112 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Function: Flexes the distal interphalangeal joints of all the fingers. 



M. pronator quadratus: This is a trapezoid-like muscle found 

 between the tendons of the flexor profundus and the distal fourth 

 of the ulna and radius. The base of the trapezoid corresponds to the 

 origin by intermingled fleshy and tendinous fibers from the anterior 

 surface of the distal fourth of the ulna (fig. 27). A sharp longitudinal 

 ridge here indicates the attachment of the pronator quadratus. 

 Proximal bundles slope down and laterally toward their ending, 

 whereas the more distal elements pass radially in a rather horizontal 

 course. The muscle thus covers the interosseous membrane before 

 having a fleshy insertion on the distal end of the radius (fig. 27). 

 The finding of Sirena (1871) in Alouatta fusca calls for no special 

 comment. 



Nerve supply: The ventral interosseous nerve reaches the muscle 

 on its deep surface to innervate it. 



Function: Pronation. 



Comparative anatomy of the forearm group. — M. pronator 

 teres of Cehus (Laugier, 1933) appears to have a shorter radial ending 

 than in any of the other four genera under consideration. Senft (1907), 

 nevertheless, describes it as covering the intermediate and distal 

 thirds of the forearm. In other details the muscle is the same in all 

 five genera. The two carpal flexors do not offer significant differences 

 (see Hill, 1962; Robertson, 1944; Senft, 1907). I found the flexor 

 superficialis of an adult female Ateles sp. divided into superficial 

 and deep parts. The first is a large belly quite like that of Alouatta in 

 origin, topographical relations and endings. The second, on the other 

 hand, is not fusiform or two-bellied, but a rather broad, flat mass 

 smaller than the superficial and originating deep to it on the ulnar 

 epicondyle and the strong aponeurotic septum together with m. 

 flexor carpi ulnaris (Schon, 1965). The same division is reported 

 by Hill (1962) for the spider monkey, where the pars profunda is 

 said to have radial and ulnar halves. Senft (1907) did not describe 

 this arrangement into superficial and deep portions in this study on 

 Ateles. The findings of Robertson (1944) in the woolly monkey are 

 comparable to ours in the howler. In his case the deep segment of 

 the superficial flexor has both an ulnar and a medial epicondylar 

 origin. He did not find this head divided by an intermediate tendon. 

 The flexor superficialis of Brachyteles (Hill, 1962) is not so split. 



M. flexor profundus of the female Ateles that I examined had 

 robust ulnar, radial, and central parts, the first two with origins com- 

 parable to those in the howler, the last arising mostly from the flexor 

 epicondyle but with some of its fibers coming from a poorly defined 

 raphe extended between the radius and that epicondyle. Its tendon 

 joins that of pars radialis. Hill (1962) gives a comparable account 



