1 82 THE SUPERIOR EXTREMITY 



extensor indicis. The dorsal interosseous nerve appears through 

 the substance of the supinator, crosses the surface of the 

 abductor pollicis longus, and dips deep to the 1 extensor pollicis 

 longus. The dorsal interosseous artery appears between the 

 adjacent borders of the supinator and the abductor pollicis 

 longus and descends along the radial border of the extensor 

 carpi ulnaris superficial to the extensor pollicis longus and the 

 extensor indicis. First clean the dorsal ( interosseous nerve and 

 secure its branches. After it pierces the supinator it gives 

 branches of supply to all the deep muscles except the supinator, 

 and to three of the superficial muscles, viz., the extensor 

 digitorum communis, the extensor digiti quinti, and the extensor 

 carpi ulnaris. The branches to the superficial muscles were 

 found as the extensor digitorum communis and the extensor 

 digiti quinti were reflected. The branch to the extensor indicis 

 is given off before the nerve dips deep to the extensor pollicis 

 longus, and it crosses the superficial surface of the long 

 extensor of the thumb. When it has been found pull the 

 extensor pollicis longus and the extensor indicis towards the 

 medial side, and follow the dorsal interosseous nerve, which 

 passes deep to both of them, to the back of the radius where it 

 enters the osteo-fascial compartment through which the extensor 

 digitorum communis and the extensor indicis pass to the dorsum 

 of the hand. Where it lies under cover of the extensor pollicis 

 longus it is joined by the perforating branch of the volar inter- 

 osseous artery, which pierces the interosseous membrane about 

 50 mm. above the wrist, and then accompanies the dorsal 

 interosseous nerve. Pull aside the tendons of the extensor 

 communis and the extensor indicis beyond the dorsal carpal 

 ligament and find the terminal part of the nerve and the con- 

 tinuation of the artery, deep to the tendons on the back of the 

 wrist joint. The nerve ends in a gangliform enlargement from 

 which twigs are distributed to the ligaments of the wrist joint, 

 and the volar interosseous artery terminates by joining the 

 dorsal carpal arterial arch. 



Now clean the deep muscles, note the osteo-fascial compart- 

 ments on the back of the radius through which they pass to the 

 hand, and trace their tendons to their insertions. 



Deep Muscles. These are (i) the supinator; (2) the 

 abductor pollicis longus ; (3) the extensor pollicis brevis ; (4) 

 the extensor pollicis longus ; and (5) the extensor indicis 

 proprius (Fig. 79). 



The supinator will be recognised from the close manner 

 in which it is applied to the proximal part of the body of 

 the radius. The other muscles take origin proximo-distally 

 in the order in which they have been named. The attach- 

 ments of the supinator cannot be satisfactorily studied at 

 present. They will be described at a later stage of the 

 dissection. 



M. Abductor Pollicis Longus (O.T. Extensor Ossis 

 Metacarpi Pollicis). The long abductor of the thumb arises 



