EXTENSOR CARPI ULNARIS— ANCONEUS 327 



3. EXTENSOR CARPI ULNARIS 



The extensor carpi ulnaris — named from its action upon the carpus and its 

 position on the uhiar l)or(ler of the forearm — is a fusiform sheet. 



Origin. — By two heads. The first head: (1) By the common tendon from 

 the back of the external condyle; (2) the front of the deep fascia of the forearm; 

 and (3_) the intermuscular septa which separate it from the extensor minimi digiti, 

 the anconeus, and the supinator brevis. 



The second head : from the posterior border of the ulna, by the aponeurosis 

 common to it, the tiexor carpi ulnaris, and the flexor profundus digitorum. 



Insertion. — The back of the base of the fifth metacarpal bone, close to its 

 ulnar l»onler. 



Structure. — The fleshy fibres converge below upon the tendon of insertion, 

 which, beginning in the interior of the muscle in the middle of the forearm, soon 

 becomes visible upon the radial border of the back of the muscle. It receives 

 fleshy fibres in penniform fashion upon its ulnar border and deep surface 

 nearly as far as the wrist, where it enters the sixth compartment of the posterior 

 annular ligament, and lies in a special groove to the outer side of the styloid 

 process of the ulna. 



Nerve-supply. — From the posterior cord of the brachial plexus (through the 

 eighth cervical nerve) by branches from the posterior interosseous division of the 

 musculo-spiral nerve which enter the deep surface of the muscle about the middle 

 of the forearm. 



Action. — (1) To extend the wrist, acting upon the three articulations involved 

 in this movement; (2) to adduct the wrist, especially when the hand is pronated; 

 (3) it will also help in the extension of the elbow- joint. 



Relations. — Sviperficially, the deep fascia of the forearm, and the posterior 

 annular ligament; deeply, the supinator brevis, the extensors of the thumb, ex- 

 tensor indicis, posterior interosseous artery, the inner half of the posterior sur- 

 face of the ulna below the middle of the forearm, the wrist, and some carpal 

 joints. On the outer side lies the extensor minimi digiti; on the inner, the 

 anconeus muscle. 



Variations. — Frequently a small slip of tendon passes downwards to join the tendon of the 

 preceding muscle, the first phalanx, or the head of the metacarpal bone. Occasionally the 

 fourth or even the third metacarpal bone may receive a slip. 



4. ANCONEUS 



The anconeus — named from its intimate relation with the elbow {ayxu)'^) — is a 

 triangular fan-sliaped sheet. 



Origin. — (1 ) The lower part of the back of the external condyle; and (2) the 

 adjacent part of the posterior ligament of the elbow-joint. 



Insertion. — The rough triangular impression upon the outer surface (1) of the 

 olecranon, and (2) of the upper third of the back of the ulna. 



Structure. — This muscle is a continuation downwards of the lower part of the 

 inner liead of the triceps. Arising by a short tendon, which is prolonged upon the 

 deep surface of the muscle and along its outer l)order, the fleshy fibres diverge in 

 a fan shape, and are inserted either directly or by short tendinous fibres into the 

 large special impression upon the ulna; the highest fibres being nearly horizontal, 

 and the lowest ajiproaching more nearly to a vertical direction. 



Nerve-supply. — From the posterior cord of the brachial plexus (through the 

 seventh and eighth cervical nerves) by a long branch from the musculo-spiral 

 nerve, which, after passing through the inner head of the triceps muscle, enters 

 the deep aspect of the anconeus close to its U})per border. 



Action. — To extend the elbow, in association with the triceps, of which some 

 authors consider it to form a fourth head. 



Relations. — Superficially, the deep fascia of the forearm; deeply, the sujiinator 



