48 Devotion to the heart of Christ is not the veneration of a single organ apart from the Person of Jesus. What we contemplate and adore is the whole Jesus Christ, the Son of God made man, represented by an image that accentuates his heart. That heart of flesh is seen as the privileged sign of the inmost being of the incarnate Son and his love, both divine and human. More than any other part of his body, the heart of Jesus is “the natural sign and symbol of his boundless love”.
Source: Dilexit Nos (Vatican.va)