ONE MEASURE OF MEDICINE’S PROGRESS IS HOW FAR INSIDE a living human body the physician can peer. Before X-rays and other imaging technologies, that job fell to ingenious devices and the naked eye. One of the most significant advances happened when a series of 19th-century innovations encountered the services of a professional sword swallower.

The idea of an endoscope—a tube that could be engineered to look deep inside the body—may have started with the “Lichtleiter,” or light conductor, conceptualized by Phillip Bozzini in 1806. This improved on the more primitive speculum by adding a mirror, a wax candle and a series of “viewing tubes” that could peer into orifices. A major advance on this idea came in 1853, when Parisian urologist Antonin Desormeaux used a tube, a kerosene lamp and a concave mirror to reflect light into the urethra, allowing for the first examination of the bladder.

Adolph Kussmaul, a physician from the University of Heidelberg in Germany, wanted to apply Desormeaux’s technique to observing the gastrointestinal tract. The son of a country doctor, Kussmaul had already, in 1845, come up with a device that inverted the lens design of a simple telescope and was supposed to show the interior of the eye. The ophthalmoscope had only one defect, he asserted—namely, that nothing could be seen with it.

Kussmaul added a straight tube, about nine inches long, to Desormeaux’s design and attempted to snake it down the throat of a man to locate a tumor. Unfortunately the limitations of the man’s esophagus prevented Kussmaul from getting the device deep enough. That’s when he turned to a man with an unusual talent: sword swallowing.

In a 1901 account by Gustav Killian, a student of Kussmaul, the doctor discovered an entertainer—allegedly known as “The Iron Henry”—who was able to relax his throat to an impressive degree. This allowed Kussmaul to insert an 18.5-inch tube with ease, thereby performing the first endoscopy of the upper gastrointestinal tract in 1868, 150 years ago.

Kussmaul then performed his innovative examination before audiences at several medical societies. One drawing that may have been based on this depicts the doctor sinking the scope down a patient’s throat, then laying him down on a raised table, head thrown back. The physician is shown squatting next to the table and peering through one end of the instrument.

Those unaccustomed to swallowing swords had a difficult time tolerating the device, and it afforded limited visibility at best. Despite these drawbacks, Kussmaul is recognized for scoring a major advance in the field.