78 



U.S. NATIONAL MUSEUM BULLETIN 2 73 



the scapula and the caudalis is probably important also in respiration. 

 Its moderate robustness suggests that the role of the serratus anterior 

 in laterally rotating the lower scapular angle, as in raising the shoul- 

 der, is still of some significance in the howler, but the wideness of 

 that angle and its short distance from the center of rotation of the 

 bone indicate that this mechanism might not be a very efficient one 

 in the animal. 



Upper subscapular nn. 



Lower subscapular n. 

 N. axillaris 

 m.coracobrachialis, pars profunda 



om.coracobrachialis, p. media 



N. musculocutaneus 



N.medianus 

 N. radialis 



N. ulnaris 



to m.pectoralis minor 

 N.cutaneusantebrachii medialis 

 N.cutaneus brachii medialis 



from Til 



to m'pectoralis major 

 N. phrenicus 



Figure 26. — Plexus brachialis as observed in the left side of one male. 



Comparative anatomy: The division of the muscle by the scalenus 

 medius is a constant feature in Alouatta (Ashton and Oxnard, 1963; 

 Campbell, 1937; this report), Ateles (Ashton and Oxnard, 1963; 

 Campbell, 1937; Hill, 1962), Lagothrix (Campbell, 1937; Hill, 1962) 

 and Cebus (Campbell, 1937; Schiick, 1913b). Hill (1962) reports it 

 also for the woolly spider monkey. Cebus (Schiick, 1913b), Lagothrix 

 (Campbell, 1937) and the howler appear to have a rather uniform 

 pars craniahs arising from the lower six cervical vertebrae and 

 cranial ribs. On the other hand, the origin from C 2 is absent in 



