thumb

The thumb, or pollex, is the short, opposable digit of the hand. In humans it sits opposite the little finger and is capable of a wide range of motion relative to the other digits. Anatomically, the thumb has two phalanges (proximal and distal) and a metacarpal base that forms the carpometacarpal joint with the trapezium. It lacks a middle phalanx, unlike the other fingers. The carpometacarpal joint is a saddle joint that allows precise movement and opposition; the metacarpophalangeal and interphalangeal joints provide flexion and extension. The thumb’s tip can touch the other fingertips, a movement called opposition, crucial for gripping and manipulation.

Functionally, the thumb coordinates with the other fingers to perform both power grips and precision grips. Its range of movement includes flexion, extension, abduction, adduction, and rotation at the carpometacarpal joint, enabling tasks from holding a hammer to manipulating small objects. The thumb plays a major role in dexterity, tool use, writing, and tactile exploration.

Muscles and nerves: The thenar muscle group—abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis—controls thumb movement, while the adductor pollicis brings the thumb toward the palm. These muscles are primarily innervated by the median nerve (recurrent branch) for the thenar group, with the adductor pollicis receiving innervation from the deep branch of the ulnar nerve.

Evolution and variation: The opposable thumb is a distinguishing feature of humans and many primates, enabling sophisticated manipulation. Variations include differences in thumb length, curvature, and joint mobility; abnormalities such as polydactyly or hypoplasia can affect grip. Thumb injuries, including fractures and tendon injuries, are common in sports and accidents.

Etymology: The term thumb derives from Old English thuma, while the anatomical term pollex comes from Latin.