William Wise Jr., age 10, of Austin, Texas, for his question:
HOW MANY BONES ARE IN THE HAND?
Hands are specially constructed for taking hold of objects. They are also used to touch and feel things.
The human hand consists of the carpus, or wrist; the metacarpus, or palm proper= and the digits, the four fingers and the thumb. There are 27 bones in the hand.
Eight carpal bones make up the carpus. They are arranged across the wrist roughly in two rows. In the row nearest the forearm, starting from the thumb side, are the navicular, lunate, triangular and pisiform bones.
In the second row are the greater multangular, lesser multangular, capitate and hamate bones. `
Five long metacarpal bones make up the palm. They connect the wrist with the fingers and the thumb.
Each of the four fingers contains three slender bones called phalanges. But the thumb has only two.
Thirty five powerful muscles move the human hand. Fifteen are in the forearm rather than in the hand itself. This arrangement gives great strength to the hand without making the fingers so thick with muscles that they would be difficult to move.
Near the wrist, the muscles become strong, slender cords called tendons. The tendons run along the palm and back of the hand to the joints of the fingers. When the muscles on the palm side of the forearm contract, the fingers close. When the muscles on the back of the forearm contract, the fingers open.
Twenty muscles within the hand itself are arranged so that the hands and fingers can make a variety of precise movements.
The hand contains at least four types of nerve endings that make the fingers and thumbs highly sensitive.
True hands, including those of humans, have opposable thumbs, or thumbs that can be moved against the fingers. This action makes it possible to grasp things in the hand and make many delicate motions. Man's progress would have been hampered if he did not have opposable thumbs.
To prove to yourself how important opposable thumbs are, try to pick up a pencil while keeping your thumb motionless alongside your hand.
In many animals, part of the forelimb corresponds to the human hand. These parts have the same basic arrangement of bones and muscles whether the animal uses them to dig, fly, swim or run.
Animals have many kinds of "hands." The mole's short, chunky "hand" is ideally suited to act like a shovel in digging tunnels. The bat's forelimb is a wing, with a web of skin spread between the fingers. A bird's "hand" is also a wing.
The seal's "hand" is its flipper. The bones have fused, or grown together, and form a broad, flat paddle useful for swimming.
The "hand" of the horse is constructed so that the animal actually stands on its middle finger. Through millions of years of development, the middle finger has become stronger and longer. It is well adapted to running.