MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 95 



clear border delimiting anconeus from the extensor carpi ulnaris. It 

 is said to be like that of man in the southern howler (Sirena, 1871). 



Nerve supply: The radial nerve. 



Function: Sirena (1871) attributes to it a function similar to that 

 of the epitrochleo-anconeus. 



M. extensor carpi ulnaris: This is the most medial of the dorsal and 

 superficial antebrachial muscles. It arises by tendinous and fleshy 

 fibers from the lower border and back of the lateral epicondyle. It 

 forms an elongated and flat muscular belly, passing distally super- 

 ficial to the supinator and the shaft of the ulna. A tendon develops 

 on its medial border upon which the fibers of the muscle are inserted 

 as far down as the lower third of the forearm. As it approaches the 

 carpus, the tendon of the extensor carpi ulnaris passes through the 

 ulnarmost of the extensor retinacular compartments and glides on a 

 groove of the distal end of the ulna. In the dorsum of the hand it is 

 covered by the other extejisor tendons and finally is inserted on the 

 ulnopalmar half of the styloid process in metacarpale V (fig. 30). 

 The ending of the muscle is also on the ulnar side of metacarpale V 

 oi Alouatta Jusca (Sirena, 1871). 



Nerve supply: The dorsal interosseous nerve innervates the muscle 

 by a branch to the deep aspect of its proximal fourth. 



Function: Abducts and dorsiflexes the metacarpus and carpus on 

 the forearm. 



M. extensor digiti quinti et quarti proprius {= extensor digiti minimi 

 of N.A.): This is also a superficial muscle of the dorsum of the fore- 

 arm found between the extensor digitorum laterally and the extensor 

 carpi ulnaris medially. Origin is by short tendinous fibers from the 

 lower end of the lateral epicondyle (fig. 27) and from the aponeurotic 

 sheath that surrounds the muscle. A belly is rapidly formed which runs 

 distally between the extensor digitorum and extensor carpi ulnaris. 

 Its breadth is reduced progressively until a tendon is formed in the 

 middle of the antebrachium and which soon spHts into a radial and 

 an ulnar subdivision just before entering the fourth compartment of 

 the extensor retinaculum (fig. 30). That of the radial side is the thinner; 

 it reaches the fourth intermetacarpal space under cover of the carpal 

 lamina of the extensor digitorum and is inserted on the deep aspect 

 of the dorsal extensor expansion at the ulnar side of the fourth meta- 

 carpophalangeal joint. The ulnar tendon is stronger and flat. At the 

 level of the fifth metacarpophalangeal articulation it broadens and is 

 inserted on the deep aspect of each of the dorsal expansions and on 

 the dorsal base of the proximal phalanx. According to Sirena (1871) 

 it corresponds to the same muscle in man. 



Nerve supply: The posterior interosseous nerve. 



