388 THE MUSCULATURE 



along the radial margin of the forearm, partly under cover of the brachio-radialis. 

 It arises from the lateral epicondylar ridge of the humerus, and is inserted into 

 the second metacarpal bone. The extensor carpi radialis brevis (fig. 367) is a 

 band-like muscle more dorsally placed than the last at the radial side of the arm. 

 It arises from the lateral epicondyle and is inserted into the bases of the second 

 and third metacarpals. These muscles are supplied by branches of the radial 

 nerve which arise proximal to the passage of the deep radial (posterior inter- 

 osseous) through the supinator muscle. Distally this set of muscles is separated 

 from the intermediate set by the long abductor and the extensors of the thumb, 

 which pass from an origin under the latter set over the tendons of the radial 

 extensors to the thumb. 



The intermediate set. — This consists of the thick, flattened extensor digitorum 

 communis and the slender extensor digiti quinti proprius (fig. 367). They arise 

 from the lateral epicondyle, and are inserted into the backs of the fingers. 



The ulnar set consists of one muscle, the fusiform extensor carpi ulnaris, 

 which arises from the lateral epicondyle of the humerus and is inserted into the 

 back of the base of the fifth metacarpal. 



The intermediate and ulnar sets of muscles are supplied by branches from the 

 ramus profundus of the radial nerve after this has passed through the supinator 

 muscle. 



In the leg the lateral set of the superficial layer is represented by the tibialis anterior. The 

 intermediate set is represented by the long extensors of the toes. The single muscle which 

 constitutes the medial set is represented by the peroneal muscles. 



The brachio-radialis (supinator radii longus) (figs. 367, 370). — Origin. — From the upper 

 two-thirds of the lateral epicondylar ridge of the humerus and from the front of the lateral 

 intermuscular septum. 



Insertion. — Into the lateral side of the base of the styloid process of the radius. 



Structure. — -The constituent fibre-bundles arise directly from the septum and by short 

 tendinous bands from the epicondylar ridge, extend downward and ventrally, and terminate 

 in a penniform manner on a tendon which extends high on the deep surface of the muscle. 

 This tendon becomes free about the middle of the forearm as a broad, flat band. This be- 

 comes narrow as the tendon winds about the radius from the volar to the lateral surface. 

 Before its insertion it expands laterally and is connected with neighbouring ligaments. The free 

 surface of the muscle faces laterally at its origin, but, owing to the torsion, ventrally in the 

 forearm. The tendon, however, is turned again so that at the insertion it faces laterally once 

 more. 



Nerve-supply. — From a branch of the radial nerve (musculo-spiral) which enters the 

 proximal third of the muscle on its deep surface. The nerve fibres arise from the fifth and sixth 

 cervical nerves. 



Action. — To flex the forearm. This action is strongest when the forearm is pronated. 

 It acts as a supinator only when the arm is extended and pronated. It then serves to put the 

 arm in a state of semi-pronation. When the forearm is flexed, it acts as a pronator. 



Relations.— The muscle is superficially placed on the ventro-lateral surface of the forearm. 

 Its tendon of insertion, however, is covered by the long abductor and the short extensor of the 

 thumb. Near its origin (fig. 367) it lies lateral to the brachialis. In the intervening tissue run 

 the radial nerve and the terminal branch of the profunda brachii artery. Dorsally and laterally 

 lies the medial head of the triceps. More distally the muscle overlies the extensor carpi radialis 

 longus. It crosses the supinator, pronator teres, and flexor poUicis longus muscles. Beneath 

 its medial edge lie the radial vessels and nerve. 



Variations. — The humeral origin may extend half-way up the shaft. The radial insertion 

 may be as high as the middle of the shaft or descend to the lesser multangular, navicular, or 

 third metacarpal. In about 7 per cent, of bodies (Le Double) the tendon of insertion divides 

 into two or three slips which are inserted on the styloid process of the radius. Occasionally 

 the radial nerve passes between these slips. An accessory slip may pass to the fascia of the 

 forearm. The muscle may be doubled throughout its length and it may be missing. It may be 

 connected by acce.ssory slips with neighbouring iiuiscles, the deltoid, brachialis, long abductor of 

 the thumb, or long radial (larpal extensor. The slip most frequently found goes to the brachialis. 



The extensor carpi radialis longus (figs. 3()7, 368, 371.) — Origin. — From the lower third 

 of the lateral oj)icondylar ridge, the lateral intermuscular septum, and from the front of the 

 tendon.s of the extensor carpi radialis brevis and the extensor communis digitorum which arise 

 from the lateral epicondyle. 



Structure and insertion. — The fibre-bundles arc inserted in a penniform manner on both 

 8urface.s of a tendon which first appears on the lateral border of the deep surfa(^e of the muscle, 

 becomes free above the middle of the forearm, and descends, closely applied to the tendon of 

 the .short radial carpal extcn.sor, to the .second compartment beneath the dorsal carj)al ligament, 

 through which it pa.s.ses to its insertion into the base of the second metacarpal near the radial 

 border. The outer surface of the muscle faces at first laterally, then ventrally. 



Nervc-sujyply. — By on(! or two br.'uiches which ari.se from the radial (musculo-spiral) nerve 

 as it passes between the bra('hialis and l)rachio-radialis. The nerve enters the deep surface 

 of the muscle in the proximal third. The nerve fibres arise from the (fifth), sixth and seventh 

 cervical nerves. 



Action. — To extend and aljduct the hand. It steadies the wrist when the flexors act on 



