EXTENSOR OSSIS METACARPI. 243 



and is inserted into the olecranon and triangular surface on the upper 

 extremity of the ulna. 



Relations. By its superficial surface with a strong tendinous apo- 

 neurosis derived from the triceps. By its deep surface with the elbow 

 joint, orbicular ligament, and slightly with the supinator brevis. 



Deep Layer. 



Supinator brevis, 

 Extensor ossis metacarpi pollicis. 

 Extensor primi internodii pollicis, 

 Extensor secundi internodii pollicis, 

 Extensor indicis. 



Dissection. The muscles of the superficial layer should be removed 

 in order to bring the deep group completely into view. 



The SUPINATOR BREVIS cannot be seen in its entire extent, until 

 the radial extensors of the carpus are divided from their origin. It 

 arises from the external condyle, from the external lateral and orbicular 

 ligament, and from the ulna, and winds around the upper part of the 

 radius, to be inserted into the upper third of its oblique line. The 

 posterior interosseous artery and nerve are seen perforating the lower 

 border of this muscle. 



Relations. By its superficial surface with the pronator radii teres, 

 supinator longus, extensor carpi radialis longior and brevior, extensor 

 communis digitorum, extensor carpi ulnaris, anconeus, the radial artery 

 and veins, the musculo- spiral nerve, radial and posterior interosseus 

 nerve. By its deep surface with the elbow joint and its ligaments, the 

 interosseous membrane, and the radius. 



The EXTENSOR ossis METACARPI POLLICIS is placed immediately 

 below the supinator brevis. It arises from the ulna, interosseus mem- 

 brane, and radius, and is inserted, as its name implies, into the base 

 of the metacarpal bone of the thumb. Its tendon passes through the 

 groove immediately in front of the styloid process of the radius. 



Relations. By its superficial surface with the extensor carpi ulnaris, 

 extensor minimi digiti, extensor communis digitorum, fascia of the fore- 

 arm, and annular ligament. By its deep surface with the ulna, inter- 

 osseous membrane, radius, tendons of the extensor carpi radialis longior 

 and brevior, and supinator longus, and at the wrist with the radial 

 artery. By its upper border with the edge of the supinator brevis. 

 By its lower border with the extensor secundi and primi internodii. The 

 muscle is crossed by branches of the posterior interosseous artery and 

 nerve. 



The EXTENSOR PRIMI INTERNODII POLLICIS, the smallest of the 

 muscles in this layer, arises from the interosseous membrane and radius, 

 and passes through the same groove with the extensor ossis metacarpi, 

 to be inserted into the base of the first phalanx of the thumb. 



Relations. The same as those of the preceding muscle with the ex- 



