MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 77 



neck. That voice-producing organs have affected other details of the 

 occipital region, in particular those of the cranium, has been shown by 

 the studies of Biegert (1963) and Leche (1912). 



M. serratus anterior (fig. 17): This is a broad muscular sheet 

 connecting the vertebral border of the scapula to the ribs and the 

 lower six cervical vertebrae. It is divided into a pars cranialis and 

 a pars caudalis separated by a narrow and long hiatus through which 

 the scalenus medius reaches its insertion. Pars cranialis arises by (1) 

 fleshy digitations from the posterior tubercles of C 2 to C 7 between 

 the origin of the scalenus posterior in front and the insertion of the 

 longissimus behind, and (2) as two muscular bands from the outer 

 surface of the bodies of ribs 1 and 2. The vertebral and costal portions 

 are separated by a short hiatus through which the deep cervical 

 artery reaches its position between the semispinales muscles. A more 

 or less continuous belly is thus formed whose fibers pass towards the 

 scapula. Costal fascicles are less oblique than the others. Insertion 

 takes place on the ventral lip of the vertebral border of the blade but 

 only over (1) the caudal half of its supraspinal part and (2) the 

 cranial fourth of its infraspinal part (fig. 24) . Pars caudalis is formed 

 by six fleshy flat bundles arising from the outer surface of the body 

 of the third to eighth rib on the lateral aspect of the thorax where 

 they alternate with those of m. obliquus externus abdominis and the 

 latissimus dorsi. These digitations join, not too far after their origin, 

 into a single sheet. Fibers from the lower ribs are more oblique in 

 direction. Insertion is on the remaining of the vertebral margin down 

 to the caudal angle. The pars caudalis of the muscle is a bit more 

 robust and longer than the cranialis. 



As explained before, Sirena (1871) treats the pars cranialis as 

 forming with the atlantoscapularis posterior a single muscle, the 

 levator anguli scapulae. The serratus has origin, according to him, 

 only on the upper nine ribs and insertion along the entire vertebral 

 border of the blade. He points out that the muscle is thin near the 

 upper angle, probably with the idea of contrasting this condition 

 with what appears to have been a thicker caudal portion. Campbell 

 (1937) explains that the muscle in Alouatta palliata is divided by the 

 insertion of the long scalene (m. scalenus medius). The proximal 

 portion of this species arises from all cervical vertebrae and the first two 

 ribs. The distal extends caudally, but he does not say how far down 

 into the thorax. 



Nerve supply: The pars craniaUs receives branches from the 

 ventral rami of C II, IV, and V; pars caudaUs is innervated by the 

 long thoracic nerve and this is usually formed by contributions from 

 C VI to VIII (fig. 26). 



Function: Both parts of the muscle should act as stabilizers of 



