Human Hands and Chicken Feet
A simple kitchen project illustrating how tendons control your hand like strings controlling a marionette.
Twenty-seven small bones give shape and sturdiness to your hand. You can feel them right underneath the covering of skin, although the pebble-bones in the base of your hand (carpals) all fit neatly together, and from the outside they feel like one big block of bone. In fact, your hand is mostly bone, except for a flexible covering of skin, and a few smallish blobs of muscle and padding. But what are these wires that keep popping out underneath the skin? You can feel a few of them running lengthwise along the top of the stick-bones in the back of the hand (metacarpals). They pop from side to side as you run your fingers across the back of your hand, and they really get taut and pop out when you try to draw your fingers back. There is one cable running down into the wrist from each finger, along the top of each metacarpal. There are similar wires at the base of the thumb, and they tighten whenever you try to draw your thumb back.
Just as there is a cord in the back of the hand for each finger, so there are corresponding cords in the palm of the hand, although they are buried under padding and are harder to find, and instead of tightening when you try to straighten (extend) your fingers, they tighten up whenever you clench (flex) your fingers. There are also numerous cords running through the wrist. Some of these cords become especially pronounced when you try to draw your wrist forwards.
All of these wires are called tendons, from a Greek noun meaning “sinew”, which in turn derives from a Greek verb meaning “to stretch”. You can see how they function as drawstrings for working the fingers and the wrist, because they tighten and pop up whenever you try to make a motion with your hand. We can't really test this idea by separating a human hand and pulling on the tendons from the outside, but there is something much like a detached human hand, with tendons sticking out, that is cheap and readily available...
Chicken feet are not common in American cuisine, but they are popular in certain ethnic cuisines. I buy mine at a local Chinese supermarket, but I have also found them at Whole Foods. They provide a wonderful analog to the human hand — they have one fewer “finger” than a human hand does, but they have very human-looking digits, with knuckles, and the claws even look like fingernails. Furthermore, the severed tendons are exposed at the cut, allowing us to pluck them and see what happens.
(Depending on exactly how the butcher separated the foot from the rest of the chicken, the tendons might be sticking out obviously, or you might have to dig a little bit with tweezers. And the bone might be cleanly cut with a tubular end, or it might be crushed with fragments sticking out. Bone fragments may resemble tendons at a casual glance, except they will be crunchy instead of limp. In a very cleanly cut leg, you should be able to find a tubular bone, with tendons on the upper side that cause the toes to extend, and tendons on the lower side that cause the toes to curl. When I demonstrated the action of chicken tendons with my classes, I could usually find one tendon that caused each toe to curl, and another somewhat larger tendon that caused the entire claw to close.)
So a chicken foot is like a marionette. It has these tendons running into it, and whenever something outside the foot pulls on a tendon, the tendon pulls one or more of the toes and causes it to move. Your hand probably works the same way — the muscles in your forearm pull on your tendons, causing them to get taut and to pull on the finger on the other end. Your hand is like a puppet, and your forearm is the puppetmaster. (If we wanted to get a better look at the puppetmaster, could we dissect a chicken wing or a chicken leg?
A Robot Finger
Each student can quite easily make a model that demonstrates this principle of operation. Break or cut a popsicle stick into three pieces, then put them back together again and wrap adhesive tape around the joints. Now you have a bendy stick, and each piece represents one of the bones (phalanges) in your finger. (If you've talked about ligaments, you can point out that the tape represents the ligaments in the knuckles.) Next you have to install a tendon to make the finger move: Tape small pieces of a soda straw to the flat side of each section, being careful not to overlap the joints. Pass a piece of yarn through the straw segments and tape it to the back of the short section on the end (i.e. the “fingertip”, or “distal phalanx”). Now when you pull on the yarn, it should make the stick curl. (The sections of straw are mainly to hold the yarn against the stick, but even these have a rough analog in a real finger: tendon tunnels. The third straw on the fingertip is probably not necessary, but don't just tape the yarn to the front of the last stick. Unless you wrap it over the top and tape it to the back, or use strong glue, it will pull away much too easily.)