EXTENSOR OSSIS METACARPI POLLICIS 



329 



digiti, extensor carpi ulnaris, anconeus, the radial vessels and nerve, the interos- 

 seous recurrent artery; deeply, the superior radio-ulnar joint. 



Variations. — A second head has been seen from the humerus near the insertion of the 

 deltoid, and it may receive fibres from the braehio-radialis. 



Fig. 278. — The Deep Layer of the Back of the Forearm. 



Supinator brevis 



Extensor ossis metacarpi poUicis 



Extensor brevis pollicis 

 Extensor longus poUicis 



Hadial extensors 



Flexor carpi ulnaris 



Flexor profundus digitoruro 



Extensor indicis 



— Extensor carpi ulnaris 



2. EXTENSOR OSSIS METACARPI POLLICIS 



The extensor ossis metacarpi pollicis, or abductor longus pollicis— 



named from its action upon the nictacari)al bone of the thumb — is a fusiform 

 sheet. 



Origin. — (1) Tlie outer division of the i)osterior surface of the ulna for a short 

 distance l)elow the junction of the upper and middle thirds of that bone; (2) the 

 adjacent portion of the interosseous memlirane; (3) the posterior surface of the 



