Thus is Mary also called. Thy neck is as a tower of ivory (Ps 86:10). Mary is called a neck; for she is the mystic neck through which the vital spirits, that is, the divine help which preserves in us the life of grace, are transmitted from Jesus Christ the Head to us the faithful, who are members of the mystic body of the Church. St. Bernardine says, “The life-giving graces flow from Christ the Head, through the Blessed Virgin, into his mystic body.” The saint then adds, “that from the time when Mary conceived the Incarnate Word, she received the great honor from God, that no one should receive any grace otherwise than through her hands.” In fine, ivory is greatly esteemed and is strong. Hence the Abbot Rupert writes of Mary, “that as a tower of ivory she is beloved by God, and terrible to the devil.”
Then, O my sovereign Lady, because thou art so beloved of God, thou canst obtain us every grace; and because thou art terrible to the evil spirits, thou canst deliver us from all their snares. Have mercy on us, who glory in living under thy protection.
From The Glories of Mary by St. Alphonsus de Liguori