326 THE MUSCLES 



tendons of the index and middle fingers above the heads of the metacarpal bones. 

 A stronger band passes tlownwards and outwards from the ring finger tendon to 

 that of the middle finger at the same level. The fourth tendon divides into two 

 parts. The one joins with the tendon of the ring finger, which immediately after- 

 wards gives off a transverse band to the extensor tendon of the little finger, and the 

 other part joins the tendon of the extensor minimi digiti upon the metacarpal bone 

 of the little finger. 



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

 seventh cervieal nerve) by several branches from the posterior interosseous division of 

 the musculo-spiral nerve which enter the upper part of the deep surface of the muscle. 



Action. — (1) Chiefly to extend the first phalanges of the fingers. (2) It has 

 some power of extending the second and third phalanges; but this part of the 

 tendon of the muscle is chiefly under the control of the lumbricales and interossei, 

 which are attached to the sides of the expansion covering the first phalanx. After 

 extending the fingers, it will also (3) help in the extension of the wrist; and (4) to 

 a slight extent, of the elliow-joint. The bands by which the tendons are attached 

 to one another upon the back of the hand hinder independent extension, especially 

 in the ring finger. The little and index fingers, having special extensors, do not 

 laliour imder this disadvantage. 



Relations. — Superficially, the deep fascia and posterior annular ligament; 

 deeply, the supinator brevis, the extensors of the thumb, the extensor indicis, the 

 dorsal interossei, the posterior and anterior interosseous arteries, the posterior inter- 

 osseous nerve, the wrist and carpal joints; on the radial side is the extensor carpi 

 radialis brevior; and on the ulnar, the extensor minimi digiti. 



Variations. — Some of the tendons may be deficient, or more frequently the muscle divides 

 into more than four tendons, two or even three of which may be attached to one finger. 

 Occasionally a tendon joins that of the extensor longus poUicis. 



2. EXTENSOR MINIMI DIGITI 



The extensor minimi digiti — named from its action upon the little finger — is 

 small and fusiform. 



Origin. — (1) The common tendon from the back of the external condyle by a 

 long fibrous process; (2) the deep fascia; and (3) the intermuscular septa which 

 intervene between it and the adjacent muscles. 



Insertion. — With the corresponding tendon of the preceding muscle. 



Structure. — Arising from the interior of the elongated case formed by the 

 various aponeuroses which diverge from the back of the external condyle, the 

 fleshy fibres, which do not begin till some distance below that ])oint, are inserted 

 into the radial border of a tendon which begins about the middle of the forearm, 

 and becomes free a short distance above the wrist-joint, a little higher than that of 

 the corresponding part of the preceding muscle. Passing through the fifth com- 

 partment of the posterior annular ligament, which lies upon the line of junction of 

 the radius and ulna, the tendon reaches the back of the fifth metacarpal bone, and 

 there blends with the fourth tendon of the extensor communis digitorum immedi- 

 ately al)ov(; the mctacarpo-phalangeal joint. 



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

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

 musculo-spiral which enters the deeji surface of the upper part of the muscle. 



Action. — (1) To extend the first phalanx of the little finger; (2) to extend the 

 second and third phalanges; (3) to extend the wrist; and (4) very slightly the 

 elbow also. 



Relations. — Superficially, the deep fascia of the forearm and the posterior 

 annular ligament; deeply, the supinator brevis, the extensors of the thumb, and 

 the extensor indicis, the inferior radio-ulnar joint, and the posterior interosseous 

 artery. On the outer side lies the extensor communis digitorum; and on the 

 inner, the extensor carpi ulnaris. 



Variations. — Tlie extensor minimi digiti is rarel}' absent, but it is sometimes blended with 

 the preceding muscle. Its tendon is often divided, sometimes into as many as three slips. 



