ABDUCTOR POLLICIS LONGUS 



393 



' Variations. — The extent of separation of the muscles into two portions varies. Accessory 

 fascieuli of origin are not uncommon. These may spring from the annular hgament, tensor liga- 

 menti annularis anterior (5 per cent, or more of bodies — Le Double), the lateral epicondyle, 

 the tendon of the bi caps, the tuberosity of the radius, etc. A sesamoid bone may lie in the 

 tendon of origin. The tensor ligamenti annularis posterior is a sUp generally present and 

 often independent of the supinator. It runs from the ulna behind the radial notch to the annular 

 ligament of the radio-ulnar joint. 



The abductor pollicis longus (extensor ossis metacarpi pollicis) (fig. 369). — Origin. — From 

 (1) the lateral margin of the dorsal surface of the ulna in the proximal portion of the middle third, 



Fig. 369. — The Deep Muscles of the B.\ck of the Forearm. 



Anconeus 



Supinator 



Abductor pollicis longus 



Extensor pollicis brevis 

 Extensor pollicis longus 



Radial extensors 



Flexor carpi ulnaris 



Flexor digitorum profundus 



Extensor indicis proprius 



Extensor carpi ulnaris 



and the adjacent interosseous membrane, (2) the dorsal surface of the radius distal and medial 

 to the attachment of the supinator, and (3) at times, from the septa lying between it and the 

 supinator, extensor carpi ulnaris, and extensor polhcis longus. 



Structure and insertion. — The fibre-bundles from this extensive area of origin converge in 

 a bipenniform manner upon a tendon which appears as an aponeurosis on the deep surface of 

 the muscle about the middle of the forearm. The tendon as it descends becomes rounded. 

 The insertion of fibre-bundles continues nearly to the wrist. Here, together with the tendon 

 of the short extensor, it enters the first osteo-fibrous canal beneath the dorsal carpal ligament 

 upon the lateral surface of the distal extremity of the radius. Upon leaving this canal the 

 tendon extends to be inserted on the radial side of the base of the first metacarpal bone. 



Nerve-supply. — By one or more branches from the deep radial (posterior interosseous) nerve 



