EXTENSOR LONG US POLLICIS 331 



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

 seventh cervical nerve) l)y a branch from the posterior interosseous division of the 

 musculo-spiral nerve, which enters the upper part of the muscle on the ulnar border 

 of its superticial aspect. 



Action. — It is a feeble muscle, the chief function of which is (1 ) to extend the 

 metacarpo-phalangeal joint of the thumb; at the same time it will assist the extensor 

 ossis metacarpi pollicis in (2) abducting and extending the first metacarpal bone; 

 and afterwards it will assist in (3) the abduction of the wrist. 



Relations. — Superficially, the extensor ossis metacarpi pollicis, extensor com- 

 munis digitorum, extensor minimi digiti, and posterior annular ligament; deeply, 

 the radial extensors, the radial artery, the Avrist and first metacarpo-phalangeal 

 joints. 



Variations. — The extensor brevis pollicis may be absent, or it may be blended with the pre- 

 ceding muscle. It may have insertions upon the metacarpal bones or the last phalanx. An 

 accessory slip has been observed from the external condylar ridge of the humeras. 



4. EXTENSOR LONGUS POLLICIS 



The extensor longus pollicis, or extensor secundi internodii pollicis — 



named from its action and length — is Hat and fusiform. 



Origin. — (1) An elongated impression upon the outer part of the posterior sur- 

 face of the ulna, extending from the middle of that l^one for three or four inches (7 "5 

 to 10 cm. ) downwards and slightly outwards close to its external border. (2) The 

 adjacent part of the back of the interosseous membrane, and a septum between it 

 and the extensor indicis. 



Insertion. — The ])ack of the base of the last phalanx of the thumb. 



Structure. — This muscle, which is stronger than the preceding, has also a pen- 

 niform arrangement. Its fibres pass downwards and outwards from their fleshy 

 origin to be inserted into the tendon upon the back of the muscle, which, appear- 

 ing about two inches (5 cm.) from its upper extremity near its radial border, 

 gradually thickens as it passes obliquely downwards and outwards. Becoming free 

 from muscular fibres at the upper border of the posterior annular ligament, it enters 

 the deep groove which forms the third compartment beneath that ligament. At the 

 lower extremity of this canal it crosses obliquely the two radial extensor tendons 

 at the back of the wrist-joint, and upon the back of the first metacarpal bone it lies 

 in close contact with the inner border of the tendon of the preceding muscle, and 

 expands into a flat and broad aponeurosis which covers the liack of the first phalanx 

 before its insertion into the last phalanx. Whilst spread over the convex surface of 

 the shaft of the first phalanx, it receives small tendinous insertions from the 

 abductor and adductor muscles of the ball of the tluiml). 



The three tendons of the extensor muscles of the thumb enclose a triangular 

 space which is visible upon the outer side of the wrist-joint, and is bounded by the 

 tendons of the extensor ossis metacarpi pollicis and the extensor brevis pollicis 

 upon its radial side, by the tendon of the extensor longus pollicis upon its ulnar 

 side, and above by the lower end of the radius. Across this triangle stretches the 

 radial artery, in its passage beneath the tendons from the anterior surface of the 

 wrist to the upper part of the back of the first interosseous space. 



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

 seventli cervical nerve), l)y branches from the posterior interosseous division of the 

 musculo-spiral nerve which enter the upper part of the muscle upon its superficial 

 aspect. 



Action. — It is (1) a strong extensor of the second phalangeal joint of the 

 thumb; and afterwards (2) of the first phalanx. It will (3) extend, and at the 

 same time adduct, the first metacarpal bone so as to draw the whole thumb in 

 the extended position backwards, and at the same time inwards. It will also (4) 

 assist in the extension of the wri.st, and (5) in the supination of the forearm. 



Relations. — Superficially, the extensor ossis metacarpi pollicis, extensor com- 

 munis digitorum, extensor minimi digiti, extensor carpi ulnaris, posterior inter- 

 osseous artery, and posterior annular ligament; deejily, the radial extensors, the 



