ABDUCTOR POLLICrS 339 



Nerve-supply. — From the inner cord of the brachial pk-xu.s (through the 

 eighth cervical nerve), by In-anches from the deep division of the ulnar nerve Avhich 

 enter the muscles at the ui)])er part <^f their anterior aspect. 



Action of the interossei muscles. — First, the common action of all the interossei 

 is (1) to tlex the tirst })halanges; and (2) to extend the second and third. Their 

 power of flexion depends upon their insertion into the sides of the first phalanges 

 as well as the edges of the aponeuroses by which the backs of these phalanges are 

 covered. The extension of the second and third phalanges depends upon th^ fact 

 that the portion of the aponeurosis into which each of them is inserted is continued 

 downwards to the bases of these phalanges. A good example of the action of these 

 muscles and of the lumljricales is given in the movements of the fingers which hold 

 the pen in writing. In forming the light upstroke it will be seen that flexion of 

 the flrst, accompanies the extension of the second and third phalanges. This 

 movement is due to the contraction of the lumbricales and interossei. On the 

 other hand, the thick downstroke is formed b}'' the extension of the first, and 

 the flexion of the second and third phalanges; this strong movement being due to the 

 action of three powerful muscles, the extensor communis, the flexor sublimis, and 

 the flexor })rofundus digitorum. Secondly, the palmar interossei adduct the index, 

 ring, and little fingers towards the middle line of the hand which passes through the 

 middle finger. Thirdly, the dorsal interossei abduct the index, middle, and ring 

 fingers from the middle line of the hand. As the middle finger can be drawn either 

 to the radial or ulnar side from the middle line of the hand, it has two dorsal inter- 

 ossei to abduct it. The first dorsal interosseous muscle will also assist in adducting 

 the thumb. 



Relations. — On the palmar surface of the interossei, separated from them by 

 the deep layer of fascia, lie the flexor profundus digitorum tendons and the lumbri- 

 cales, the adductor and part of the short flexor of the thumb, the deep palmar arch 

 above, and the deep transverse ligaments beloAv; behind, lie the tendons of the 

 extensor communis digitorum and extensor indicis, separated from the dorsal inter- 

 ossei by the deep dorsal fascia. 



Variations. — Sometimes the dorsal interossei receive small accessory heads from the posterior 

 carpal ligaments, the lower end of the radius, or the adjacent deep fascia. A jjart of the first 

 dorsal interosseous has been seen running separately from the front of the metacari)0-phalangeal 

 joint of the thumb to the tendon of the first lumbricalis. Occasionally the second dorsal inter- 

 osseous is inserted upon the ulnar side of the index finger, so as to resemble the same muscle in 

 the foot. 



Muscles of the Thenar Eminence 



These are four in number, and they form a fleshy mass at the radial border of 

 the palm, covered by the thenar portion of the palmar fascia, and called the thenar 

 eminence or ball of the thumb. They are the abductor pollicis, the opponens 

 pollicis, the flexor brevis pollicis, and the adductor i)ollicis. 



1. ABDUCTOR POLLICIS 



The abductor pollicis — named from its action upon the thuml) — is a small 

 thick triangular sheet. 



Origin. — (1) The tuberosity of the scaphoid Ijone; (2) the upper ]iart of the 

 ridge on the trapezium; (3) the outer part of the front of the anterior annular 

 ligament; (4) the outer or thenar division of the ]ialmar fascia by which the ball 

 of the thumb is covered; and (5) the slip from the tendon of the extensor ossis 

 metacarpi pollicis which joins this part of the palmar fascia. 



Insertion. — (1) With the outer tendon of the flexor brevis pollicis into the 

 radial side of the base of the first ]ihalanx of the thumb; and into (2) the outer edge 

 of the aponeurosis of the extensor longus pollicis upon the back of the first phalanx. 



Structure. — Arising 1w fleshy or short tendinous fibres from the anterior 

 annular ligament and some or all of the other four origins, the muscle converges 

 upon a short tendon which l)lends with the adjacent tendon of the fiexor brevis. 



