Full Idea
Democritus and Epicurus say the soul has two parts, one which is rational and is situated in the chest area, and the other which is non-rational and is spread throughout the entire compound of the body.
Gist of Idea
Atomists say soul has a rational part in the chest, and a diffused non-rational part
Source
report of Democritus (fragments/reports [c.431 BCE]) by Aetius - fragments/reports 4.4.6
Book Reference
'Hellenistic Philosophy: Intro Readings', ed/tr. Inwood,B. /Gerson,L. [Hackett 1988], p.62
A Reaction
The spread part corresponds to such things as feeling fear in the stomach, or excitement throughout the limbs. I can't think what grounds there would be for choosing the chest as the home of reason. I suppose you can hear reason thumping in there..