118 U.S. NATIONAL MUSEUM BULLETIN 273 



M. contrahens digiti quinti: This is smaller than either of the other 

 two members of this layer (fig. 34). Its origin is by fleshy fibers from 

 the palmar aspect of the fourth metacarpal base and the adjacent 

 surface of the hamate. Other fibers join the muscle from the sagittal 

 septum to metacarpale IV, but these do not have a horizontal direction 

 as those of the other two contrahentes. All its fascicles run distomedi- 

 ally and cross the dorsal side of the deep transverse metacarpal ligament 

 joining the heads of metacarpals IV and V. They end on the radial side 

 of the first phalanx base and corresponding articular capsule of V. 

 Sirena (1871) found a similar contrahens digiti quinti in Alouatta 

 fusca. 



Nerve supply: The deep ulnar nerve. 



Function: Adduction of the fifth digit. 



DEEP SERIES, DEEP LAYER 



M. flexor pollicis brevis, caput profundum (figs. 33a, 34): I was 

 able to find in both hands of one of my male specimens what I believe 

 to be a modification of this muscle. It is represented by a small but 

 clearly identifiable muscle arising on the ulnar half of the first meta- 

 carpal base and ligaments of the adjacent joint. Its fleshy bundles 

 are inserted on the ulnar side of the first metacarpal shaft. It lies 

 completely under cover of the ulnar head of m. flexor pollicis brevis 

 and is innervated by a twig from the deep ulnar nerve. 



Mm. interossei: These muscles are arranged into four dorsal and 

 three palmar interossei both m Alouatta seniculus (fig. 33) and 

 Alouatta fusca (Sirena, 1871). Every dorsal interosseous originates 

 from the adjacent sides of the metacarpals between which it is located 

 (fig. 33a). Its fibers end on the expansion of the extensor tendon to 

 the finger on whose metacarpal the interosseous takes the more 

 extensive origin. The ventral interossei arise from only one meta- 

 carpal (fig. 33a). Their insertion is on the extensor expansion of the 

 finger on whose metacarpal they arise. 



The first dorsal interosseous lies between the first and second 

 metacarpal bones and is formed by a radial head mth fleshy origins 

 from the dorsal half of the ulnar side of the shaft and base of meta- 

 carpale I and an ulnar head, of more extensive origin, from the entire 

 radial half of the base and shaft of metacarpale II. The large single 

 belly crosses the radial side of the second metacarpophalangeal joint 

 where some of its deep fibers are anchored. The insertion is on the 

 same side of the corresponding dorsal extensor expansion proximal to 

 the first lumbrical. The second dorsal interosseous originates from 

 the second and third metacarpals. It is also formed by two heads. 

 The radial head arises from the dorsal half of the ulnar side of the 

 second metacarpal shaft and base. The larger ulnar head has its 



