In 1822, William Beaumont, a doctor stationed at Fort Mackinac, Mich., rushed to aid a man who had been accidentally shot in the abdomen. The wound was horrendous. “Not many people would be expected to survive an injury of this magnitude,“ writes F. González-Crussi in “Carrying the Heart: Exploring the Worlds Within Us,“ a fascinating look at our attempts to understand the human body’s inner workings.