392 THE MUSCULATURE 



common extensor, from the enveloping fascia, and from the septa between it and the extensor 

 digiti quinti, anconeus, and supinator (brevis); the other from the proximal three-fourths of 

 the dorsal border of the ulna. 



Structure and insertion. — The fibre-bundles descend in an osteo-fascial compartment 

 bounded by the dorsal surface of the ulna, the fascia of the forearm, the dense fascia overlying 

 the ulnar origin of the muscles of the thumb, and the origin of the extensor indicis. The tendon 

 commences high in the muscle and appears on the radial border of the middle third of the back 

 of its belly. The fibre-bundles are inserted in a penniform manner on the ulnar border and 

 deep surface of the tendon as far as the wrist. Here the tendon enters the sixth osteo-fibrous 

 canal beneath the dorsal carpal ligament in a special groove on the outer side of the styloid 

 process of the ulna. It is inserted into the base of the fifth metacarpal. 



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

 as this emerges from the supinator (brevis) muscle. Several filaments enter the deep surface 

 of the muscle in the middle third. The nerve fibres arise from the sixth, seventh and eighth 

 cervical nerves. 



Action. — To extend and abduct the hand ulnarward. 



Relations. — It occupies a superficial position on the ulnar side of the extensors of the fore- 

 arm. Beneath it lie the deep muscles of the back of the forearm and the posterior surface of 

 the ulna. 



Variations. — It may receive a slip from the triceps or be fused with the anconeus or with 

 the extensor of the little finger. More frequently it is doubled, partially or completely. An 

 accessory tendon may go to the first phalanx of the little finger, to the head of the fifth meta- 

 carpal, to the fourth metacarpal, to the extensor tendon of the little finger, or to the fascia over 

 the opponens digiti quinti. The muscle may be reduced to a fibrous band. The ulnaris digiti 

 quinti is a rare muscle arising from the dorsal surface of the ulna and inserted into the base of 

 the first phalanx of the little finger. It may be represented by a fasciculus or an extra tendon 

 from the ulnar extensor. 



b. Deep Layer 



(Fig. 369) 



The muscles of this group extend from the ulna to the radius, thumb, and index- 

 finger. They are the supinator, abductor pollicis longus, extensor pollicis longus 

 and brevis, and extensor indicis proprius. The supinator is a rhomboid muscle 

 which arises from the lateral epicondyle of the humerus and the supinator crest of 

 the ulna winds laterally around the radius and is inserted into its volar surface. 

 The abductor pollicis longus is a fusiform muscle which arises from the middle 

 third of the ulna, the interosseous membrane, and the radius, and is inserted 

 into the base of the first metacarpal. The extensor pollicis brevis arises from the 

 radius distal to the preceding muscle, and is inserted into the base of the first 

 phalanx of the thumb. The extensor pollicis longus is a narrow muscle which 

 arises from the middle third of the dorsal surface of the ulna and is inserted into 

 the base of the second phalanx of the thumb. The extensor indicis proprius is a 

 narrow, fusiform muscle arising from the shaft of the ulna and inserted into the 

 dorsal aponeurosis of the index-finger. These muscles are supplied from branches 

 of the deep radial (posterior interosseous) nerve while this is passing through or 

 after its exit from the supinator. 



The extensor pollicis longus is represented by the extensor hallucis longus of the leg. The 

 abductor pollicis longus and extensor polhcis brevis are represented by the abnormal abductor 

 hallucis longus and extensor primi internodii hallucis muscles, the rudiments of which are 

 perhaps normally present in the tibialis anterior. The supinator and the extensor indicis 

 muscles are not represented in the leg. On the other hand, the extensor digitorum brevis, 

 normal in the foot, is only occasionally found on the back of the hand. 



The supinator (brevis) (figs. 366, 369, 372). — Origin. — From (1) the inferior dorsal portion 

 of the lateral epicondyle by a tendinous band which is adherent to the deep surface of the 

 tendons of origin of the radial and common extensors and to the radial collateral ligament of 

 the joint; and (2) the ulna by a superficial aponeurosis and by fibre-bundles attached directly 

 to the depression below the radial notch and to the supinator crest. 



Insertion. — The lateral and volar surfaces of the radius from the tuberosity to the attach- 

 ment of the pronator teres. 



Structure. — From their origin the fibre-bundles descend spirally in a muscular sheet which 

 enwraps the radius (fig. 366). The attachment extends to the oblique line. The muscle is 

 divided into a superficial and a deep plane by a septum in which the deep radial (posterior 

 interosseous) nerve runs. The radial attachments of these two portions are separated by an 

 osseous area into which no fibre-bundles are inserted. The fibre-bundles of the superficial layer 

 have a much more vertical cour.se and are longer than those of the deep layer. 



Nerve-supply. — By branches wlii('h arise from the deep radial (posterior interosseous) 

 nerve before it pa.sKes between the two layers of the supinator muscle. The nerve fibres arise 

 from the fifth, sixth, and seventh cervical nerves. 



Action. — To su[)inate the forearm. 



Relations. — The supinator is covered by the superficial group of extensor muscles above 

 described and by the anconeus. 



