400 THE MUSCULAK SYSTEM. 



the radius; and from the intervening portion of the interosseous membrane 

 (Fig. 355, p. 397). 



Becoming superficial in the distal part of the forearm, along with the extensor 

 pollicis brevis, between the extensors of the wrist and the common extensor of the 

 fingers, its tendon passes, with the latter muscle, under cover of the dorsal carpal 

 ligament, to be inserted into the lateral side of the base of the first metacarpal bone 

 (Fig. 356, p. 399). From the tendon, close to its insertion, a tendinous slip passes 

 to the abductor pollicis brevis and the fascia over the thenar eminence, and 

 another is frequently attached to the greater multangular bone. 



Nerve-Supply. The dorsal interosseous nerve (C. (5.) 6. 7. 8.). 



Actions. The muscle abducts the metacarpal bone of the thumb, and assists in abduction 

 and extension of the wrist 



M. Extensor Pollicis Brevis. The extensor pollicis brevis (O.T. extensor 

 primi internodii pollicis), an essentially human muscle, is a specialised portion of 

 the previous muscle. It arises from a rhomboid impression on the dorsal surface of 

 the radius, and from the interosseous membrane, distal to the abductor pollicis 

 longus (Fig. 355, p. 397). It is closely adherent to that muscle, and accompanies it 

 deep to the dorsal carpal ligament and over the radial artery to the thumb. 



Its tendon is then continued along the dorsal surface of the first metacarpal 

 bone, to be inserted into the dorsal surface of the base of the first phalanx of the 

 thumb. Before reaching its insertion the tendon helps to form the capsule of the 

 metacarpo-phalangeal joint. 



Nerve-Supply. The dorsal interosseous nerve (C. (5.) 6. 7. 8.). 



Actions. The muscle extends the wrist and thumb (or the metacarpo-phalangeal joint), 

 and assists in abduction of the wrist and thumb. 



3VL Extensor Pollicis Longus. The extensor pollicis longus (O.T. extensor 

 secundi internodii pollicis) arises from the lateral part of the dorsal surface of 

 the ulna, in its middle third, and from the interosseous membrane, distal to the 

 abductor pollicis longus (Fig. 355, p. 397). Its tendon grooves the dorsal surface 

 of the radius, and occupies a special compartment under cover of the dorsal carpal 

 ligament. 



Extending obliquely across the dorsal surface of the hand, the tendon crosses 

 the radial artery, helps to form the capsule of the first metacarpo-phalangeal 

 articulation, and is inserted into the dorsal surface of the base of the second phalanx 

 of the thumb. 



At the wrist the tendons of the muscles of the thumb, the abductor pollicis longus 

 and extensor pollicis brevis laterally, and the extensor pollicis longus medially, 

 bound a hollow (the " anatomical snuff-box ") best seen in extension and abduction 

 of the thumb, which corresponds to the position of the radial artery as it winds 

 round the wrist to reach the palm of the hand. 



Nerve-Supply. The dorsal interosseous nerve (C. (5.) 6. 7. 8.). 



Actions. This muscle is an extensor and an abductor of the thumb, and of the wrist. 



M. Extensor Indicis Proprius. The extensor indicis proprius (O.T. extensor 

 indicis) arises, distal to the extensor pollicis longus, from the most distal impression! 

 on the dorsal surface of the ulna, extending distally from the middle of the body! 

 to within two inches of its distal end, and sometimes also from the interosseoufl 

 membrane (Fig. 355, p. 397). Its tendon passes through a compartment of thel 

 dorsal carpal ligament along with the tendons of the extensor digitorum communis i 



On the dorsum of the hand the 'tendon lies on the medial side of the tendon o: 

 the common extensor destined for the forefinger, and is inserted into the forefinger | 

 joining the membranous expansion of the tendon of the extensor digitorunn 

 communis on the dorsum of the first phalanx. 



Nerve-Supply. The dorsal interosseous nerve (C. (5.) 6. 7. 8.). 

 Actions. The muscle is an extensor of the wrist and forefinger. 



