I answer that, Some have fallen into various errors about the matter of this sacrament. Some, known as the Artotyrytae, as Augustine says (De Haeres. xxviii), "offer bread and cheese in this sacrament, contending that oblations were celebrated by men in the first ages, from fruits of the earth and sheep." Others, called Cataphrygae and Pepuziani, "are reputed to have made their Eucharistic bread with infants' blood drawn from tiny punctures over the entire body, and mixed with flour." Others, styled Aquarii, under guise of sobriety, offer nothing but water in this sacrament.
Now all these and similar errors are excluded by the fact that Christ instituted this sacrament under the species of bread and wine, as is evident from Mt. 26. Consequently, bread and wine are the proper matter of this sacrament. And the reasonableness of this is seen first, in the use of this sacrament, which is eating: for, as water is used in the sacrament of Baptism for the purpose of spiritual cleansing, since bodily cleansing is commonly done with water; so bread and wine, wherewith men are commonly fed, are employed in this sacrament for the use of spiritual eating.
Secondly, 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.
Thirdly, 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 the species of wine "for the health of the soul," according to Lev. 17:14: "The life of the animal [Vulg.: 'of all flesh'] is in the blood."
Fourthly, as to the effect with regard to the whole Church, which is made up of many believers, just "as bread is composed of many grains, and wine flows from many grapes," as the gloss observes on 1 Cor. 10:17: "We being many are . . . one body," etc.