thumbs

The thumb is the short, thick first digit of the hand, set apart from the other fingers by its unique range of motion and its role in grasping. It consists of two phalanges (proximal and distal) attached to the first metacarpal bone. The thumb’s base forms a saddle-shaped carpometacarpal joint that enables a wide arc of movement, including opposition, where the thumb tip can touch the other fingertips. The thumb also participates in flexion, extension, abduction, and adduction at multiple joints, including the metacarpophalangeal and interphalangeal joints.

Opposition and precision grip are the defining functions of the thumb. Together with the other fingers, the thumb allows both powerful grips and fine, controlled pinches. Movements are produced by several muscle groups, especially the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) and the adductor pollicis. The intrinsic muscles are innervated mainly by the median nerve through the recurrent branch, except for the adductor pollicis, which receives motor input from the ulnar nerve.

Evolutionarily, the opposable thumb is a key feature of humans and many primates, enabling sophisticated manual tasks and tool use. The degree of thumb mobility varies among species, with humans generally possessing a highly versatile base joint and longer, more opposable thumbs.

Clinical relevance includes conditions such as basal joint (thumb base) osteoarthritis, which can impair grip and pinch, and De Quervain tenosynovitis, which affects the tendons near the thumb. Thumb injuries and congenital anomalies like polydactyly also affect dexterity and hand function.

Culturally, the thumbs-up gesture commonly signals approval in many societies, though meanings can vary across cultures.