288 DISSECTION OF THE FOREARM. 



but it becomes superficial nearer the lower end of the radius. Below the 

 annular ligament its tendon crosses the radial artery, and the extensors of 

 the wrist. 



Action. Its use is similar to that of the extensor of the first phalanx. 

 "When the phalanges are straight, the two extensors will assist in carrying 

 back the metacarpal bone. 



The EXTENSOR INDICIS, H (indicator), arises from the ulna for three or 

 four inches, usually beyond the middle, and internal to the preceding mus- 

 cles ; and from the interosseous ligament below. Near the annular liga- 

 ment the tendon becomes free from muscular fibres, and passing through 

 that band with the common extensor of the fingers, is applied to, and blends 

 with, the external tendon of that muscle in the expansion on the phalanx 

 of the forefinger. 



Until this muscle has passed the ligament it is covered by the superfi- 

 cial layer, but it is afterwards subaponeurotic. 



Action. The muscle can point the fore finger even when the three inner 

 fingers are bent, inclining it towards the others at the same time. And it 

 will help the common extensor of the digits in pulling back the hand. 



Dissection. To lay bare the supinator brevis, it will be necessary to 

 detach the anconeus from the external condyle of the humerus, and to cut 

 through the supinator longus and the radial extensors of the wrist. After 

 those muscles have been divided, the fleshy fibres of the supinator are to 

 be followed forwards to their insertion into the radius ; and that part of 

 the origin of the flexor profundus digitorum, which lies on the outer side 

 of the insertion of the brachialis anticus, is to be removed. 



The SUPINATOR BREVIS, D, surrounds the upper part of the radius, 

 except at the tubercle and along a slip of bone below it. It arises from a 

 depression below the small sigmoid cavity of the ulna, from the external 

 margin of the bone for two inches below that pit, and from the orbicular 

 ligament of the radius and the external lateral ligament of the elbow joint. 

 The fibres pass outwards, and are inserted into the upper third or more of 

 the radius, except at the inner part, reaching downwards to the insertion 

 of the pronator teres, and forwards to the hollowed anterior surface. 



The supinator is concealed altogether at the posterior and external 

 aspects of the limb by the muscles of the superficial layer; and anteriorly 

 the radial vessels and nerve lie over it. The lower border is contiguous 

 to the extensor ossis metacarpi pollicis, only the posterior interosseous 

 artery (a) intervening. Through the substance of the muscle the posterior 

 interosseous nerve ( 3 ) winds to the back of the limb. 



Action. When the radius has been moved over the ulna as in prona- 

 tion, the short supinator comes into play to bring that bone again to the 

 outer side of the ulna. 



The posterior interosseous artery (fig. 88, a) is an offset from the com- 

 mon interosseous trunk (p. 269), and reaches the back of the forearm 

 above the ligament between the bones. Appearing between the contiguous 

 borders of the supinator brevis and extensor ossis metacarpi, the artery 

 descends at first between the superficial and deep layers of muscles ; and 

 afterwards, with a superficial position in the lower third of the forearm, 

 along the tendon of the extensor carpi ulnaris as far as the wrist, where it 

 ends by anastomosing with the carpal and anterior interosseous arteries. 

 It furnishes muscular offsets to the contiguous muscles, except the two or 

 three outer ; and the following recurrent branch : 



The recurrent branch (b) springs from the artery near the commence- 



