MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 119 



fibers originating from the whole radial half of the shaft and proximal 

 end of the third metacarpal. The common belly flattens against the 

 radial side of the third metacarpophalangeal joint where some fibers 

 are inserted. It ends on the radial aspect of the dorsal extensor ex- 

 pansion of digit III proximal to the tendon of the second lumbrical. 

 The third dorsal interosseous is also a double-headed muscle. It lies 

 between the third and fourth metacarpals and is the mirror image of 

 the second dorsal interosseous. Its larger radial head arises from the 

 entire ulnar half shaft and base of metacarpale III. The ulnar head 

 originates only from the dorsal half of the radial side of the shaft and 

 l)roximal end of metacarpale IV. The muscle passes distally against the 

 ulnar side of the third metacarpophalangeal joint on whose capsule 

 some of its fibers attach. Insertion is on the ulnar side of the dorsal 

 extensor expansion to the third digit. The fourth dorsal interosseous 

 has an arrangement similar to that of the previous muscle. Its radial 

 head is also larger and arises from the entire ulnar half of the shaft 

 and base of the fourth metacarpal. The less developed ulnar head 

 takes origin on the dorsal half of the radial side of the fifth metacarpal 

 shaft and base. The muscle runs distally against the ulnar side of the 

 capsule of the fourth metacarpophalangeal joint to which it gives 

 some fibers. The insertion is on the dorsal extensor expansion to the 

 fourth digit. 



The first pahnar interosseous occupies only the ulnar half of the 

 second intermetacarpal space. It arises from the ventral half of the 

 ulnar side of the shaft and base of metacarpale II. The fibers run 

 distally and the muscle flattens against the ulnar aspect of the capsule 

 of the second metacarpophalangeal joint. Some fibers end here, but 

 most of them are inserted on the ulnar side of the dorsal extensor 

 expansion to the second digit. Tliis muscle is of the same approximate 

 size as the third palmar interosseous. The second is the best developed 

 of the thiee and occupies only the radial half of the third intermeta- 

 carpal space. Origin is from the palmar half of the radial side of shaft 

 and base of metacarpale IV. It lies against the radial side of the 

 fourth metacarpophalangeal joint on whose capsule some of its fibers 

 end. The muscle blends with the dorsal extensor expansion of the 

 radial side of digit IV proximal to the third lumbrical. The third 

 palmar interosseous is on the radial half of the fourth intermetacarpal 

 space and originates from the palmar half of the radial side of the 

 shaft and base of metacarpale V. The muscle crosses the radial 

 aspect of the last metacarpophalangeal joint. Some of its fibers end 

 abductors and the here, but the majority are inserted on the dorsal 

 extensor expansion to the fifth digit above the fifth lumbrical ending. 



Nerve supply: The deep ulnar nerve. 



Function: Every interosseous is an extensor of the metacarpo- 



