EXTENSOR PRIMI INTERNODII POLLICIS. 



237 



Dissection. The muscles of the superficial layer Fig 125. 



should be removed in order to bring the deep group 

 completely into view. 



The SUPINATOR BREVIS cannot be seen in its en- 

 lire extent until the radial extensors of the carpus 

 are divided from their origin. It arises from the 

 external condyle, from the external lateral and or- 

 bicular 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 pos- 

 terior interosseous artery and nerve are seen perfo- 

 rating 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 corn- 

 munis 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 membrane, 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, interosse- 

 ous 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 nerv-e. 



The EXTENSOR PRIMI INTERNODII POLLICIS, the smallest of the muscles 

 in this layer, arises from the interosseous membrane and ulna, 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 excep- 

 tion 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 phalanx of the 

 thumb. 



* The deep }ayer of muscles'on the posterior aspect of the fore-arm. 1. The lowei 

 part of the humerus. 2. The olecranon. 3. The ulna. 4. The anconeus muscle. 5 

 The supinator brevis muscle. 6. The extensor ossis metacarpi pollicis. 7. The exten- 

 sor primi internodii pollicis. 8. The extensor secundi internodii pollicis. 9. The ex 

 tensor indicis. 10. The first dorsal interosseous muscle. The other three dorsal inter 

 ossei are seen between the metacarpal bones of their respective fir.gers. 



