"Hilary says (De Trin. viii): 'There is no room for doubt regarding the truth of Christ's body and blood; for now by our Lord's own declaring and by our faith His flesh is truly food, and His blood is truly drink.' And Ambrose says (De Sacram. vi): 'As the Lord Jesus Christ is God's [...]
"In Baptism water is used for the purpose of ablution: but in this sacrament [Eucharist] it is used by way of refreshment, according to Ps. 22:3: 'He hath brought me up on the water of refreshment.'" Source: St. Thomas Aquinas, Summa Theologica
"Water ought to be mingled with the wine which is offered in this sacrament. First of all on account of its institution: for it is believed with probability that our Lord instituted this sacrament in wine tempered with water according to the custom of that country: hence it is written (Prov. 9:5): 'Drink the [...]
"As Ambrose says (De Sacram. v), just as Christ's sacrifice is denoted by the offering of Melchisedech, so likewise it is signified by the water which flowed from the rock in the desert, according to 1 Cor. 10:4: 'But they drank of the spiritual rock which came after them.'" Source: St. Thomas Aquinas, Summa [...]
"As our Lord compared Himself to the grain of wheat, so also He compared Himself to the vine, saying (Jn. 15:1): 'I am the true vine.' But only bread from wheat is the matter of this sacrament, as stated above. Therefore, only wine from the grape is the proper matter of this sacrament." [...]
"Barley bread serves to denote the hardness of the Old Law; both on account of the hardness of the bread, and because, as Augustine says: 'The flour within the barley, wrapped up as it is within a most tenacious fibre, denotes either the Law itself, which was given in such manner as to be vested [...]
"Christ is contained in this sacrament, and He compares Himself to a grain of wheat, saying (Jn. 12:24): 'Unless the grain of wheat falling into the ground die, itself remaineth alone.' Therefore bread from corn, i.e. wheaten bread, is the matter of this sacrament." Source: St. Thomas Aquinas, Summa Theologica
"There is no quantity, however small, of the bread and wine which cannot be consecrated. Therefore, neither is there any quantity, however great, which cannot be consecrated." Source: St. Thomas Aquinas, Summa Theologica
"As to the effect, considered in each of the partakers. For, as Ambrose (Mag. Sent. iv, D, xi) says on 1 Cor. 11:20, this sacrament 'avails for the defense of soul and body'; and therefore 'Christ's body is offered' under the species of bread 'for the health of the body, and the blood' under [...]
"In relation to Christ's Passion, in which the blood was separated from the body. And therefore in this sacrament, which is the memorial of our Lord's Passion, the bread is received apart as the sacrament of the body, and the wine as the sacrament of the blood." Source: St. Thomas Aquinas, Summa Theologica [...]