Claude Bernard did not get off to a promising start. Born in 1813 in Saint-Julien, France, this son of poor vineyard workers attended a simple village school and originally dreamed of becoming a writer. At 21, he'd already written several plays and set off to Paris, France, to show them around. A well-known literary critic, however, strongly suggested he try a different career and, after some thought, Bernard took the man's advice. He entered the Faculty of Medicine, finished almost at the bottom of his class, but managed to obtain a medical degree in 1843. Four years later, the young man's fortune changed. He became an assistant to François Magendie, one of France's most prominent--and controversial--physiologists.

Unlike most of his contemporaries, Magendie firmly believed that researchers could study the body's reactions in the same way that they studied the reactions of inorganic material. Bernard agreed, learned a great deal...