Your gut has more neurons than your spinal cord

1899. British physiologist John Newport Langley coins the term "enteric nervous system" for the network of neurons in the digestive tract. He puts the word into his treatise, and almost no one reads it.

1965. Michael Gershon, an American neurogastroenterologist, starts counting them. He finds gut neurons that do not depend on the brain to function: reflexes, movements, chemical memory of what you eat.

1998. Gershon publishes "The Second Brain". The name sticks. The figure behind it sticks harder: an adult's gut holds about 500 million neurons. The spinal cord, five times fewer: 100 million.

2011. Another team measures the body's serotonin. Ninety percent of it is made in the gut, not the brain. The same molecule that governs your mood is cooked downstairs and rises through the vagus nerve.

Today. Medicine puts it plainly: there are not two brains, there is one and a half. The half is 500 million neurons that respond to what you had for dinner before you notice.