MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 49 



lies betw een the artery and the root of C VIII in front and the deep 

 cervical artery behind. The last vessel separates this scalenus from the 

 levator costae primus. Sirena (1871) indicates the presence of a similar 

 muscle A\'hich he names scalenus medius. 



Nerve supply: A branch from the anterior ramus of C VIII. 



M. scalenus brevis posterior: This muscle is constant but of variable 

 development in different specimens and even in the same indi^ddual 

 from one side to the other. It lies dorsolateral to the line of the cervical 

 transverse processes, between the scalenus medius in front and both 

 m. serratus anterior and the atlantoscapularis posterior behind (fig. 

 17). This scalenus forms a fusiform belly which, arising from the 

 transverse crest of C 1 and the anterior tubercle of C 2 and 3, descends 

 and is firmly anchored by fleshy fascicles to the posterior tubercles 

 of C 4 to 7. In the right side of one male there was an additional 

 attachment to the head of rib 1. 



Sirena (1871) does not describe this muscle but the scalenus 

 brevis posterior can be easily missed in cases where its development 

 is reduced. I found one such instance in one male. 



Nerve supply : Branches from the anterior rami of C II to V. 



Function of the scalene muscles. — -The scalenes are known to 

 be elevators of the upper ribs and abductors of the neck. Due to the 

 ventrally convex bending of the cer\acal spine contraction of the 

 scalenus brevis posterior helps to maintain that curvature and there- 

 fore acts as an extensor of the neck, a function which may explain 

 the development attained by these muscles in two of the males. 



Comparative anatomy of the scalene muscles. — The complex 

 relations of these muscles and the various terminologies used by the 

 authors to refer to them make their comparative study difficult. It is 

 perhaps for such reasons that not too many students have paid 

 attention to the scalenes. Tschachmachtschjan (1912) studied the 

 scalenus medius in two specimens of Cebus "flavus" and one of Cehus 

 apella, all males, and one female Ateles ater {=Ateles paniscus). His 

 description of the muscle is rather sketchy, but enough to allow its 

 identification with what I have called scalenus medius in Alouatta. 

 The origin in Cebus is said to be from the transverse process of C 2 

 and the insertion on ribs 3, 4, 5, or on 3 and 4 only. The same muscle 

 in the spider monkey extends from the transverse process of C 2, 3, 

 and 4 to ribs 4 and 5. According to Tschachmachtschjan, the muscle 

 is supplied by the long thoracic nerve in both genera. Forster (1916a) 

 describes the scalenus medius in one Ateles geojfroy as lying between 

 the tuberculum anterior of C 2 to 5, reinforced by bundles from C 1, 

 and ribs 1 to 4. The attachment to rib 2 is said by him to be absent 

 on the right side, that to rib 3 on the left. The scalenus anterior of 

 the Forster specimen is just like that in the howler. Furthermore, a 



