The thenar muscles. New findings.

Examination of the thenar muscles in 30 anatomical preparations of the hand have shown that the abductor pollicis brevis, the opponens pollicis, and the adductor pollicis muscles are made up of several muscle bellies. The number and insertions of these bellies are varied. Both heads of flexor pollicis brevis do not originate from any particular muscle belly. The superficial head of this muscle always inserted into the head of the thumb metacarpal, either completely, or, some of the fibres of the dorsal aponeurosis of the thumb were attached to the base of the proximal phalanx. Furthermore the anatomy of the abductor pollicis brevis muscle was related to the presence of a tendinous slip from abductor pollicis longus. These variations could have an influence on proprioception in the thumb ray.