Heart.
The heart was once thought to be the center of human thought. Each though was guided by the pulsating rhythm of the fleshed, blood-pumpinh organ.
Humans had grown to know better, but didn't diminish the importance of the heart.
Its true purpose was discovered...
It was responsible for the circulation of nutrients around the body, put simply.
Cain's {Mana Heart} operated on the same principles, just that what they circulated was not physical, but more spiritual.
The gap between the physical and spiritual realms could be bridged by the mind. With mana, one could interact with the spiritual, which in turn could affect the physical world.