332 THE MUSCLES 



anterior interosseous and radial arteries, wrist, metacarpo-phalangeal, and phalan- 

 geal joints. 



Variations. — Tiiis muscle may seiul a slip to the base of the first phalanx of the tliumb. It 

 may also receive a teudiuous slip from the following muscle. 



5. EXTENSOR INDICIS 



The extensor indicis — named from its action upon the index finger — is flat 

 and fusiform. 



Origin. — (1) An elongated impression -which stretches downwards and slightly 

 outwards from the junction of the middle and lower thirds of the outer division of 

 the posterior surface of the ulna, inmiediately internal to the impression for the 

 preceding muscle, but at the same time rather lower down, to within a short dis- 

 tance of the lower extremity of that bone. (2) For a short space from the part of 

 the interosseous ligament which is adjacent to the lower part of this impression. 

 (3) The septum between it and the preceding muscle. 



Insertion. — The inner side of the aponeurosis of the first tendon of the extensor 

 connnunis digitorum. 



Structure. — This penniform muscle arises by fleshy fibres which pass oliliquely 

 downwards and outwards to the anterior surface of a tendon which is first seen 

 upon the radial border of its posterior surface in the lower third of the forearm, and, 

 becoming free just above the Avrist, passes beneath the tendon of the extensor 

 minimi digiti, and enters the fourth compartment of the posterior annular ligament, 

 where it lies beneath the inner tendons of the extensor communis digitorum. It 

 is then inserted into the inner edge of the aponeurosis of the tendon of the com- 

 mon extensor belonging to the index finger, at about the level of the metacarpo- 

 phalangeal joint. 



Nerve-supply. — From the posterior cord of the brachial plexus (through the 

 seventh cervical nerve), by a branch from the posterior interosseous division of the 

 musculo-spiral nerve Avhich enters the upper part of the muscle near the radial 

 border of its superficial aspect. 



Action. — It assists the extensor communis digitorum in (1) extending the 

 index finger, and especially its first phalanx. At the same time, on account of its 

 direction, it Avill (2) adduct the index finger, while the action of the extensor 

 communis digitorum upon this finger is that of extension combined with abduction. 

 It will then assist feebly in (3) the extension of the Avrist. 



Relations. — Superficially, the extensor communis digitorum, the extensor 

 minimi digiti, extensor carpi ulnaris, and posterior annular ligament. Deeply, 

 the dorsal interosseous muscle of the second space, the wrist, and some carpal 

 joints. 



Variations. — The extensor indicis may give slips to the extensor longus pollicis and to the 

 middle finger. It may receive slips from the posterior carpal ligaments or the bases of the meta- 

 carpal bones, and these slips may be inserted into several digits, so as to form an extensor brevis 

 digitorum maniis. 



THE FASCIA OF THE HAND 



The dorsal fascia is a thin layer, continuous with the posterior annular liga- 

 ment, and, like it, composed chiefly of transverse fibres. It covers the extensor 

 tendons, and connects them together upon the back of the hand; and upon the first 

 phalanges it blends with the aponeuroses which succeed to the tendr)ns. Between 

 the fingers it di])s down to join the superficial transverse ligament which forms the 

 web by which the liases of the fingers are connected. 



A deeper layer of the dorsal fascia covers the back of the dorsal interossei, and 

 is attached to the l)ack of the metacarpal Ijones. 



