MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 135 



far the most abundant) attach to its cranioventral aspect and medial 

 border. The part of the tendon not covered by muscle is very short. 

 Insertion is on the ventral slope of the greater trochanter, above the 

 endings of the gluteus minimus and scansorius (fig. 40). 



Sirena (1871) says that the origin extends to the transverse processes 

 of the first caudal vertebrae and the last portion of the sacral crest. 

 There is no difference in the insertion. 



Nerve supply: A branch from the superior gluteal nerve reaches 

 the muscle on its deep siu"face and near the origin from the posterior 

 margin of the ilium. 



Function: When the pelvis is fixed, the gluteus medius is a lateral 

 rotator of the thigh by means of its fibers arising on the laterally 

 directed part of the iliac blade (fig. 42). The contraction of these same 

 bundles will produce some rotation of the pelvis if the femur is the 

 fulcrum. Medial gluteal fibers ^^ith origin on the dorsally facing area 

 of the gluteal plane would help to flex the thigh on the pelvis when 

 the leg is extended as in the rare occasions when the animal stands 

 on its hind limbs. On the other hand, if the thigh is flexed and fixed 

 in that position as happens in squatting, contraction of these same 

 fibers would tend to prevent forward tilting of the pelvis and trunk 

 in general. Waterman (1929) has already mentioned the importance 

 of these fascicles in Cebus when the animal squats. She did not ac- 

 count, however, for the pronounced development of the horizontal 

 part of the iliac blade in Alouatta. It seems to me reasonable to think 

 that this osteological character might be related to the function of 

 the gluteus medius of this animal in maintaining the trunk semierect 

 when sitting on its haunches. According to Carpenter (1934), there 

 is considerable variability in the posture of the howlers. He observed 

 that one of their frequent postures was that of resting in a sort of 

 squatting position. In such a stance the trunk tends to bend forward 

 as an arch due to its own weight. This is prevented by the action of 

 the erector spinae, whose contraction transforms the spine and the 

 whole trunk into a rather rigid column on top of which the head and 

 neck are balanced (fig. 42). In consequence of this, the body tends 

 to tilt forward at the hip joint. The enlarged hyolaryngeal apparatus 

 at the cranial end of the rigid column enhances this tendency. It is 

 the contraction of those fibers of the middle gluteus with origin on 

 the horizontal area of the gluteal plane that pulls the pelvis and there- 

 fore the whole trunk backward, thus preventing the forward tilting 

 (fig. 42). 



M. tensor fasciae laiae: This muscle is found right where the 

 boundaries of three regions meet, the gluteal, inguinal, and femoral 

 (fig. 43). It is superficial, weak, and lies in a narrow compartment 

 formed \\ithin both the gluteal aponeurosis and the fascia lata, these 



