174 THE SUPERIOR EXTREMITY 



forearm. Now continue the reflection until the attachment of 

 the flap to the posterior border of the ulna is reached. As the 

 reflection proceeds, the intermuscular septa which pass from the 

 deep surface of the deep fascia between the muscles of the dorsum 

 of the forearm must be divided. 



Superficial Muscles. The muscles in this region consist 

 of a superficial and a deep group. The superficial muscles, 

 named from the lateral to the medial border of the 

 forearm, are: (i) the brachio - radialis ; (2) the extensor 

 carpi radialis longus; (3) the extensor carpi radialis brevis; 

 (4) the extensor digitorum communis ; (5) the extensor digiti 

 quinti proprius ; (6) the extensor carpi ulnaris; and (7) the 

 anconaeus. This group therefore comprises one flexor of 

 the elbow, three extensors of the wrist, two extensors of the 

 fingers, and a feeble extensor of the forearm at the elbow 

 joint, viz., the anconaeus. In the distal part of the forearm 

 the extensor digitorum communis is separated from the 

 extensor carpi radialis brevis by a narrow interval, in which 

 appear two muscles belonging to the deep group. The two 

 muscles in question turn round the lateral margin of the fore- 

 arm, superficial to the radial extensors of the wrist, and end in 

 tendons which go to the thumb. The proximal of the two 

 muscles is the abductor pollicis longus, the distal is the 

 .extensor pollicis brevis, but they lie in such close contact, 

 and so intimately are their tendons connected, that in many 

 cases they appear, at first sight, to be blended together by their 

 margins. After they have become superficial, the abductor 

 pollicis longus and the extensor pollicis brevis turn round 

 the radial border of the forearm superficial to the radial 

 extensors of the wrist, and as they descend to the thumb 

 they lie in the groove on the lateral surface of the styloid 

 process of the radius in which they are retained by the 

 most lateral part of the dorsal carpal ligament (Fig. 74). 



A short distance proximal to the dorsal carpal ligament 

 on the radial side of the tendons of the extensor digitorum 

 a third muscle of the deep group also comes into view ; it is 

 the extensor pollicis longus. Its tendon crosses the radial 

 extensors of the wrist at the distal border of the dorsal 

 carpal ligament. 



Dissection. The skin of the dorsum of the hand has already 

 been removed, and the cutaneous nerves and veins have been 

 displayed. The thin deep fascia is still in position, but it will 

 not prevent a successful demonstration of the mucous sheaths 



