The Magnificat is the song of the most holy Heart of Mary

I call the Magnificat the true Canticle of the most holy Heart of the Blessed Virgin Mary for several reasons.

First of all, it originated in her Immaculate Heart and its melody vibrated her heart-strings before it found voice on her lips.

Secondly, the words were sung only through the impetus and inspiration flowing from her Heart—from her corporeal, spiritual and divine Heart. The corporeal Heart of Mary, being filled with acute and extraordinary joy, impelled her most holy mouth to sing this Magnificat with extraordinary fervor and jubilation; her spiritual Heart, being wholly delighted and enraptured in God, brought forth from her hallowed lips these ecstatic words: "And my spirit hath rejoiced in God my Savior." But Her divine Heart, that is to say, the divine Child who was hidden in her blessed womb and dwelling in her Heart, the soul of her soul, the spirit of her spirit and the Heart of her Heart, is the primary author of this canticle. It is this divine Heart which inspired the mind of the holy Mother with the thoughts and truths contained in the canticle. It is the Heart of Jesus who pronounces through her, its prophetic utterance of praise.

Thirdly, the Magnificat is the canticle of the Heart of the Mother of uncreated love, that is, the canticle of the Holy Spirit, Who is the spirit and the Heart of the Eternal Father and the Divine Son. The Holy Spirit is also the Heart and the Spirit of the Virgin Mother, permeating and possessing her so completely that her very presence and voice filled with the same spirit St Zachary, St Elizabeth and her unborn child, St John the Baptist.

Saint John Eudes, The Admirable Heart of Mary, 1681, #277