THE STOMACH

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.