MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 75 



Sirena (1871) describes this muscle together with the pars cranialis 

 of serratiis anterior and calls them m. levator anguli scapulae. It is 

 clear from his text that the higher digitation of this levator corresponds 

 to the muscle I have just studied in Alouatta seniculus. He apparently 

 did not see any separation that would justify their independent treat- 

 ment. Ashton and Oxnard (1963) have also included one such muscle 

 with the serratus anterior, pars craniaUs, in their single howler speci- 

 men. Yet it is clear from their text that an atlantoscapularis pos- 

 terior, as I have interpreted it, is also present in this case. Campbell 

 (1937) does not separate this muscle from the serratus anterior in 

 Alouatta palliata. I do emphasize the distinctness of the two muscles 

 on the basis of the peculiar scapular attachment of the first in my 

 red howlers, where its fascicles are fixed to both lips of the cranial 

 half of the supraspinous portion of the vertebral border (fig. 25) . This 

 arrangement is very evident on plate 3, figure 12, of Ashton and 

 Oxnard's (1963) study. Otherwise, the atlantoscapularis posterior can 

 hardly be separated from the serratus anterior, pars cranialis. This 

 emphasis of mine will be further explained when I deal with the 

 functions of the muscle. 



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



Function: It elevates the scapula. In addition, I think that when 

 the scapula is fixed the muscle is an accessory extensor of the neck. 

 By such an action it produces, together with the rhomboideus, an 

 extra traction upon the margin of the bone, and to this influence the 

 scapula has responded with an enlargement of the corresponding 

 portion of its vertebral border. The extraordinarily large supraspinous 

 fossa in Alouatta is a character already indicated by several authors 

 (Erikson, 1963; Hill, 1962; Schultz, 1930). I do not know if this char- 

 acter occurs also in the very young howlers where the hyolaryngeal 

 apparatus has not yet attained its maximum size. 



Comparative anatomy: The atlantoscapularis posterior is an in- 

 dependent muscle in one Ateles and two Lagothrix (Campbell, 1937; 

 Robertson, 1944) in all of which it arises only from C 1. Schuck 

 (1913b) describes its origin from C 1, 3, and 4 in another spider 

 monkey. It arises from C 1-4 in Brachyteles (Hill, 1962), where the 

 muscle is described as different from the serratus anterior. In all 

 other spider and woolly monkeys reported (Campbell, 1937), it forms 

 a single muscidar plate with the serratus. 



In general, the muscle tends to be fused with the serratus anterior, 

 pars cranialis, in all the five genera imder consideration ; but it appears 

 as if in Alouatta it might be responsible for the large supraspinous 

 fossa due to its possible function of extending the neck in this animal. 



M. trapezius: This is a triangular muscle extending from the 

 occiput and midhne of the cervical and upper thoracic regions to the 



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