The network of neurons found in the wall of the vertebrate gut that controls and coordinates its muscular activity. In humans it contains an estimated 100 million neurons organized into two layers: a submucosal layer that controls secretions from the gut wall and also has sensory functions, and a second layer between the circular and longitudinal muscle layers, whose contractions it controls. The enteric nervous system operates relatively independently of the central nervous system but transmits signals about the state of the gut to the brain via the vagus nerve. It may play a part in influencing mood and in immunity. The range of enteric neurotransmitters matches that used by the brain and includes serotonin, which is produced in large amounts in gut tissues.