The stomach, which resembles a bag, is
about a foot long and six inches wide. Its capacity is about two and a half
pints, and a heavy meal may take as long as six hours to pass through it. The
stomach wall has three layers of muscles-circular, Longitudinal], and oblique.
tach contracts in a different direction, permitting the stomach to squeeze,
twist, and churn its contents, actions that are important.
In addition to mucous glands, it contains other
glands which secrete hydrochloric acid and several enzymes. One enzyme, rennin,
acts on casein, a protein in milk, forming a curd to be digested by other
enzymes. Lipase, an enzyme which splits some fats, including those in cream and
egg yolk, plays a small role in the stomach, a larger one in the intestine.
Hydrochloric acid helps in the digestion of proteins and has other useful
chemical effects.
Another enzyme produced by the stomach, pepsin, helps digest
the milk curd resulting from the action of rennin. What emerges from the
stomach after the activity there is a semifluid material called enthymeme. It
takes little time, a matter of minutes, for fluids -water, beverages of various
kinds-to pass through the stomach. But the rest of a meal spends from three to
as much as six hours in the stomach.
The time is affected somewhat by the nature of the
food. Carbohydrates pass through most quickly; proteins take longer; fats
require the most time. Some fats, in fact, slow the digestive process in the
stomach for other foods by slowing secretion of gastric juices, thus somewhat
prolonging stomach emptying time.
At both ends, stomach muscles form
sphincters, ringlike valves. At the junction of stomach and esophagus, there is
the cardiac sphincter. A similar but stronger valve, the pyloric sphincter,
lies at the lower end of the stomach where it joins the small intestine. The
two valves close the stomach during digestive activities. When chyme is ready
to move on to the intestine, the pyloric sphincter opens and closes several
times to allow the stomach to gradually empty.
One phenomenon associated with
the stomach is worth noting here. Somehow, the stomach, which secretes
hydrochloric acid to digest proteins in foods, is not itself digested by the
acid. How it resists the action of an acid that is capable of dissolving even
iron is not fully understood. Yet it does resist, and it is normal to have a
usual quota of acid in the stomach-this, despite the concern of millions of Americans
who, with the help of constant reminders from the manufacturers of various ant-
acids, spend about $100 million a year to neutralize stomach acid.
