MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 97 



the rest to the sesamoid. In one male they were equally distributed 

 to the two bones, but never did I see them reaching metacarpale I. 



Sirena (1871) observed the tendon of the muscle inserted by two- 

 thirds of its fibers on the trapezium and the rest on the radial side of 

 the first metacarpal. He also pointed out the great development of 

 the tendon ^v'ith respect to man. Grand (1967) has found in Alouatta 

 caraya the same ending I have described for Alouatta seniculus. 



Nerve supply: The dorsal interosseous nerve sends a branch to 

 the muscle. 



Function: Abductor of the carpus and hence of the whole hand. 



Comparative anatomy: Hill (1962) indicates the presence in 

 Ateles of an abductor pollicis longus and an extensor poUicis brevis. 

 They are said to be well developed and theu' tendons, after crossing 

 those of the carpal extensors, are inserted on the base of the first and 

 rudimentary metacarpal. Senft (1907) regards a single belly in his 

 specimen of Ateles ater {= Ateles paniscus) as representing the two 

 muscles. It has origin from the radius, interosseous membrane, and 

 ulna; insertion on the palmar side of metacarpale I. Straus (1941) 

 studied one Ateles geo^royi {= Ateles geofroy) and found the abductor 

 pollicis longus arising from the ulna and radius. Insertion Avas on 

 the first metacarpal. The same muscle of Brachyteles (Hill, 1962) 

 arises from both antebrachial bones to end on the trapezium and 

 trapezoid. The double origin occurs also in Lagothrix where Robertson 

 (1944) calls it abductor carpi radialis because of its ending on the 

 trapezium and trapezoid bones. The abductor pollicis longus of 

 Cebus comes from the ulna and radius, ending on the trapezium and 

 metacarpale I (Straus, 1941). Senft (1907) states that in the capuchin 

 monkey the fibers arising from the idna correspond to m. extensor 

 pollicis brevis and those from the interosseal membrane to the abductor 

 pollicis longus of man, even if there is only one belly. According to 

 him, the insertion is restricted to the first metacarpal. 



The type of insertion that Grand (1967) and I have found in 

 Alouatta, exclusively on the carpus (trapezium and radial sesamoid), 

 Robertson (1944) in the woolly monkey, and Hill (1962) in Brachyteles 

 (trapezium and trapezoid) corresponds to the type I of Straus (1941). 



M. extensor digitorum profundus: This primitively single muscular 

 layer is here divided into two muscles, the extensor digiti tertii 

 proprius and the extensor pollicis et indicis longus. Both of them 

 have only ulnar origins and are inserted into the corresponding 

 digits. 



M. extensor digiti tertii proprius (fig. 29) is the smaller and more 

 distal of the two deep extensors. It springs from the lateral margin of 

 the ulna along its distal half by fleshy fibers directed laterally and dis- 

 tally toward the axis of the forearm. It is covered by both the extensor 



