136 THE UPPER EXTREMITY 



The anconaeus is frequently more or less directly continuous with the 

 triceps, and this, together with the fact that it gets a special branch of 

 supply from the radial nerve, has led some anatomists to regard it as 

 a piece of the triceps muscle. This is not the case, however ; it belongs 

 to, and is therefore properly classified with, the group of muscles on the 

 extensor aspect of the forearm. 



The nerve of supply to the anconaeus has already been 

 dissected. It is a long slender branch from the radial 

 nerve (musculo-spiral), which descends to its destination in 

 the substance of the medial head of the triceps. In addition 

 to this, the distal part of the muscle usually receives a twig 

 from the dorsal interosseous nerve. 



Dissection. Reflect the extensor digitorum communis and the extensor 

 digiti quinti proprius. Divide the fleshy belly of each about its middle, and 

 throw them proximally and distally. In doing this care must be taken to 

 secure and preserve the nerve twigs from the dorsal interosseous nerve 

 which enter these muscles on their deep surface. The dorsal interosseous 

 artery and nerve, together with the deep muscles, are now exposed, and 

 may be fully dissected. In the distal part of the forearm the terminal part 

 of the dorsal interosseous nerve dips under cover of the extensor pollicis 

 longus, to reach the interosseous membrane and the back of the carpus. 

 In following this part of the nerve, the terminal or perforating branch of 

 the volar interosseous artery will be seen appearing on the dorsum of the 

 forearm, under cover of the extensor pollicis longus. 



Deep Muscles. These are (i) the supinator ; (2) the 

 abductor pollicis longus; (3) the extensor pollicis brevis; (4) 

 the extensor pollicis longus ; and (5) the extensor indicis 

 proprius. 



The supinator will be recognised from the close manner 

 in which it is applied to the proximal part of the body of 

 the radius. The other muscles take origin proximo-distally 

 in the order in which they have been named. The attach- 

 ments of the supinator cannot be satisfactorily studied at 

 present. They will be described at a later stage of the 

 dissection. 



M. Abductor Pollicis Longus (O.T. Extensor Ossis 

 Metacarpi Pollicis). This muscle arises from both bones 

 of the forearm, and from the interosseous membrane which 

 stretches between them. Its origin from the radius corre- 

 sponds to the middle third of its dorsal surface ; its origin 

 from the ulna is more proximal, from the lateral part of the 

 dorsal surface of the body immediately distal to the oblique line 

 which marks the distal limit of the insertion of the anconaeus. 

 The muscle proceeds distally and laterally, and comes to the 

 surface in the interval between the extensor digitorum com- 



