UPPER EXTREMITY AND THORAX, POSTERIOR. 385 



Extensor Indicis. Fig. 79. 



Origin, From the posterior surface of the ulna and the 

 interosseous membrane just below the extensor longus pol- 

 licis. 



Insertion. Into the tendon of the common extensor for 

 the index finger. 



Nerve Supply- Same as the above. 



Action. To extend the index finger and hand. 



Supinator Brevis. Figs. 67, 79. 



Origin. By the common tendon from the external (epi) 

 condyle of the humerus, from the external lateral ligament 

 of the elbow-joint, from the orbicular ligament, from the 

 ridge and triangular surface of bone below the lesser sig- 

 moid cavity of the ulna, from the intermuscular septa. 



Insertion. Into the upper third of the radius from its 

 neck to the insertion of the pronator radii teres below, and 

 limited by the oblique line in front. 



Nerve Supply. The posterior interosseous, which passes 

 through the muscle from the front to the back of the fore- 

 arm. (The fibres come from the sixth cervical nerve.) 



Action. To supinate the forearm. 



The Posterior Interosseous Artery. (See page 297.) 



Fig. 79. 



This is a branch from the common interosseous in the 

 anterior part of the forearm. It turns backward, passing 

 between the radius and ulna on either side, and the interos- 

 seous membrane below, and the oblique ligament above, and 

 appears in the posterior part of the forearm between the 

 supinator brevis and the extensor ossis metacarpi pollicis 

 muscles. It descends between the deep and superficial 

 layers of muscles until just above the wrist it terminates in 

 branches which anastomose with the anterior interosseous 

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