Interesting article on thumbs in Massage Today.
The thumb, or first digit, is made up of two phalanges and associated soft tissue. It is attached to the wrist at the first carpometacarpal (CMC) joint. This is a classic saddle joint where the base of the proximal first phalanx and the distal surface of the trapezium are reciprocally saddle-shaped. This allows for considerably more degree of motion than found with any of the other four digits.
The motions allowed at this joint are flexion/extension; abduction/adduction; opposition/apposition/reposition; and circumduction.