MUSCLES OF THUMB. 277 



Between botli flexor tendons and the bones are thin membranes, one for 

 each. By means of this each tendon is connected with the capsule of the 

 joint, and the fore part of the phalanx immediately behind the bone into 

 which it is inserted. 



The lumbricales muscles (fig. 84, i) are four small fleshy slips, which 

 arise from the tendons of the deep flexor near the annular ligament ; 

 and are directed to the radial side of each finger, to be inserted into an 

 aponeurotic expansion on the dorsal aspect of the metacarpal phalanx 

 (fig. 82, c). 



These muscles are concealed for the most part by the tendons and ves- 

 sels that have been removed; but they are subcutaneous for a short distance 

 between the processes of the palmar fascia. The outer two arise from 

 single tendons, but each of the others is connected with two tendons. 



Action. By their insertion into the long extensor tendon they will aid 

 it in straightening the two last phalangeal joints ; and when the metacarpo- 

 phalangeal joints are much bent they may assist in maintaining the flexion 

 of these articulations. 



Tendon of the flexor pollicis longus. Beneath the annular ligament this 

 tendon is external to the flexor profundus, and turns outwards between the 

 heads of the flexor brevis pollicis (fig. 84), to be inserted into the last 

 phalanx of the thumb. The common synovial membrane surrounds it be- 

 neath the annular ligament, and sends a prolongation, as before said, into 

 its digital sheath. 



Dissection (fig. 84). The deep palmar arch of the radial artery, with 

 the deep branch of the ulnar nerve, and the interossei muscles, will come 

 into view if the flexor profundus is cut above the wrist, and thrown with 

 the lumbricales muscles towards the fingers ; but in raising the tendons 

 the student should preserve two fine nerves and vessels entering the two 

 inner lumbrical muscles. 



The dissection of the muscles of the ball of the thumb and the little 

 finger is next to be prepared. Some care is necessary in making a satis- 

 factory separation of the different small thumb muscles ; but those of the 

 little finger are more easily defined. 



SHORT MUSCLES OF THE THUMB (fig. 84). These are four in number, 

 and are named from their action on the thumb. The most superficial is 

 the abductor pollicis ; beneath it is the opponens pollicis, which will be 

 recognized by its attachment to the whole length of the metacarpal bone. 

 To the inner side of the last is the short flexor. Arid the wide muscle 

 coming from the third metacarpal bone is the adductor of the thumb. 



The ABDUCTOR POLLICIS, A, is about an inch wide, and is thin, and 

 superficial to the rest. It arises from the upper part of the annular liga- 

 ment at the radial side, and from the ridge of the os trapezium ; and is 

 inserted into the base of the first phalanx of the thumb. 



The muscle is subcutaneous, and rests on the opponens pollicis : it is 

 connected oftentimes at its origin with a slip from the tendon of the ex- 

 tensor ossis metacarpi pollicis. 



Action. It removes the metacarpal bone of the thumb from the other 

 digits ; and when it has so acted it may assist slightly the short flexor in 

 bending the metacarpo-phalangeal joint. 



Dissection. The opponens pollicis will be seen on cutting through the 

 abductor. To separate the muscle from the short flexor on the inner side, 

 the student should begin near the fore part of the metacarpal bone, where 

 there is usually a slight interval. 



