Objection 1. It seems that the sacramental species are not broken in this sacrament, because the Philosopher says in Meteor. iv that bodies are breakable owing to a certain disposition of the pores; a thing which cannot be attributed to the sacramental species. Therefore the sacramental species cannot be broken.
Objection 2. Further, breaking is followed by sound. But the sacramental species emit no sound: because the Philosopher says (De Anima ii), that what emits sound is a hard body, having a smooth surface. Therefore the sacramental species are not broken.
Objection 3. Further, breaking and mastication are seemingly of the same object. But it is Christ's true body that is eaten, according to John 6:57: "He that eateth My flesh, and drinketh My blood." Therefore it is Christ's body that is broken and masticated: and hence it is said in the confession of Berengarius: "I agree with the Holy Catholic Church, and with heart and lips I profess, that the bread and wine which are placed on the altar, are the true body and blood of Christ after consecration, and are truly handled and broken by the priest's hands, broken and crushed by the teeth of believers." Consequently, the breaking ought not to be ascribed to the sacramental species.
On the contrary, Breaking arises from the division of that which has quantity. But nothing having quantity except the sacramental species is broken here, because neither Christ's body is broken, as being incorruptible, nor is the substance of the bread, because it no longer remains. Therefore the sacramental species are broken.
I answer that, Many opinions prevailed of old on this matter. Some held that in this sacrament there was no breaking at all in reality, but merely in the eyes of the beholders. But this contention cannot stand, because in this sacrament of truth the sense is not deceived with regard to its proper object of judgment, and one of these objects is breaking, whereby from one thing arise many: and these are common sensibles, as is stated in De Anima ii.
Others accordingly have said that there was indeed a genuine breaking, but without any subject. But this again contradicts our senses; because a quantitative body is seen in this sacrament, which formerly was one, and is now divided into many, and this must be the subject of the breaking.
But it cannot be said that Christ's true body is broken. First of all, because it is incorruptible and impassible: secondly, because it is entire under every part, as was shown above (Question 76, Article 3), which is contrary to the nature of a thing broken.
It remains, then, that the breaking is in the dimensive quantity of the bread, as in a subject, just as the other accidents. And as the sacramental species are the sacrament of Christ's true body, so is the breaking of these species the sacrament of our Lord's Passion, which was in Christ's true body.
Reply to Objection 1. As rarity and density remain under the sacramental species, as stated above (2, ad 3), so likewise porousness remains, and in consequence breakableness.
Reply to Objection 2. Hardness results from density; therefore, as density remains under the sacramental species, hardness remains there too, and the capability of sound as a consequence.
Reply to Objection 3. What is eaten under its own species, is also broken and masticated under its own species; but Christ's body is eaten not under its proper, but under the sacramental species. Hence in explaining John 6:64, "The flesh profiteth nothing," Augustine (Tract. xxvii in Joan.) says that this is to be taken as referring to those who understood carnally: "for they understood the flesh, thus, as it is divided piecemeal, in a dead body, or as sold in the shambles." Consequently, Christ's very body is not broken, except according to its sacramental species. And the confession made by Berengarius is to be understood in this sense, that the breaking and the crushing with the teeth is to be referred to the sacramental species, under which the body of Christ truly is.
Source: Summa Theologica (New Advent)