NESTLED ALONG THE CLEAR BLUE STRAITS BETWEEN the great lakes called Michigan and Huron is an oblong and verdant island. Centuries ago, the Chippewa and Ottawa tribes named it Michilimackinac, or “the great turtle.” Referred to today as Mackinac but pronounced “Mackinaw,” this tiny land mass attracts tourists by the millions seeking its calming waters, grand hotels, attractive vistas, bicycle paths, and tons of a sticky, sweet confection made of butter, milk, sugar, cocoa, and a touch of vanilla, called Mackinac fudge. In 1670, the French Jesuit missionary Father Jacques Marquette and his intrepid interpreter Louis Joliet fled St. Ignace, Michigan, to settle on the “Great Turtle” in the name of their homeland. Less than a century later, in 1759, the British took control of the island from the French and, in 1783, after signing the Treaty of Paris, Great Britain relinquished it to the fledgling United States, which had to defend and regain it during the War of 1812. The French pursued fur trapping there with vigor and cunning. But it was John Jacob Astor, the first US multimillionaire, who put Mackinac Island on the map in 1817, when Michigan was admitted to the Union and Astor chose the island as the main trading post of his fabled American Fur Company, a pelt empire that spanned from the Great Lakes to the Mississippi River. Every June, the American Fur Company hosted a convention on Mackinac for thousands of trappers eager to sell or barter the bounty they had hunted the previous winter. The rest of the year, the island’s population hovered at a mere 500. Situated on the southeast cliff a few hundred feet above the shoreline was a limestone fortress built by the British in 1761 and subsequently occupied by the US Army to protect the island’s commerce and trade. In 1820, one of the military officers stationed there was a young physician named William Beaumont. In 1810, he began a 2-year apprenticeship to a well-established Vermont physician named Benjamin Chandler and in 1812 passed his state’s qualification examination. That same year, Beaumont enlisted with the US Army in search of adventure and clinical experience and served as a surgeon’s mate in the War of 1812. After the end of that conflict in 1815, he resigned his post to set up a private practice in Plattsburgh, New York. Five years later, he turned his practice over to a cousin and re-enlisted in the Army, which assigned him to Fort Michilimackinac. At this distant frontier outpost, Beaumont began the work that culminated in a remarkable, if not outright revolutionary, book—Experiments and Observations on the Gastric Juice and the Physiology of Digestion. But he hardly accomplished this gargantuan task alone. Indeed, Beaumont had the help and the body of a French Canadian fur trapper named Alexis St. Martin. The story of Beaumont and St. Martin has been recounted so often it has acquired the finely burnished patina of hagiography. Yet even when stripped of its most sensational layers, their collaboration remains an inspiring and cautionary tale about the boundaries between physician and patient and medical investigator and human participant. On the morning of June 6, 1822, as the annual pelt swapping jamboree was under way, a 20-year-old St. Martin waited in line at the American Fur Company’s store to make some trades. What began as a rudimentary exercise in capitalism erupted into a medical emergency when a shotgun accidentally discharged. As one eyewitness described, “the muzzle was not over three feet from [St. Martin]—I think not more than two. The wadding entered, as well as pieces of his clothing; his shirt took fire; he fell, as we supposed, dead.” The gun’s contents entered just under St. Martin’s left breast, fractured several ribs, lacerated his diaphragm, and left a gaping hole through which portions of the lung and stomach protruded. Beaumont quickly arrived and administered first aid amid the stunning sights and smells of St. Martin’s breakfast leaking from the wound. Thanks to Beaumont’s quick-witted and precise ministrations, St. Martin did survive the event, albeit with a fistula leading directly into his stomach that remained patent until his last day of life. It was a medical intervention that would soon change medicine. For much of the next few years, Beaumont provided daily care and attention to his fascinating patient. Although the wound partially healed, St. Martin remained weak and miserable and refused any attempts by Beaumont to somehow suture the hole shut. As a result of his critical injuries and lengthy rehabilitation, St. Martin was also penniless.