According to this Scientific American article, there is a nervous control center similar to the brain located in the abdomen.  So when Stephen Colbert talks jokingly about Truthiness and Thinking With His Gut, he was more right than wrong.

As Olympians go for the gold in Vancouver, eventhe steeliest are likely to experience that familiar feeling of “butterflies” in the stomach. Underlying this sensation is an often-overlooked network of neurons lining our guts that is so extensive some scientists have nicknamed it our “second brain”.

Apparently, this mass of neurons does it’s own autonomous control, has it’s own serotonin system of secretion and uptake, and is affected by medications that affect mood and mind.

Technically known as the enteric nervous system, the second brain consists of sheaths of neurons embedded in the walls of the long tube of our gut, or alimentary canal, which measures about nine meters end to end from the esophagus to the anus. The second brain contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system

So, in some cases, this brain may actually be more capable than the one in the head, may actually rule the roost, so to speak. Some voters may actually be able to think with their guts, as Colbert jokes.

“A big part of our emotions are probably influenced by the nerves in our gut,” Mayer says. Butterflies in the stomach—signaling in the gut as part of our physiological stress response…is but one example.

And of course, how does this brain interact with the bacteria that live there?  There is no blood/brain barrier to keep them away from this one…

U.C.L.A.’s Mayer is doing work on how the trillions of bacteria in the gut “communicate” with enteric nervous system cells (which they greatly outnumber). His work with the gut’s nervous system has led him to think that in coming years psychiatry will need to expand to treat the second brain in addition to the one atop the shoulders.