The front border of the shaft of the third metacarpal bone.
- The inner side of the base of the first phalanx of the thumb ;
- the inner side of the aponeurosis of the extensor longus pollicis which cover the first phalanx.
Arising by short tendinous or fleshy fibres, the muscle converge upon a short tendon, which blends on its outer side with the inner tendon of the flexor brevis pollicis, and sends a fibrous slip to the inner side of the aponeurosis of the extensor longus pollicis.
From the lower cord of the brachial plexus (through the eight cervical nerve), by the deep branch of the ulnar which sends filaments to the upper border and deep surface near the origin.
- To flex the first phalanx of the thumb ;
- to adduct and fle the carpo-metacarpal joint of the thumb ;
- to extend the second phalanx of thi thumb.
This latter action of the adductor and abductor pollicis is useful in man of the movements of the thumb, in which it will be found that the second phalan has to be kept extended, while its palmar aspect is being opposed to the tips o the other digits.
Superficially, some of the tendons of the flexor profundus digitorui and the two outer lumbricales ; deeply, the abductor indicis and the interossei (the space between the second and third metacarpal bones). Part of the lower border is subcutaneous.
The adductor pollicis is often difficult to separate from the inner head o the flexor brevis poUicis, the adductor obliquus, as it is sometimes called.