The vagus nerve is the longest cranial nerve. It contains motor and sensory fibers and, because it passes through the neck and thorax to the abdomen, has the widest distribution in the body. It contains somatic and visceral afferent fibers, as well as general and special visceral efferent fibers. Vagus means “wandering” in Latin, and true to its name, the nerve meanders around the chest and abdomen, connecting most of the key organs—heart and lungs included—to the brain stem. It’s like a back door built into the human physiology, allowing you to hack the body’s systems. Vagus nerve stimulation, or VNS, got its start in the 1990s, when Cyberonics, of Houston, developed an implanted stimulator to treat particularly tough cases of epilepsy. That application was just the beginning. Researchers soon found that stimulation had the potential to treat a variety of ailments, including painful neurological conditions such as migraine headaches and fibromyalgia, inflammatory p...