It didn't look like Alexis St. Martin would survive when he was shot accidentally in the side at close range on Mackinac Island in Michigan in 1822. "In a few weeks, however, the wound began to heal and a year later, St. Martin was in good health — with one notable peculiarity. His skin had fused to the gunshot wound through the wall of his stomach, leaving an opening into his body. At first, the fistula was source of awe to Beaumont who could peer through it to the inner workings of a living man. Over the better part of the next decade, the doctor made this medical oddity the source of 238 experiments, which contributed previously unknown insights into the process of digestion."
Read more about this unique study in my article for Atlas Obscura by clicking here.