The life inside The human gut is an amazing piece of work. Often referred to as the "second brain," it is the only organ to boast its own independent nervous system, an intricate network of 100 million neurons embedded in the gut wall. So sophisticated is this neural network that the gut continues to function even when the primary neural conduit between it and the brain, the vagus nerve, is severed. (Citing the enteric nervous system's autonomy and apparent infallibility, comedian Stephen Colbert once christened the gut "the pope of your torso.")