal origins 

 of the piriformis, attributing its origin to only the margin of the 

 sciatic notch. He also noted the fact that the muscle has no intra- 

 pelvic part. 



Nerve supply: Nerve to the piriformis. 



Function: Lateral rotation of the thigh. 



M. biceps jemoris, caput brevis: This portion of the biceps will 

 be treated together with its long component in a following section. 



Comparative anatomy of the postpelvic coxofemoral group. — 

 The arrangement of the postpelvic muscles in the howler does not 

 appear too different from the patterns found in the four other large 

 cebids, excluding the capuchin. The studies of Bodini (1965) on the 

 lower limb of Alouatta, Ateles, and Lagothrix, of HUl (1962) on Brach- 

 yteles, of Klaatsch (1900) on Ateles and Lagothrix, and of Waterman 

 (1929) in Alouatta sustain this generalization. A tensor fasciae latae 

 is not differentiated, however, in the spider (Bodini, 1965; Hill, 

 1962), woolly (Bodmi, 1965; Hill, 1962) and woolly spider monkey 

 (Hill, 1962), but it is found in Cebus where the muscle is better 

 developed than in Alouatta (Bodini, 1965). The gluteus maximus of 

 Ateles (Bodini, 1965; Hill, 1962), Brachyteles (Hill, 1962), and Lago- 

 thrix (Bodini, 1965; HUl, 1962) appears as an extensive triangular 

 sheet with fleshy fibers arising from the proximal caudal vertebrae 

 and undersurface of the gluteal aponeurosis up to the iliac crest. 



