thumb /thum/ [AS, thuma] , the first, most lateral digit on the radial side of the hand, classified by most anatomists as one of the fingers because its metacarpal bone ossifies in the same manner as those of the phalanges. Other anatomists classify the thumb separately, noting that it has a much different articulation with the metacarpal bone (a saddle joint) and is composed of one metacarpal bone and only two phalanges. The nerves that innervate the various muscles controlling the thumb include branches of the radial nerve, branches of the median nerve, and the deep palmar branch of the ulnar nerve.
