MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 105 



Nerve supply: The musculocutaneous nerve contributes several 

 branches to the muscle. 



Function : Flexor of the elbow. 



Comparative anatomy of the upper arm group. — M. biceps does 

 not have any significant variation among the five genera, with the 

 exception of Ateles dariensis (=Ateles fuscipes rohustus) where 

 Campbell (1937) found a reduced long head with a supplementary 

 humeral origin. The fusion of the two heads takes place far distally in 

 the arm of Ateles (Howell and Straus, 1931) and Brachyteles (Hill, 

 1962). An aponeurosis biceps brachii is present in the spider monkey 

 (Hill, 1962; Howell and Straus, 1931), Brachyteles (Hill, 1962) and 

 Lagothrix (Robertson, 1944). It is absent in one Cebus dissected by 

 Howell and Straus (1931). Senft (1907) apparently did not find it 

 either in Cebus or Ateles. M. brachialis shows the same basic arrange- 

 ment in all these five genera. 



Forearm Group 



SUPERFICIAL SERIES, RADIAL DIVISION 



The five muscles belonging in this subdivision arise from the ventral 

 surface of the ulnar epicondyle. Then- fibers form a powerful, common 

 fleshy mass \\dth internal tendinous partitions separating and providing 

 additional origins to the bundles of the different muscles. By careful 

 dissection the precise topographical relations of their respective origins 

 can be determined. 



M. pronator teres (fig. 32): This robust and elongated muscle 

 forms the medial boundary of the antecubital fossa. Origin is from 

 (1) the ventral and upper part of the ulnar epicondyle, (2) the lower 

 portion of the medial supracondylar ridge (fig. 27), (3) its own apo- 

 neurotic sheath, and (4) a tendinous septum separating the deep 

 surface of the muscle from the superficial head of the flexores digi- 

 torum superficialis and carpi ulnaris. There is no ulnar head. The 

 belly is oriented lateromedially, reaching the upper three-fourths of 

 the radial shaft below the bicipital tuberosity. Insertion takes place 

 on the ventral aspect of the bone between the origin of the radial 

 head of the deep flexor medially and the antebrachial intermuscular 

 septum laterally (fig. 27). This ending is formed by a mixture of 

 tendinous and fleshy fascicles. The first forms laminae, both on the 

 superficial and deep aspects of the muscle; the deep one in particular 

 has continuity with the septum between the pronator and sublimis 

 already mentioned. The rest of the insertion is all muscular. Some of 

 the distal fibers follow the lateral intermuscular septum for some 

 distance. 



In Alouatta fusca, the muscle is said to be as in man (Sirena, 1871) ; 

 but I wish to emphasize that it appears to be longer in the howler. 



