In structure, the hand has a bony basis of eight small carpal bones in the wrist, ﬁve metacarpal bones in the ﬂeshy part of the hand, and three phalanges in each ﬁnger – two only in the thumb. From the muscles of the forearm, 12 strong tendons run in front of the wrist. Of these, nine go to the ﬁngers and thumb and are bound down by a strong band, the ﬂexor retinaculum, in front of the wrist. They are enclosed in a complicated synovial sheath, and pass through the palm and down the ﬁngers. Behind the wrist, 12 tendons likewise cross from forearm to hand.
Forming the ball of the thumb and that of the little ﬁnger, and ﬁlling up the gaps between the metacarpal bones, are other muscles, which act to separate and bring together the ﬁngers, and to bend them at their ﬁrst joints (knuckles).