The challenge for Dr. Nicholas Senn, a US Army surgeon during the late 19th Century, was how to quickly assess abdominal gunshot wounds before the invention of x-ray machines. Was the path of a bullet a flesh wound, or had it penetrated the intestines, thus requiring a more complicated surgery?
His solution was to pump hydrogen gas up the patient's rectum*, then hold a flame to the wound. If the gas ignited, that indicated that the gunshot wound had indeed damaged the intestines.
To test this method, Senn first pumped gas up his own rectum to see what it would feel like. Then, in separate experiments, he performed the surgery on animals that had suffered gunshot wounds. You can read his book on the subject here.
Dr. Senn's methods were apparently successful and he went onto other medical accomplishments. Kaushik Patowa of Amusing Planet writes:
Although the first human patient, a 27-years-old black man with a 38 caliber pistol shot wound to his stomach, did not survive, the procedure itself was a success and eventually found use in military surgery to detect bowel wounds in soldiers. Dr. Senn’s methods only became obsolete when X-rays became standard diagnostic tool.
Nicholas Senn was a pioneer in surgical medicine. Aside from his work on gastrointestinal perforation, Doctor Senn was involved in experimental research on acute pancreatitis, plastic surgery, head and neck oncology, and the treatment of leukaemia with x-rays. He strongly supported the early operation for appendicitis, which was not the practice of the time. He also stressed the importance of first aid, first introduced to the military by the German surgeon Friedrich von Esmarch in 1870. Senn frequently quoted the German dictum, “The fate of the wounded rests with the one who applies the first dressing”.
*"Rectum? 'Pert near killed him!"