The Mystical Geek

On the feast of the Sacred Heart the Church asks us to focus our attention on the Heart of Jesus broken by a lance, the source of all Christian life.  It is a heart that He showed to a nun, Margaret Mary Alacoque, as if to say "Tell everyone how much I love them."
A monk of the adoration offers us a webpage which illustrates the history of the devotion to the Sacred Heart from the time of the Fathers of the Church.  It is interesting reading and very informative in that from the text of the Scriptures the early Fathers have associated Jesus' heart with the Eucharist and the Waters of Baptism.  Interesting is a note about St. Clare who "greeted many times a day the Sacred Heart of Jesus in the Most Blessed Sacrament."  The connection between the Heart of Jesus and the Blessed Sacrament would seem strange.  And yet, we know that the word for "flesh" as used by Jesus in John 6 was for the Jewish tongue "leb", which can also mean heart.  Perhaps, this association can become reasonable in the light of the recent findings about the Lanciano Eucharistic miracle.  The consecrated host from which blood flowed in the 8th Century AD has been analyzed to have turned into human flesh, or to be more exact, to a cross-sectional piece of the human hear and displays "the myocardium, the endocardium, the vagus nerve and also the left ventricle of the heart for the large thickness of the myocardium."

The online Catholic Encyclopedia has a page on the Devotion to the Sacred Heart.  The author makes this note about the Devotion to the Sacred Heart:

In approving the devotion to the Sacred Heart, the Church did not trust to the visions of St. Margaret Mary; she made abstraction of these and examined the worship in itself. Margaret Mary's visions could be false, but the devotion would not, on that account, be any less worthy or solid. However, the fact is that the devotion was propagated chiefly under the influence of the movement started at Paray-le-Monial; and prior to her beatification, Margaret Mary's visions were most critically examined by the Church, whose judgment in such cases does not involve her infallibility but implies only a human certainty sufficient to warrant consequent speech and action.