MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 93 



narrow, and long tendons which pass into the hand through the third, 

 from radial to ulnar, compartment of the extensor retinaculum. It 

 is here accompanied below by the tendons of the deep extensor mus- 

 cles. This narrow and deep compartment is formed at each side by 

 the nonarticular portions of the adjacent and distal ends of the two 

 antebrachial bones. The tendons of the extensor digitorum, which can 

 be numbered 1 to 5 from the radial to ulnar side, reach the dorsum 

 of the carpus under cover of the local blood vessels and nerves. Here 

 they are united by connexus intertendineus into a triangular lamina 

 at the distal or basal end of which four flattened bands extend toward 

 digits II to V. Those to II and V are smaller. Of the five original 

 tendons, number 1 contributed its fibers to digits II and III, number 

 2 to III, 3 to III and IV, 4 to IV and V, 5 to V. The common tri- 

 angular lamina is tightly contained within a subdivision of the super- 

 ficial layer of the deep manual fascia. The arrangement of every one of 

 the tendinous bands to the digits is similar. Each runs distally over 

 the two proximal phalanges of the corresponding finger and is inserted 

 at the base of the distal phalanx on its dorsal aspect. It is closely 

 bound to the capsule of the first interphalangeal joint, and after 

 crossing the metacarpophalangeal articulation, it sends toward the 

 palmar aspect of the hand a pair of expansions, one at each side. 

 These are connected to the capsular ligaments and receive distally 

 the insertions of the interossei and lumbrical muscles. 



The observations in Alouatta fusca (Sirena, 1871) are like mine 

 except that the tendons are said to end as in man. 



Nerve supply: The dorsal interosseous nerve supplies several 

 branches to the deep aspect of the muscle. 



Function: Extensor of the distal phalanges of digits II to V. 



Comparative anatomy: The extensor digitorum is equal in Alouatta, 

 Ateles (Senft, 1907; Hill, 1962), Brachyteles (Hill, 1962), Cebus (Senft, 

 1907; Straus, 1941) and Lagothrix (Robertson, 1944). The distribution 

 of the five original tendons of the muscle in the last genus resembles 

 very closely the one I described for the howler, except that the fifth 

 tendon in the woolly monkey contributes to both IV and V digits. 

 Information in this respect was not available about the other animals. 

 Robertson (1944) includes the belly of an extensor digiti minimi with 

 that of the extensor digitorum in his description of the latter. 



SUPERFICIAL SERIES, ULNAR DIVISION 



M. anconeus (fig. 28): It was present on both sides of only one 

 of my specimens and could be seen as a triangular muscle with the 

 apex at the dorsum of the lateral epicondyle and the base on the 

 radial border of the olecranon. The upper fibers were not easily 

 differentiated from those of the triceps, but the lower ones formed a 



