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Scientists are claiming to have discovered a second brain - in the human stomach.

The breakthrough, involving experts in the US and Germany, is believed to play a major part in the way people behave.

This 'second brain' is made up of a knot of brain nerves in the digestive tract. It is thought to involve around 100 billion nerve cells -- more than held in the spinal cord.

Researchers believe this belly brain may save information on physical reactions to mental processes and give out signals to influence later decisions. It may also be responsible in the creation of reactions such as joy or sadness.

The research is outlined in the latest issue of German science magazine, Geo, in which Professor Wolfgang Prinz, of the Max Planck Institute for Psychological Research in Munich, says the discovery could give a new twist on the old phrase "gut reaction".

He said: "People often follow their gut reactions without even knowing why, its only later that they come up with the logical reason for acting the way they did. But we now believe that there is a lot more to gut feelings than was previously believed." Professor Prinz thinks the stomach network may be the source for unconscious decisions which the main brain later claims as conscious decisions of its own.

The second brain was rediscovered by Michael Gershorn, of the University of Colombia in New York, after it was forgotten by science. He says it was first documented by a 19th century German neurologist, Leopold Auerbach.

He discovered two layers of nerve cells near a piece of intestine he was dissecting. After putting them under the microscope he found they were part of a complex network.

Recent research has already raised the idea that many reactions may be made in the stomach. Benjamin Libet, of the University of California found the brains of volunteers asked to raise their arms only registered activity about half a second after the movement had been made. He believes his work implies another part of the body may have been involved in making the decision.