MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 73 



(1962), apparently following Sirena (1871), also describes the aponeu- 

 rotic expansion to the mandible, but he regards this variation as part 

 of the muscular origin. Ashton and Oxnard (1963) studied one howler, 

 Alovatta sp., in their monograph on the shoulder muscles of the 

 primates. The atlantoscapularis anterior in this individual coincides 

 with that of the three species already alluded to but, in addition, more 

 closely resembles the Central America variety in having two heads of 

 insertion, one attached to the acromion, the other to the clavicle. 



Nerve supply: A branch from the anterior division of either C III 

 or IV. 



Function: It elevates the shoulder. 



Comparative anatomy: The acromioclavicular insertion of Alov- 

 atta is always superficial to that of m. trapezius (fig. 24). This arrange- 

 ment is also found in Lagothrix (Ashton and Oxnard, 1963; Campbell, 

 1937; Miller, 1932; Robertson, 1944) and Ateles (Campbell, 1937; 

 Miller, 1932; Schiick, 1913b). Ashton and Oxnard (1963) report one 

 specimen of the spider monkey where the ending of m. atlantoscapu- 

 laris anterior lies deep to that of the trapezius. This insertion is always 

 deep to m. trapezius in Cehus (Campbell, 1937; Miller, 1932, Schiick 

 1913b). It is also in the howler, Ateles, and Lagothrix that the shoulder 

 attachment of the muscle constantly includes the acromion and the 

 clavicle (Ashton and Oxnard, 1963; Campbell, 1937; Miller, 1932; 

 Schiick, 1913b). This is not the case for the ring- tailed monkey where 

 its insertion is restricted to the acromion and/or the spine (Campbell, 

 1937; Miller, 1932; Schiick, 1913b). Hill (1962) does not describe the 

 insertion of the trachelo-acromialis (m. atlantoscapularis anterior) in 

 the shoulder of Brachyteles. The distal division of the atlantoscapularis 

 anterior into two heads, one for the clavicle and the other for the 

 acromion, has been found in only two howlers, one Alouatta palliatxi 

 (Campbell, 1937) and one Alouatta sp. (Ashton and Oxnard, 1963). 

 It also occurs in two Ateles (Campbell, 1937; Schiick, 1913b), but 

 apparently never in Cehus or Lagothrix. 



The muscle of the howler conforms in its origin, insertion, and 

 general arrangement more with the three brachiators than with the 

 versatile capuchin monkey. 



M. atlantoscapularis posterior (figs. 17, 19, 25): This muscle was 

 constantly present as a fleshy band extended from the transverse 

 process of the atlas to the cranial angle and upper part of the spinal 

 border of the scapula. Origin is from the tip of the first cervical 

 transverse process. Its belly descends caudolaterally in contact with 

 the first digitation of serratus anterior, pars cranialis. As it approaches 

 the scapula it expands and is inserted on the cranial half of the supra- 

 spinous part of the scapular vertebral border up to the cranial angle 

 (fig. 24). This ending invades both surfaces of the bone near the 



