129 The Eucharistic Food contains, as all are aware, "truly, really and substantially the Body and Blood together with soul and divinity of our Lord Jesus Christ." It is no wonder, then, that the Church, even from the beginning, adored the body of Christ under the appearance of bread; this is evident from the very rites of the august sacrifice, which prescribe that the sacred ministers should adore the most holy sacrament by genuflecting or by profoundly bowing their heads.
Source: Mediator Dei (Vatican.va)