96 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Function: Extends the fifth metacarpophalangeal joint and helps 

 to extend and abduct the fourth. 



Comparative anatomy of the ulnar division. — Senft (1907) 

 mentions the presence of an anconeus in Cehus, but does not explain 

 whether or not it was found in Ateles. Hill (1962) says it is absent in 

 the spider monkey and Robertson (1944) reports it in Lagothrix. 

 When discussing the musculature of the cebids in general, Hill (1960) 

 claims that the muscle is arranged exactly as in man. I suspect that 

 there might be individual variability. 



The ending of m. extensor carpi ulnaris is on the medial side of the 

 fifth metacarpal base in Ateles (Senft, 1907) and Brachyteles (Hill, 

 1962), on its volar surface in Lagothrix (Robertson, 1944) and Cehus 

 (Senft, 1907; Straus, 1941). In all these animals, as well as the howling 

 monkey, the muscle is probably more an abductor of the carpus than 

 an extensor. 



Only Brachyteles (Hill, 1962) seems to have the two tendons of the 

 extensor digiti (quarti et) quinti proprius ending exclusively on the 

 fifth digit, one inserting on the expansion of the extensor digitorum, 

 the other on the base of metacarpale V. 



DEEP SERIES 



M. abductor pollicis longus (fig. 29) : This is the more laterally 

 located of all the deep and dorsal antebrachial muscles. It has ex- 

 tensive origins from the radius, interosseous membrane, and ulna. 

 Fleshy fibers spring from (1) the dorsomedial upper three-fourths 

 of the radial diaphysis below the insertion of m. supinator, (2) the 

 entire dorsal aspect of the upper three-fourths of the interosseous 

 membrane, and, finally, (3) from a corresponding stretch of the ulnar 

 shaft right behind the attachment of said membrane. The radial origin 

 is marked by a rugosity in line with the interosseous crest. Ulnar 

 fibers ascend up to the lateral side of the olecranon behind the sigmoid 

 notch where a well-marked depression is found between its margin 

 and the lateral border of the olecranon. All fibers of the abductor 

 longus are oriented toward the radial side of the arm. A central, 

 broad, and superficial tendon is formed very high up and receives 

 the fascicles of the muscle. It goes around the distal end of the radius 

 under partial cover of the brachioradialis, but itself lies over the 

 tendons of the two radial carpal extensors. The long abductor enters 

 the more radial compartment of the extensor retinaculum along a 

 deep groove in the distal end of the radius. After crossing the lateral 

 aspect of the carpus, it is inserted on both the trapezium and the 

 radial sesamoid, which is wedged between the scaphoid and the trape- 

 zium (fig. 30). In three out of four specimens (two males, one female), 

 three-quarters of the tendinous fibers are fixed to the trapezium, 



