6o2 



HUMAN ANATOMY. 



Nerve-Supply. — By the posterior interosseous branch of the musculo-spiral 

 nerve from the sixth cervical nerve. 



Action. — To supinate the forearm. 



Variations. — The posterior interosseous nerve perforates the supinator and occasionally 

 marks the line of separation of the muscle into two portions, which correspond to the epicon- 

 dylar and ulnar portions of the muscle. The muscle is indeed a composite one, a portion of it 

 being derived from the superficial extensor layer and the rest of it from the deep layer. 



Fig 



Brachio-radialis 



Flexor sublimis digitorum 



2. Extensor Ossis Metacarpi Pollicis (Fig. 580). 



Attachments. — The extensor of the metacarpal bone of the thumb (m. abduc- 

 tor pollicis longus) arises from the middle third of the posterior surfaces of the ulna, 



the interosseous membrane, and 

 the radius. It passes down- 

 ward and laterally, and its ten- 

 don passes through the first 

 compartment beneath the pos- 

 terior annular ligament to be 

 inserted into the outer side of 

 the base of the first metacarpal 

 bone. 



Nerve-Supply. — By the 

 posterior interosseous branch of 

 the musculo-spiral nerve from 

 the sixth, seventh, and eighth 

 cervical nerves. 



Action. — To abduct and 

 slightly extend the thumb and, 

 continuing its action, to abduct 

 the hand. 



Relations. — It is covered 

 by the muscles of the superficial 

 layer and is crossed obliquely by 

 the dorsal interosseous artery. 

 Below it crosses obliquely the 

 tendons of the extensores carpi 

 radiales and the radial artery. 



Variations. — It may be par- 

 tially or wholly fused with the ex- 

 tensor brevis pollicis. Occasionally 

 it possesses two tendons, one of 

 which may be inserted into the dor- 

 sal carpal ligament, the abductor 

 brevis pollicis, or the trapezium. 



Extensor iiidicis 



Tendons of 



extensor 



communis 



digitorum 



Extensor ossis 

 metacarpi pollicis 

 Extensor brevis pollicis 

 Extensor carpi radialis longior 

 Extensor longus pollicis 

 . carpi radialis brevior 



Radial artery 



First dorsal 

 interosseus 

 Radialis indicis 

 artery 



\ \ 



\ 



Superficial dissection of hand, viewed from radial side, showing 

 extensor tendons of thumb. 



3. Extensor Brevis Polli- 

 cis (Fig. 580). 



Attachments. — The short 

 extensor of the thumb (m. exten- 

 "^ sor pollicis brevis), also termed 



the exteyisor primi iyiternodii pollicis, lies along the medial border of the extensor 

 ossis metacarpi pollicis. It arises from the interosseous membrane and the pos- 

 terior surface of the radius, partly under cover of the extensor longus pollicis, and its 

 tendon, after passing with that of the abductor through the first compartment of the 

 posterior annular ligament, is ifiserted into the base of the first phalanx of the thumb. 

 Nerve-Supply. — By the posterior interosseous branch of the musculo-spiral 

 nerve from the sixth, seventh, and eighth cervical nerves. 



Action. — To abduct the thumb and extend its first phalanx. 



