hand, grasping organ at the end of the forelimb of certain vertebrates that exhibits greatmobility and flexibility in the digits and in the whole organ. It is made up of thewrist joint, thecarpal bones, themetacarpal bones, and thephalanges. The digits include a medial thumb (when viewed with the palm down), containing two phalanges, and four fingers, each containing three phalanges.

The major function of the hand in all vertebrates excepthuman beings is locomotion; bipedal locomotion in humans frees the hands for a largely manipulative function. In primates the tips of the fingers are covered byfingernails—a specialization that improves manipulation. The palms and undersides of the fingers are marked by creases and covered by ridges calledpalm prints andfingerprints, which function to improvetactile sensitivity and grip. The friction ridges are arranged in general patterns that are peculiar to each species but that differ in detail. No two individuals are alike, and in humans the patterns are used for identification. Thethumb is usually set at an angle distinct from the other digits; in humans and the great apes it rotates at thecarpometacarpal joint, and it is therefore opposable to the other fingers and may be used in combination with them to pick up small objects.

Among theapes and some New Worldmonkeys, the hand is specialized forbrachiation—hand-over-hand swinging through the trees. Digits two to five are elongated and used in clasping tree limbs; the thumb is reduced and little used in swinging. Terrestrial monkeys, such as thebaboon, do not have reduced thumbs and can carry out precise movements with fingers and opposing thumb. The development ofdexterity in the hands and increase in brain size are believed to have occurred together in theevolution of humans.

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The Human Body