In “Gulp: Adventures on the Alimentary Canal,” Mary Roach explores the much-maligned but vital tube from mouth to rear that turns food into the nutrients that keep us alive. She introduces us to scientists who tackle questions no one else thinks to ask. Why doesn't the stomach digest itself? Can wine tasters really tell a $10 bottle from a $100 bottle? Why do Americans eat, on average, no more than thirty different foods on a regular basis? “Gulp” is as much about human beings as it is about human bodies. We meet a “disgust” researcher, a saliva expert, and one of medicine’s oddest couples: Alexis St. Martin, a French Canadian trapper with a hole gut-shot in his stomach, and William Beaumont, the army surgeon who achieved fame by placing food inside St. Martin to see what happened. We revisit our conversation with Mary Roach on Wednesday’s AU.