How your stomach digests food: the man with a window into his stomach

Your lunch lands in your stomach, a bag of muscle that stretches to make room. Two hundred years ago, doctors still argued about what a stomach did to food. Some thought it ground food, like a mill.

In 1822, at a trading post in North America where people bought and sold furs, a gun went off by accident. It hit Alexis St. Martin, a young canoe man of the fur trade, in the side. The army doctor, William Beaumont, did not think he would live.

He lived. But the wound never closed all the way. A small opening stayed, from his skin straight into his stomach.

So Beaumont used it as a window. From 1825, he tied bits of food to a silk string, put them into the stomach, and pulled them out every few hours to see what was left.

Then came his most famous test. He took some juice out of the stomach and put it in a cup with a piece of meat. He kept the cup warm, and the meat came apart in the cup too. No stomach, no grinding: the juice alone did it. Digestion is chemistry.

Today we know what is in the juice. It holds a strong acid and an enzyme called pepsin. The protein of your chicken is a chain folded up in a tight ball. The acid unfolds it, and pepsin cuts it into shorter pieces.

So why does the acid not digest the stomach itself? A thick layer of mucus covers the wall and keeps the acid away. And the cells under it are replaced every few days.

For hours, the stomach squeezes and churns, and your lunch turns into a thick soup called chyme. A ring of muscle at the exit lets it out a little at a time, in small squirts.

After all that work, almost none of your lunch has crossed the stomach wall. It is still in the tube, so it is still outside your body, like the bean in the donut hole. The real work waits in the next part.