330 THE MUSCLES 



radius near the middle of that bone for about two inches (5 cm.) below the inser- 

 tion of the su})inat()r brevis; and (4) the septa which separate it from the supinator 

 brevis, extensor car})i uhiaris, and extensor longus pollicis. 



Insertion. — (1 ) The small impression upon the radial side of the base of the 

 first metacarpal bone; (2) the fascia covering the ball of the thumb; and frequently 

 (3) the l^ack of the trapezium. 



Structure. — A somewhat bipenniform muscular sheet arising by an origin 

 which stretches obliquely downwards and outwards from the back of the ulna, at 

 the upi)er part of its middle third, to the middle of the back of the radius. The 

 tendon ap})ears first as an aponeurosis upon the anterior aspect of the muscle just 

 below the middle of the forearm. The fleshy fibres pass obliquely downwards and 

 outwards to be inserted upon the posterior face of this aponeurosis. This as it 

 descends thickens into a rounded tendon, which, becoming free from its muscular 

 fibres just above the posterior annular ligament, crosses the back of the two radial 

 extensor tendons, and enters the first compartment of the posterior annular liga- 

 ment upon the outer surface of the lower end of the radius. After leaving this 

 compartment, the tendon passes vertically downwards, lying upon the external 

 lateral ligament of the wrist-joint and the radial artery, to be inserted into the first 

 metacarpal l)one. From its anterior border a strong aponeurosis is given off to 

 that part of the palmar fascia which covers the ball of the thumb, and which 

 forms a part of the origin of the abductor pollicis. Frequently this division of the 

 insertion is indicated by a groove running up the tendon, or by a more complete 

 separation which may extend as high as the fleshy fibres of the muscle. 



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

 seventh cervical nerve) by filaments derived from the posterior interosseous 

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

 of its superficial aspect. 



Action. — (1) It abducts and extends the first metacarpal bone, drawing the 

 thuml^ away from the middle line of the hand, and slightly forwards. (2) It 

 abducts the wrist. It should be remembered that on account of the plane of the 

 thumb ))eing different to that of the palm and fingers, its abduction involves a 

 considerable forward movement, which must not be confounded with flexion. 



Relations. — Superficially, the extensor communis digitorum, extensor minimi 

 digiti, extensor carj^i ulnaris, the posterior interosseous artery, and posterior 

 annular ligament; deeply, the extensor brevis and the extensor longus pollicis, 

 the two radial extensors, the radial artery, and the wrist-joint. 



Variations. — The extensor ossis metacarpi pollicis may receive an accessory slip from the radial 

 group of extensors. 



3. EXTENSOR BREVIS POLLICIS 



The extensor brevis pollicis — sometimes called the extensor primi inter- 

 nodii pollicis, from its action upon the first phalanx (internodius) of the thumb — 

 is flat and fusiform. 



Origin. — (1) The middle of the posterior surface of the interosseous membrane 

 just below the preceding muscle; (2) an elongated impression upon the inner part 

 of the posterior surface of the radius extending from the middle of that bone for 

 about three inches (7 '5 cm.) downwards and slightly outwards; (3) an aponeurosis 

 which s('])arates it from the preceding muscle. 



Insertion. — The back of the base of the first phalanx of the thumb. 



Structure. — A pennifonn muscle arising by short tendinous fibres. These soon 

 become fleshy, and pass downwards and outwards to a tendon which begins in the 

 lower third of the forearm upon the radial liorder of its superficial aspect. Lying 

 beneath the tendon of the preceding muscle and in close connection with it, this 

 tendon passes over the tendons of the radial extensors, runs through the first com- 

 partment of the posterior annular ligament, crosses the first metacarpo-phalangeal 

 joint on the ulnar side of the tendon of the ]ireceding muscle, and then expands into 

 a broad aponeurosis which is inserted into the whole of the posterior surface of the 

 base of the first phalanx. 



