Sex Sleep Eat Drink Dream by Jennifer Ackerman

nineteen-year-old Canadian trapper named Alexis St. Martin. Beaumont, a U.S. Army surgeon, was called one June morning in 1822 to treat St. Martin for a large wound in the abdomen. The poor trapper had found himself at the wrong end of a shotgun. The gun had fired accidentally and struck him at a distance of only three feet, "literally blowing off integuments and muscles of the size of a man's hand," wrote Beaumont. The gaping wound was such that the trapper's death seemed certain. Despite great loss of blood and days of high fever, St. Martin survived. But the injury left a permanent hole in his stomach, a kind of valve the size of a forefinger that had to be plugged so food wouldn't ooze out during meals. The hole allowed Beaumont to see inside St. Martin's stomach to a depth of five or six inches and to conduct more than a hundred groundbreaking experiments on the workings of the stomach, its secretions, and the process of digestion.
    "Pure gastric juice ... is a clear, transparent fluid; inodorous; a little saltish; and very perceptibly acid," wrote Beaumont. "It is the most general solvent in nature ... even the hardest bone cannot withstand its action." It's true. The gastric juice sloshing about in your core is one powerful brew, made of pepsin, an enzyme that breaks down the proteins in food, and hydrochloric acid—a substance so caustic it can demolish bacteria and dissolve iron—which provides the acidic environment that pepsin requires to do its work. Smelling or tasting food, or just thinking about it, stimulates cells in the lining of the stomach to secrete hydrochloric acid. Among the stomach's more famous feats is its ability to digest, say, boiled beef, with the help of this acid without burning up its own tissue or digesting itself—a talent it owes to its inner walls, which possess a layer of mucus and bicarbonate that shields it from its own corrosive contents. When gastric juice leaves the protected environment of the stomach and backs up into the esophagus, the result is the painful sensation of heartburn. If occasional, this backup is only bothersome, but if frequent, it's dangerous, as gastric juices can erode or destroy the lining of the esophagus. The production of these juices is lowest in the morning and peaks from about 10 P.M. to 2 A.M., which explains why peptic ulcers act up and heartburn flares during these hours.
    Despite its special equipment, your stomach is dispensable. An effective storage facility and preparer of food for digestion, kneading it into small particles, pulverizing and sterilizing it, the stomach otherwise plays just a small part in the actual process of digestion and virtually no part in absorption (except of certain drugs, such as alcohol and aspirin). The work of absorbing takes place through finger-like projections in the intestines called villi.
    These days, detailed study of digestion no longer requires a bullet hole; with special scopes and chemical tools we can observe in the murky fundus or dim niches of the duodenum, even in the tiny villi, events occurring at the level of individual cells and molecules. We can track these activities over time, listen to signals flying to and from the gut, and gawp at its unexpected "intelligence."
    That we digest our meals without taxing the brain is largely due to an independent, self-sufficient "brain within the belly," according to Michael Gershon of Columbia University. The brain in the head controls what goes on at the top and bottom of the digestive system, but what happens in between is managed primarily by what Gershon calls the "brain gone south."
    Inside the thirty-two-foot tube of your intestinal tract lies an intricate web of millions of nerve cells that runs things, controlling both the movement and the chemistry of digestion. Only in the past few years have scientists begun to unravel the secrets of this intelligent network, known as the enteric nervous system. Gershon was among the first to suggest that the system