The Brain in Your Belly

There is a second brain inside you, and it is not in your skull. Coiled through your gut sits a web of some five hundred million neurons — more than a cat's whole brain — wired so independently that it keeps running even if its line to the head is cut. It digests, reacts, and decides on its own. It has moods.

And it talks upward. Down a thick nerve and through a stream of chemicals, your gut shapes how you feel; most of your body's serotonin is made there, not in your head. The microbes lodged in it join in, tilting your cravings toward whatever feeds them. The lurch of dread before bad news, the "gut feeling" you can't justify — that is the second brain, reporting in.

You think your decisions are made up top. Some were settled in your belly before your head was even told.

And if your gut gets a vote — who else living inside you does?