384 A MANUAL OF ANATOMY, 



inches above the middle, from the middle third of the 

 posterior surface of the radius, and from the intervening 

 portion of the interosseous membrane between its bony 

 origins ; from the intermuscular septa. 



Insertion. Into the outer aspect of the base of the first 

 metacarpal bone, and into the deep fascia of the ball of the 

 thumb. 



Nerve Supply. The posterior interosseous (from the 

 seventh cervical nerve). 



Action. Extensor and abductor of thumb, abductor of 

 hand, slight supinator of forearm. 



Extensor Brevis Pollicis. (Extensor primi internodii 

 pollicis.) Fig. 79. 



Origin. From the posterior surface of the interosseous 

 membrane and the radius below the above muscle, with 

 which it is continuous. 



Insertion. Into the outer surface of the base of the 

 first phalanx of the thumb. 



Nerve Supply. Same as the above. 



Action. To extend and abduct the thumb, to abduct the 

 hand. 



Extensor Longus Pollicis. (Extensor secundi internodii 

 pollicis.) Fig. 79. 



Origin. From the posterior surface of the ulna for its 

 middle third, from the adjoining portion of the interosseous 

 membrane. 



Insertion. Into the base of the second phalanx of the 

 thumb. 



Nerve Supply. Same as the above. 



Action. To extend the thumb and hand, to supinate the 

 forearm. 



