Full Idea
The soul is united with the body in just one place, a gland (the pineal) in the centre of the brain. It is placed so that its slightest movement will affect the passions, and it plays the essential role of uniting the two symmetrical sides of the body.
Gist of Idea
The pineal gland links soul to body, and unites the two symmetrical sides of the body
Source
report of René Descartes (The Passions of the Soul [1649], §31) by PG - Db (ideas)
Book Reference
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.307
A Reaction
See Idea 4862 for Spinoza's nice response to Descartes' proposal. If Descartes had followed brain research for the last four hundred years, at what point would he have wavered? If every single part of the brain seems to 'interact', dualism looks unlikely.
Related Idea
Idea 4862 Can the pineal gland be moved more slowly or quickly by the mind than by animal spirits? [Spinoza on Descartes]