MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 115 



diictor, decreasing in size as it approaches its end. Beyond the middle 

 of the first metacarpal a stout and round tendon is formed and this 

 attaches to the ventroradial base of the first phalanx. On the right 

 side of one male and on both sides of another male this insertion is in 

 common with that of the abductor pollicis brevis. A caput ulnare, 

 narrower proximally but as strong as that of the radial side, arises 

 by fleshy fibers from the fibrous sheath of the flexor carpi radialis 

 and the trapezoid. It passes distally along the ulnar side of meta- 

 carpale I to be inserted by short and tendinous fibers on the radial 

 tubercle of the metacarpal head. The deep flexor tendon to digit I 

 travels between the two portions of the muscle. 



Origin of the radial head in Alouatta fusca (Sirena, 1871) is from 

 the flexor retinaculum and the trapezium, its insertion on the radial 

 base of the first phalanx. The ulnar head is said to come from the 

 trapezoid and capitate bones and to be inserted together with the 

 rest of the muscle. 



Nerve supply: The radial head is innervated by a continuation of 

 the branch of the median to the abductor poUicis brevis. The ulnar 

 receives a twig from the deep branch of the ulnar. 



Function: By its attachment to landmarks around the metacar- 

 pophalangeal joint, but only on the radial side, these two heads can 

 probably cause some degree of medial pollical rotation in addition 

 to their role as flexors. 



M. opponens pollicis (figs. 33, 34) : It is a small but conspicuous 

 muscle located deep to the flexor brevis and arising from (1) the palmar 

 surface of the trapezium and (2) the flexor retinaculum. Its fibers 

 follow a distolateral direction, cross the pollical carpometacarpal joint, 

 and are inserted on metacarpale I for the entire proximal three-quar- 

 ters of its radial shaft. Sirena (1871) claims that his howler's opponens 

 pollicis is also just like in man. 



Nerve supply: It receives a twig from the ramus of the ulnar 

 nerve which supplies the ulnar head of the previous muscle. 



Function: Its feeble development corresponds with the lack of 

 opposability in the thumb of Alouatta. Indeed, the muscle probably 

 acts only as a weak medial rotator. 



M. palmaris brevis: This muscle is represented by transverse mus- 

 cular fibers found in the proximal part of the hypothenar eminence, 

 almost at the heel. They are mixed with the fatty lobules enmeshed 

 within the fibrous strands of the subcutaneous tissue of the hand. 

 Their origin is from the medial border of the palmar aponeurosis 

 with insertion on the skin of the medial manual margin. Sirena (1871) 

 describes in addition a small m. palmaris brevis externus on the hy- 

 pothenar side. 



Nerve supply: The superficial ramus of the ulnar nerve. 



