92 U.S. NATIONAL MUSEUM BULLETIN 2 73 



M. supinator: This is a deeply set muscle found around the back, 

 lateral side and front of the upper end of the radius over which it lies 

 as a half cyHnder. Supinator fascicles arise from the dorsal, lateral, and 

 ventral aspects of the fibrous capsule of the elbow joint where they 

 take a firm grip, particularly in the lateral ligament. The most medial 

 of its dorsal fibers originate from a rugosity found behind the lesser 

 sigmoid cavity in the ulna. Some of the lateral fascicles have a tendi- 

 nous origin from the dorsal and lower end of the lateral epicondyle. All 

 its bundles follow a spiral direction around the radius and they end on 

 the lateral surface of this bone lateral to the bicipital tuberosity and 

 between the insertions of the capsular ligaments of the joint above and 

 the pronator teres below (fig. 27) . The superficial and lateral fibers of 

 the supinator are tendinous and form a thin cover to the dorsal in- 

 terosseous nerve which penetrates the muscle on its way to the dorsal 

 antebrachial compartment. 



Once more Sirena (1871) compares the howler with man and finds 

 no difference between the muscles of the two animals. 



Nerve supply: A branch of the dorsal interosseous nerve. 



Function: Supination. 



Comparative anatomy of the radial division. — The brachio- 

 radialis muscles of the spider (Hill, 1962), woolly (Hill, 1962; Robert- 

 son, 1944) and woolly spider monkey (Hill, 1962) are entirely similar 

 to that of the howler. The fibrous connection with the sheath of m. 

 brachialis is described by Robertson (1944) as an additional origin in 

 Lagothrix. Cehus (Hill, 1960; Straus, 1941) offers no difference from 

 this pattern. The two carpal extensors also appear to be similar in all 

 these animals (Hill, 1962; Robertson, 1944), except in Cehus where 

 Straus (1941) found the brevis tendon inserted in both metacarpals 

 II and III. Senft (1907) was not able to differentiate the two muscles 

 proximally in Ateles ater {=Ateles paniscus). He describes them as 

 arising by a common belly from both the humerus and the epicondyle. 

 All the authors agree that the tendon of the extensor brevis is always 

 more robust than that of the longus. No significant difference can be 

 pointed out about m. supinator either. Robertson (1944) noted a large 

 epicondylar origin for this muscle in Lagothrix. 



SUPERFICIAL series, INTERMEDIATE DIVISION 



M. extensor digitorum: From the tip (fig. 27) and dorsum of the 

 lateral epicondyle short tendinous fibers take origin and soon become 

 muscular to form a flat and elongated belly which runs distally be- 

 tween m. extensor carpi radialis brevis laterally and the fifth digital 

 extensor medially. Many fascicles of the extensor digitorum arise 

 from the aponeurotic septa between it and the other two muscles. 

 At the distal third of the forearm the belly transforms into five roimd. 



