EXTENSOR MUSCLES. 217 



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, exjen- 

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

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

 interosseous 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, interosseous 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 ulna- 

 ris, extensor minimi digiti, extensor communis digitorum, fascia of 

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

 ulna, interosseous 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 poste- 

 rior 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 

 exception of the extensor carpi ulnaris. The muscle accompanies 

 the extensor ossis metacarpi. 



The Extensor secundi internodii pollicis arises from the ulna and 

 interosseous membrane. Its tendon passes through a distinct canal 

 in the annular ligament, and is inserted into the base of the last pha- 

 lanx of the thumb. 



Relations. By its external surface with the same relations as the 

 extensor ossis metacarpi. By its deep surface with the ulna, inter- 

 osseous membrane, radius, wrist joint, radial artery, and metacarpal 

 bone of the thumb. The muscle is placed between the extensor 

 primi internodii and extensor indicis. 



The Extensor indicis arises from the ulna as high up as the exten- 

 sor ossis metacarpi pollicis, and from the interosseous membrane. 

 Its tendon passes through a distinct groove in the radius, and is 

 inserted into the aponeurosis formed by the common extensor tendon 

 of the index finger. 



Relations. The same as those of the preceding muscle, with the 

 exception of the hand, where the tendon rests upon the metacarpal 

 bone of the fore-finger and interosseous muscle, and has no relation 

 with the radial artery. 



The tendons of the extensors, as of the flexor muscles of the fore- 



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