hand

In primates, one of a pair of structures at the ends of the two arms, each having four fingers, a thumb, and a palm. Primates, especially humans, are unique among animals in having a thumb that can make contact with all the fingers, allowing them to grip objects.

Bones The skeleton of the human hand consists of 27 bones: eight carpals in the wrist, roughly arranged in two rows of four each; five metacarpals, forming the bony support of the palm; 14 phalanges of the fingers, the thumb containing two bones and the others three each.

How the hand works The movements of the hand and fingers depend jointly on various muscles of the forearm, which end in tendons attached to bones in the hand, and on small intrinsic muscles which are totally included within the hand. In humans, the thumb is functionally independent and its fleshy palmar surface can be brought into contact with (or opposed to) the palmar surfaces of all the other fingers. This movement, which, among the primates, is most expertly performed by humans, as they have proportionally the longest thumb set at the widest angle to the index finger, forms the basis of the precision grip used to carry out delicate manipulations. The other working position of the hand is the power grip, in which the thumb and fingers hold the object against the palm. This is used when force is required. The wrist is extended (the hand bent backwards) and the long flexor muscles of the forearm contract to flex the fingers towards the palm and close them around the object to be gripped. The thumb is placed along the shaft of the tool and reinforces the pressure of the fingers.