90 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Nerve supply: The radial nerve gives off three branches as it 

 travels between the other humeroradial muscles and brachiahs. The 

 highest goes to brachioradiaUs, the intermediate to the extensor 

 carpi radialis longus and the lowest to the brevis. 



Function: Forearm flexor. 



M. extensor carpi radialis longus: This is also a long and trans- 

 versely flattened muscle. It connects the distal end of the humerus 

 to the hand, is fleshy on its upper half and tendinous on the lower. 

 The origin is by muscular flbers from the lower half of the lateral 

 supracondylar ridge, between those of brachioradiaUs above and the 

 extensor brevis below (fig. 27). The fascicles of the longus run distal- 

 ward covered by brachioradialis and over those of the extensor brevis. 

 The belly narrows down to a flat and long tendon in the middle of 

 the forearm. This is crossed superficially by the abductor polUcis 

 longus in the distal third of the radius. Then, it enters the second 

 compartment of the extensor retinaculum together with the extensor 

 brevis. The tendon now^ advances on the dorsolateral aspect of the 

 carpus below those of the extensores polUcis et indices until it reaches 

 the dorsum of the second metacarpal base where it is inserted on 

 the radial half (flg. 29). Sirena (1871) found this muscle to be just 

 as in man. 



Nerve supply: It has already been described with the brachio- 

 radialis. 



Function: Extends both carpus and metacarpus. 



M. extensor carpi radialis brevis: Of a similar appearance but 

 better developed than the other radial carpal extensor, the brevis 

 arises by fleshy flbers from the lowest part of the lateral supracon- 

 dylar ridge and the epicondyle itself (flg. 27). It runs distaUy toward 

 the metacarpus, being fleshy in its proximal half and tendinous in 

 the distal. It is covered by the other two humeroradial muscles and 

 related on its deep surface first to m. supinator and lower down to 

 the radius. A flat tendon is formed on the lateral aspect of the muscle 

 upon which all its fibers converge. The tendon is covered by that of 

 the abductor pollicis longus and then reaches the second compart- 

 ment in the extensor retinaculum where it is medial to the tendon 

 of the longus. In the carpus it proceeds distally over the scaphoid, 

 OS centraUs and capitate to insertion on the well-developed styloid 

 process of the third metacarpal (fig. 29). This structure projects 

 radially and dorsally from the dorsum of the base. 



The second radial extensor of the southern howler is also said to 

 be hke that of man, but Sirena (1871) notes that its tendon is larger 

 than that of the first extensor. 



Nerve supply: It has already been described. 



Function: Extensor of the carpus and metacarpus. 



