Theresa Roche, age 10, of Collingdale, Pennsylvania, for her question:

Why doesn't the stomach digest itself?

This logical question has baffled medical scientists for centuries. The hard working pouch we call the stomach is flooded with juices that digest meat and reduce all sorts of tough fiberous food to mushy pulp. Its walls are living muscular tissue. Yet a healthy stomach manages to do its daily duty    without digesting itself along with its meaty food.

In the last year or so, a group of medical researchers set about finding the solution to this conundrum    once and for all. So far they have verified the known facts about how the stomach performs its impossible work. They also discovered a few new facts and suggested a theory that might explain the mystery. But a definite answer to the old riddle they did not find. They still are trying    and hoping that other teams will help to investigate the many possibilities.

The latest theory suggests that the stomach protects itself from itself with a remarkable chemical barrier. It was already known that glandular tissues deep in the muscular walls pour in hydrochloric acid, which is fine for pulverizing fiberous foods. But this corrosive chemical can destroy living cells and even dissolve zinc. Yet as far as we know, the only living cells annihilated inside a healthy stomach are bacteria. Another mighty chemical is pepsinogen, which becomes the pepsin that breaks tough proteins apart.

These two powerful digestive juices certainly would demolish the muscular stomach    if it had no way to defend itself. If the latest theories are correct, its secret defenses are chemical barriers within the jellified mucus that lines the inside walls of the stomach. This mucosal layer seems to be impervious to the strong digestive juices. Tiny tubelets lead them through it into the stomach and other factors shut off their retreat.

It is thought that the chemical barrier may be operated by positive and negative ions. Hydrochloric acid is ionized in the stomach when its molecules lose hydrogen atoms. The mucus layer contains ions of sodium and ionizes other substances by adding or removing electrons.

Charged particles of this sort attract or repel each other. These ion reactions possibly create chemical barriers by repelling corrosive digestive juices that try to pass through the mucus to the meaty stomach walls.

At present, we cannot give a definite answer to why a healthy stomach does not digest itself. But we know that its defenses can be weakened to allow its strong juices to corrode agonizing ulcers in its muscular walls. This occurs when the mucosal layer becomes unhealthy. The miserable damage may be caused by illness or poor diet, by certain medicines, emotional shocks or stresses.