FOREARM AND HAND 181 



M. Extensor Carpi Ulnaris. The extensor carpi ulnaris 

 arises, by means of the common extensor tendon, from 

 the lateral epicondyle of the humerus ; from the fascia 

 of the forearm, and from the intermuscular septum between 

 it and the extensor digiti quinti proprius. In the middle 

 third of the forearm it receives some fibres from the strong 

 fascial layer which binds it to the dorsal border of the 

 ulna. The tendon does not become free from the fleshy 

 fibres until it approaches close to the wrist. It occupies 

 the groove on the dorsal aspect of the distal end of the 

 ulna, between the head and styloid process, and, passing 

 under cover of the dorsal carpal ligament, is inserted into the 

 tubercle on the base of the metacarpal bone of the little 

 finger. The muscle is supplied by the dorsal interosseous 

 nerve. It is an extensor of the wrist and elbow, and it takes 

 part in the movement of adduction at the wrist, when it acts 

 simultaneously with the flexor carpi ulnaris. 



M. Anconaeus. The anconaeus lies at the lateral part of 

 the dorsal aspect of the elbow joint. It is a small, short 

 muscle, of triangular outline, which arises, by a relatively 

 small origin, from the back of the lateral epicondyle of the 

 humerus. The insertion is into the lateral border of the 

 olecranon of the ulna and the proximal third of the dorsal 

 surface of the body of the ulna. The nerve of supply has 

 already been dissected. It is a long slender branch which is 

 given off from the trunk of the radial nerve, behind the 

 middle third of the humerus, and it descends through the 

 substance of the medial head of the triceps before it reaches 

 the anconaeus. In addition, the distal part of the muscle 

 sometimes receives a branch from the dorsal interosseous 

 nerve. It is an extensor of the elbow joint. 



Dissection. To expose the deep muscles of the back of the 

 forearm and the dorsal interosseous vessels and nerve the 

 extensor digitorum communis and the extensor digiti quinti 

 must be reflected. Divide the fleshy portion of each, about the 

 middle of its length, and turn the proximal parts of the divided 

 muscles towards their origins and the distal parts towards the 

 insertions. As the muscles are reflected care must be taken to 

 secure and preserve the twigs from the dorsal interosseous nerve 

 which enter their deep surfaces. When the reflection is com- 

 pleted pull aside the extensor carpi ulnaris ; then the greater 

 parts of the dorsal interosseous vessels and nerve and the deep 

 muscles of the forearm will be exposed. The muscles which 

 are exposed are, from above downwards, the supinator, the 

 abductor pollicis longus, the extensor pollicis brevis, and the 

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