MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 81 



attachment extends over the lateral three-fourths of the superior 

 nuchal hne. Hill (1962) specifies T 4 as the last vertebrae of origin 

 in the spine. Campbell (1937) also notes the extensive occipital attach- 

 ment in relation to the nuchal crest. Furthermore, he remarks that 

 the rhomboideus capitis is present but fused to the rest of the muscle. 

 Ashton and Oxnard (1963, pi. 3, fig. 12) attribute only a short origin 

 to the muscle from the superior occipital hne in semibrachiators 

 including Alouatta, but their specimen of this genus suggests that the 

 muscle possesses, in fact, a more extensive origin. 



Nerve supply: The rhomboideus receives its innervation from 

 the ventral rami of C III and IV. I never found it supplied by branches 

 of the brachial plexus. 



Function: Besides its action as a stabilizer of the shoulder, the 

 rhomboideus appears to have importance in maintaining the head 

 and neck extended. This function would be accomplished by the 

 occipital and higher cervical fibers. 



Comparative anatomy: In both Cebus (Campbell, 1937; Schiick, 

 1913b) and the howler (Campbell, 1937; Sirena, 1871; this report) 

 the rhomboideus is undivided and forms a continuous muscular 

 lamina between the scapula, the occiput, and the spine. According 

 to Schiick (1913b), its caudal extension reaches T 6 in the capuchin. 

 Campbell (1937) appears to have found variability in this genus 

 because he says it is T 5 in one place and a few lines further states 

 it to be T 7. Schiick (1913b) explains, in addition, that the origin in 

 Cebvs from the linea occipitalis is very thin. This obviously contrasts 

 with the strong appearance in Alouatta. M. rhomboideus is divided 

 in Ateles (Schiick, 1913b; Campbell, 1937). One portion is represented 

 by a slender band springing from the lateral part of the nuchal crest; 

 the rest and larger portion comes from the spines between C 6 and T 7 

 (Schiick, 1913b). Hill (1962) describes in Brachyteles a rhomboideus 

 minor with origin from the midcervical region of the ligamentum 

 nuchae and the rhomboideus major reaching the spine of T 6. The 

 subdivision of the muscle is also found in Lagothrix (Campbell, 1937), 

 and Robertson (1944) describes a rhomboideus dorsi reaching T 4 

 or 5, a cervicis, and a capitis in this genus. The last is formed by two 

 parts which merge after their independent origin. 



INTRINSIC SERIES 



M. teres major (fig. 25): Its robust and round belly is fixed by 

 fleshy fibers to the caudal angle and the lower half of the axillary 

 scapular margin (fig. 24). Its own sheath also affords origin to several 

 fascicles. The voluminous belly passes toward the floor of the axilla 

 with the latissimus tendon spiraling around it. On approaching the 

 humerus, teres major changes into a frontally compressed, broad. 



