For centuries, scientists had believed that nerves were hollow conduits, like pipes, and some fluid, or air—pneuma—flowing through them carried a wave of information from one nerve to the next, and from the nerve to a muscle, ultimately causing that muscle to contract. According to the “balloonist” theory, as it was called, the muscle was a balloon, and when it was filled with pneuma, it swelled like an air-filled bladder.
