DEAR DOCTOR K:
A friend told me that we have no muscles in our fingers. Is that true? If so, then how do our fingers do all that they do?
It is true, but our hands work wonderfully anyway. That’s because even though there are no muscles in the fingers, 34 muscles in the palms and forearms make the fingers work.
And our fingers perform a remarkable variety of feats, from the practical (opening doors and typing), to conveying information (through sign language or applause), to gathering information about the environment through our sense of touch.
It’s no wonder, then, that our hands have a complex design. Each hand has 27 bones and a corresponding number of joints (the spaces where two bones meet). Together, the bones in our hands make up nearly a quarter of the total number of bones in our body.
Of course, the muscles in our palms and forearms make the fingers work only when the brain tells them to. So many subtle and important things are done by our hands that about one-fourth of the part of the brain that controls body movement is devoted to controlling our hands.
So how do we hold a pot, open a door or play the piano? It begins in the brain, which sends messages down the nerves that connect to the muscles in our palms and forearms. Those messages tell specific muscles to tighten and others to relax. Three major nerves control the movements of our hands.
The muscles are all connected to tendons. Those tendons connect each muscle to specific bones in our fingers. Tendons are strong, connective tissue fibers. They are the things that finally move your fingers the way you want them to move.
When a muscle contracts, it pulls on the tendon, which then pulls on the bone and moves it. (I’ve put an illustration showing the anatomy of our hands below.)
Try this: Stick out your right hand directly in front of you, with the palm facing the floor and fingers outstretched. Now, keeping your hand straight out in front of you, make a fist. Did you feel the muscles in your forearm, just below the elbow, get tense?
Ligaments, bands of fibrous tissue that bind bones together and keep them properly lined up, are also important to the function of your hand. Ligaments on either side of the fingers prevent them from bending too far to the side. Those that stretch across the palm keep your fingers from bending too far back.
You may have heard that an opposable thumb is a key anatomical difference between humans and animals. An opposable thumb means that the thumb can oppose, or touch, the index finger. Actually, many animals — chimpanzees, koalas, even opossums — have opposable thumbs.
However, it actually is our unique ability to also oppose our other fingers to the thumb — and to thereby strongly grip and grasp objects — that makes us special.
Inside the hand
Under the skin, the hand’s 27 bones and 34 muscles work in synchrony to perform a range of movements, from a powerful hammer blow to a gentle caress.
The main bone of the hand is the metacarpal, which connects to the finger bones, or phalanges. The knuckle joint that connects these two bones is the metacarpophalangeal (MCP) joint. Each finger has three phalanges and the thumb has two. The middle joint of the finger is the proximal interphalangeal (PIP) joint. The joint near the end of the finger is the distal interphalangeal (DIP) joint. The thumb has only an interphalangeal (IP) joint.
The small but powerful muscles of the hand are located entirely in the palm and forearm. The fingers have no muscles. Fingers are controlled by tendons attached to the muscles of the hand that help to flex and extend them. Two major nerves are the median nerve (leading to the thumb, index, and middle fingers and the inner half of the ring finger) and the ulnar nerve (leading to the small finger and the outer half of the ring finger). The bursa helps cushion the hand.