In 1921, the German physiologist Otto Loewi performed an unnerving experiment that would win him and a colleague the Nobel Prize for Medicine: He removed the heart from a live frog; placed it, still beating, into a beaker containing a nutrient solution to keep it alive; and poured juices extracted from another frog’s vagus nerve onto it. The heartbeat slowed, proving that the heart rate is chemically governed. The chemical in question was later found to be acetylcholine, a neurotransmitter also present in humans, which regulates our heart rate, muscle contractions, digestion, and attention. Disrupt it with, say, a cobra’s strike, and your diaphragm will stop working properly, as will your breath.