When Aristotle speaks of the soul 'in the heart' he has in mind the heart as primary and proximate material organ of the soul, the controlling organ originally informed and activated by the soul, upon which all the other organs depend for their formation and activation by the soul. They too live, are informed and activated by the soul, but in a way that is secondary and more remote. To speak of the soul as existing 'in the heart' is not to deny its presence in the other organs, but to indicate the primary and proximate subject it informs and activates.
Tracy, Theodore S.J., "Heart and Soul in Aristotle" (1974). The Society for Ancient Greek Philosophy Newsletter. 90.