Objection 1. It seems that the substantial form of the bread remains in this sacrament after the consecration. For it has been said (5) that the accidents remain after the consecration. But since bread is an artificial thing, its form is an accident. Therefore it remains after the consecration.
Objection 2. Further, the form of Christ's body is His soul: for it is said in De Anima ii, that the soul "is the act of a physical body which has life in potentiality". But it cannot be said that the substantial form of the bread is changed into the soul. Therefore it appears that it remains after the consecration.
Objection 3. Further, the proper operation of a things follows its substantial form. But what remains in this sacrament, nourishes, and performs every operation which bread would do were it present. Therefore the substantial form of the bread remains in this sacrament after the consecration.
On the contrary, The substantial form of bread is of the substance of bread. But the substance of the bread is changed into the body of Christ, as stated above (2,3,4). Therefore the substantial form of the bread does not remain.
I answer that, Some have contended that after the consecration not only do the accidents of the bread remain, but also its substantial form. But this cannot be. First of all, because if the substantial form of the bread were to remain, nothing of the bread would be changed into the body of Christ, excepting the matter; and so it would follow that it would be changed, not into the whole body of Christ, but into its matter, which is repugnant to the form of the sacrament, wherein it is said: "This is My body."
Secondly, because if the substantial form of the bread were to remain, it would remain either in matter, or separated from matter. The first cannot be, for if it were to remain in the matter of the bread, then the whole substance of the bread would remain, which is against what was said above (Article 2). Nor could it remain in any other matter, because the proper form exists only in its proper matter. But if it were to remain separate from matter, it would then be an actually intelligible form, and also an intelligence; for all forms separated from matter are such.
Thirdly, it would be unbefitting this sacrament: because the accidents of the bread remain in this sacrament, in order that the body of Christ may be seen under them, and not under its proper species, as stated above (Article 5).
And therefore it must be said that the substantial form of the bread does not remain.
Reply to Objection 1. There is nothing to prevent art from making a thing whose form is not an accident, but a substantial form; as frogs and serpents can be produced by art: for art produces such forms not by its own power, but by the power of natural energies. And in this way it produces the substantial forms of bread, by the power of fire baking the matter made up of flour and water.
Reply to Objection 2. The soul is the form of the body, giving it the whole order of perfect being, i.e. being, corporeal being, and animated being, and so on. Therefore the form of the bread is changed into the form of Christ's body, according as the latter gives corporeal being, but not according as it bestows animated being.
Reply to Objection 3. Some of the operations of bread follow it by reason of the accidents, such as to affect the senses, and such operations are found in the species of the bread after the consecration on account of the accidents which remain. But some other operations follow the bread either by reason of the matter, such as that it is changed into something else, or else by reason of the substantial form, such as an operation consequent upon its species, for instance, that it "strengthens man's heart" (Psalm 103:15); and such operations are found in this sacrament, not on account of the form or matter remaining, but because they are bestowed miraculously upon the accidents themselves, as will be said later (77, 3, ad 2,3; 5,6).
Source: Summa Theologica (New Advent)