The Thenar Muscles
The ball of the thumb is called the thenar (THEEN-ar
) eminence. The thenar group is responsible for drawing the thumb in front of the palm and rotating it to oppose the fingers, and thus includes flexion, adduction, abduction, and rotation of the thumb.
The surface muscles of the ball of the thumb are abductor pollicis brevis and flexor pollicis brevis. (Pollicis, pronounced PAHL-iss-iss,
refers to the thumb.) They originate on the tubercles of the scaphoid and trapezium bones, as well as a wide bracelet of tendon over the carpus called the flexor retinaculum (ret-in-NAC-you-lum
). They insert onto the base of the first phalanx of the thumb. These muscles are covered by an appreciable fat pad.
Also part of the thenar group is adductor pollicis. This muscle originates on the third metacarpal from the base to the head, and inserts on the base of the proximal phalanx of the thumb. Action: adducts the thumb and draws it in front of the palm.
On thumb-side of the palm the straight form of first dorsal interosseus is crossed by the diagonal form of adductor pollicis. It appears this way on either side of the flattened hand.
Adductor pollicis is the muscular component of the webbing
between the palm and the thumb, although most of the visible form here is skin.