FOREARM AND HAND 183 



from both bones of the forearm, and from the interosseous 

 membrane, which stretches between them. Its origin from 

 the radius corresponds to the middle third of the dorsal 

 surface of that bone j its origin from the ulna is more proximal, 

 from the lateral part of the dorsal surface of the body, im- 

 mediately distal to the oblique line which marks the distal 

 limit of the insertion of the anconseus. The muscle proceeds 

 distally and laterally, and comes to the surface in the interval 

 between the extensor digitorum communis and the extensor 

 carpi radialis brevis. Then it crosses the two radial extensors, 

 closely accompanied by the extensor pollicis brevis. The 

 tendon which issues from it, as it becomes superficial, is 

 continued distally, over the lateral side of the expanded 

 distal end of the radius, and under cover of the dorsal carpal 

 ligament, and is inserted into the lateral side of the base of 

 the metacarpal bone of the thumb. The muscle is supplied 

 by the dorsal interosseous nerve. In addition to being an 

 abductor of the thumb it assists in producing supination and 

 abduction of the hand (Figs. 74, 75, 76, 79). 



M. Extensor Pollicis Brevis (O.T. Extensor Primi Inter- 

 nodii Pollicis). The short extensor of the thumb is placed 

 along the distal border of the preceding muscle. It arises 

 from a small portion of the dorsal surface of the radius, and 

 also from the interosseous membrane. Its tendon is closely 

 applied to that of the abductor pollicis longus, and accom- 

 panies it deep to the dorsal carpal ligament. It must be 

 traced, on the dorsal aspect of the metacarpal bone of the 

 thumb, to the base of the proximal phalanx, into which it is 

 inserted. The muscle is supplied by the dorsal interosseous 

 nerve. It is an extensor of the first interphalangeal joint 

 and of the metacarpo-phalangeal joint of the thumb, and it 

 takes a subsidiary part in the production of abduction of 

 the hand. 



M. Extensor Pollicis Longus (O.T. Extensor Secundi Inter- 

 nodii Pollicis). The long extensor of the thumb takes origin 

 from the lateral part of the dorsal surface of the body of the 

 ulna, in its middle third, and also from the interosseous 

 membrane. It overlaps, to some extent, the preceding 

 muscle, and it ends in a tendon which passes under cover 

 of the dorsal carpal ligament, where it occupies a deep narrow 

 groove on the dorsum of the distal end of the radius. 

 On the carpus it takes an oblique course, and, after crossing 



i 12 c 



