Hand (anatomy)

Hand (anatomy), terminal portion of the arms or anterior limbs of humans and other primates, especially adapted for grasping. The grasping appendages of other mammals and lower forms of animals are sometimes called hands in order to distinguish them from the feet of the hind limbs, but true hands appear only in the primates.

Superficially the hand consists of a broad palm attached to the forearm by a joint called the wrist. At one side and at the outer edge of the palm are five digits, the thumb and four fingers. The thumb in humans and some other primates is articulated so that it can be brought more or less opposite to the fingers and thus be employed for grasping tree branches and small objects. The fingers themselves can be folded forward over the palm for the holding of objects.

The human hand has 27 bones: the 8 bones of the carpus, or wrist, arranged in two rows of four; the 5 bones of the metacarpus, or palm, one to each digit; and the 14 digital bones, or phalanges, 2 in the thumb and 3 in each finger. The carpal bones fit into a shallow socket formed by the bones of the forearm.

The movements of the human hand are accomplished by two sets of muscles and tendons: the flexors, for bending the fingers and thumb, and the extensors, for straightening out the digits. The flexor muscles are located on the underside of the forearm and are attached by tendons to the phalanges of the fingers. The extensor muscles are on the back of the forearm and are similarly connected. The human thumb has two separate flexor muscles that move the thumb in opposition and make grasping possible.

The articulation of the human hand is more complex and delicate than that of comparable organs in any other animals. Because of this articulation, only humans are able to use and manipulate a wide variety of tools and implements.