394 THE MUSCULATURE 



after it has emerged from the supinator. The branches enter the muscle on the superficial 

 surface in the proximal third. The nerve fibres come from the sixth, seventh (and eighth) 

 cervical nerves. 



Action. — It carries the first metacarpal radialward and forward. At the height of its 

 contraction it flexes and abducts the hand at the wrist. 



Relations. — Near its origin the muscle is covered by the superficial extensors of the forearm. 

 More distal!}^, accompanied bj^ the short extensor, it passes radially, becomes superficial, and 

 crosses the tendons of the two radial carpal extensors. 



Variations. — The muscle or its tendon may be doubled. An accessory tendon may be 

 applied to the multangulum majus (trapezium), the transverse ligament of the carpus, the 

 superficial muscles of the thenar eminence, or the first metacarpal. Of these, the attachment 

 to the short abductor and short flexor is the most frequent (7 out of 36 bodies — Wood). There 

 may be three or more tendons. The muscle may be fused with the short extensor. 



The extensor poUicis brevis (fig. 369). — Origin. — From the distal part of the middle 

 third of the medial portion of the dorsal surface of the radius and from the neighbouring portion 

 of the interosseous membrane. Rarely its origin extends to the ulna. 



Structure and insertion. — The fibre-bundles converge on a tendon which appears on the 

 radial border. The fibres are inserted as far as the dorsal carpal (posterior annular) hgament. 

 The tendon Hes parallel to the ulnar side of that of the abductor poUicis longus, and, in close 

 connection with it, passes through the first compartment beneath the dorsal carpal ligament, 

 and crosses the metacarpo-phalangeal joint on the radial side of the long extensor tendon. It 

 is inserted on the base of the first phalanx of the thumb or into the capsule of the metacarpo- 

 phalangeal joint. 



Nerve-supply. — From a branch derived from the deep radial (posterior interosseous) nerve. 

 This branch is usually given off in common with or near the nerve to the abductor poUicis 

 longus, and many traverse that muscle to reach the extensor pollicis brevis, which it enters in 

 the proximal third of its radial border. The nerve fibres come from the sixth, seventh (and 

 eighth) cervical nerves. 



Action. — To extend the thumb at the metacarpo-phalangeal joint and to abduct the first 

 metacarpal. It hkewise acts as a weak supinator of the forearm. 



Relations. — It hes between the abductor polUcis longus and the extensor pollicis longus, 

 by which its origin is partly overlapped. In company with the former muscle it passes medially 

 from beneath the common extensor of the fingers and over the tendons of the radial carpal 

 extensors to reach its osteo-fibrous canal under the dorsal carpal ligament. 



Variations. — The head of the muscle may be fused with the long abductor. Its tendon 

 of insertion may give rise to a slip inserted on the first metacarpal (in 2 out of 85 bodies — Le 

 Double) or into the terminal phalanx. Its tendon is often united with that of the long extensor. 

 It may be fused with the long abductor of the thumb and has been found missing. It may be 

 doubled. 



The extensor pollicis longus (fig. 369). — Origin. — From the middle third of the lateral 

 part of the dorsal surface of the ulna; from the neighbouring part of the interosseous membrane; 

 and from the septa between it and the extensor indicis proprius, and the extensor carpi ulnaris. 



Structure and insertion. — The fibre-bundles converge in a bipenniform manner on the two 

 sides of a tendon which appears high on the dorsal surface of the muscle. They extend as far 

 as the dorsal carpal (posterior annular) hgament. The fusiform body of the muscle descends 

 somewhat obliquely on the dorsal surface of the forearm. The tendon enters the third osteo- 

 fibrous canal beneath the dorsal carpal (posterior annular) ligament. On emerging from the 

 canal it passes very obliquely across the dorsal surface of the carpus, over the tendons of 

 the radial extensors, to the ulnar side of the first metacarpal. It passes along this and on the 

 dorsal surface of the first phalanx, expands to be inserted into the base of the second phalanx. 

 The aponeurosis of insertion receives tendinous slips from the short muscles of the volar surface 

 of the thumb. 



Nerve-supply. — By a twig from the deep radial (posterior interosseous) nerve. The branch 

 gives rise to twigs which enter the proximal third of the radial border of the muscle. The 

 fibres arise from the sixth, seventh, and eighth cervical nerves. 



Action. — To extend the second phalanx on the first, and this on the metacarpal. . It also 

 draws the whole thumb when extended toward the second metacarpal. It may have a slight 

 supinator action on the forearm. 



Relations. — The head of the muscle is partly overlapped by the long abductor of the thumb. 

 It lies between this and the extensor pollicis brevis on one side, and the extensor indicis proprius 

 on the other. Over it lie the extensors of the fingers and the ulnar carpal extensor. 



Variations. — The tendon may give a slip to the base of the first phalanx of the thumb, 

 to the dorsal carpal ligament, or to the index finger. It may receive an accessory slip from the 

 common cxten.sor of the fingers or the short extensor of the thumb. It is frequently! doubled. 

 .\n additional extensor is found in about 6 per cent, of bodies between the extensor! of the index 

 fingor and that of the thumb. It has a double tendon and insertion into both digits (extensor 

 communis pollicis et indicis). 



The extensor indicis proprius (fig. 369). — Origin. — From the proximal part of the distal 

 third f)f tho postcjrior siirfaco of the ulna, medial and distal to that of the preceding muscle, from 

 the adjacent intero.ssoous membrane, and from the septum between it and the extensor pollicis 

 longus. 



Structure and insertion. — The fibre-bundles are inserted on a tendon which first appears on 

 the radial border of the muscle. The insertion of fibre-bundles extends nearly to the dorsal 

 carpal (posterior annular) ligament. Here the tendon passes beneath that of the extensor 

 of the little finger and enters the fourtii osteo-fibrous canal beneath tiic lateral tendons of the 

 common extensor. It passes across the wrist beneath the tendon from the extensor communis 

 to the index finger, and is in.scrted on the ulnar side of this into the dorsal aponeurosis of the 

 index finger opposite the base of the first phalanx. 



