The soul does not reside in the body as an organ nor does it use any organ specially as its dwelling place. The whole body is its instrument but the heart and the emotional centers respond fastest to its intimations since it is in close proximity to the soul that dwells behind the emotional center in the middle of the chest.
However we should not mistake it to be in the three-dimensional structures or frames built by material nature as forms for habitation of the Divine and as container for the contents of consciousness. The soul, like the Divine Presence is in the fourth dimension and it is only when by the force of concentration and aspiration or by a special Grace that we pass into the fourth dimension that we can see the soul. Otherwise we can sense its presence through subjective states such as peace and joy and devotion and faith and the urge to give oneself to the Divine, the urge for progress etc.
The reason why it is advised to concentrate in the centre of the chest is because through a subtle point here we can pass through an inner door within the material body into the subtle realms and thereby going further and deeper come in contact with our secret soul.
About Savitri | B1C2-10 Condition of the World (p.18)