Near the dawn of the 19th century medicine advanced inwards. Doctors began once more to diagnose problems with the heart and lungs by placing their ears against the bodies of patients and listening intently. This practice had been used since the time of the Greeks but recent advances had returned to frequent usage. This new body of science was in its infancy and doctors had great trouble listening to internal problems and keeping abreast of developments in the understanding of the human interior. Then it was improved by chance and embarrassment.
The Doctor René Theophile-Hyacinthe Laënnec was busy at work in the nineteenth century, doing nineteenth century things when he was presented with a chest problems. It was a young and rather plump lady, who was followed by her family. They lined the room as the young lady told of her suffering. For a decent diagnosis, he needed to listen to her lungs.
Under the watchful gaze of the family and the pressure of nineteenth century sensibilities he felt suddenly aware of how uncomfortably close he would have to place his head to her bosom, so he improvised. He grabbed a nearby piece of paper and rolled it up into a tube and placing the paper purposefully on her skin. To his shock when he listened, the sounds were much clearer. The lazy lungs breathing and the nervous heart beating.
That day, in 1816, the stethoscope was invented. Over time they became less papery and more trumpet-like. So it was until 1851 when a binaural stethoscope, one allowing the use of both ears, was introduced. Designs similar to the ones still used today, or so I hear.