120 U.S. NATIONiAL MUSEUM BULLETIN 2 73 



phalangeal joint of the corresponding digit. This is brought about 

 by the insertion on the dorsal extensor expansion proximal to the 

 ending of the lumbrical and after crossing the dorsal side of the deep 

 transverse metacarpal ligament. In addition, the dorsal interossei are 

 abductors and ventral adductors. In relation to the zygodactylous 

 grasp, the second palmar interosseous is the best developed of the three. 



M. opponens digiti quinti {^opponens digiti minimi of N.A.): It is 

 the deepest of all the hypothenar muscles and occupies the radial 

 side of the ventral aspect of the fifth metacarpal where it is inserted 

 on the whole ventral shaft up to the head (fig. 34). Its fibers originate 

 on the palmar aspect of the fifth metacarpal base and nearby hamate. 



Nerve supply: Deep branch of the ulnar nerve. 



Function: Medially rotates metacarpale V. 



Comparative anatomy of the hand muscles. — The intrinsic 

 thumb muscles in the spider monkey are sometimes well differenti- 

 ated with an arrangement comparable to that of the howler with 

 which they coincide even in the presence of a flexor poUicis brevis 

 profundus. More often they are rudimentary in this genus (see Straus, 

 1942, and Straus, unpublished data). Senft (1907) found no abductor 

 brevis or opponens to the thumb and the corresponding flexor rep- 

 resented only by a broad musculotendinous strand between the flexor 

 retinaculum and the head of metacarpale I and the rudimentary 

 phalanx. The thenar muscles in Brachyteles form only an undiffer- 

 entiated mass (Hill, 1962), but in Lagothrix (Robertson, 1944) they 

 are like those of Alouatta. The abductor pollicis brevis of the capuchin 

 monkey studied by Senft (1907) was fused with the flexor brevis to 

 the extent that he regarded the first as merely a part of the second. 

 This went from the flexor retinaculum to its usual phalangeal ending, 

 but Senft (1907) does not say whether or not it had two heads. He 

 did not find an opponens pollicis. Other observations in Cebus (Straus, 

 unpublished) report a pattern of thenar muscles like that of the 

 howler. 



A relatively well-developed palmaris brevis occurs in Lagothrix 

 (Robertson, 1944) and Cebus (Straus, unpublished), but was not 

 found in Ateles (Straus, unpublished). The observation of Sirena 

 (1871) of one such muscle on the thenar side of Alouatta remains unique. 



The three hypothenar muscles of Ateles (Senft, 1907; Straus, un- 

 published) match quite accurately the features of those in Alouatta, 

 but Senft (1907) apparently did not find the opponens. The abductor 

 of the fifth digit in Brachyteles is said to be bicipital and one of these 

 heads is supposedly homologous to the short flexor of the fifth digit, 

 as described by Robertson in 1944 (see Hill, 1962). Its opponens to 

 V is feeble (Hill, 1962). The three muscles in Lagothrix (Robertson, 

 1944) are just like those in the howler. In Cebus (Senft, 1907) the 



