In Communion you receive the Whole Christ. Substance and form are philosophical terms. But here they are used in a special way we don’t need to go into because you wouldn’t understand them. In this instance the Substance of Christ includes his human form and his body and the Divine Nature ( in philosophy we would call "form, "substantial form). Now the form of the human person is the soul ( but in Christ there is no human person but a human nature composed of body and soul only). The human nature of Christ has been assumed by the Divine Person. So you receive the Second Person of the Trinity with His human and Divine natures. *
So you do receive the substance and form of Christ. You are also receiving the Whole Christ, that is with all his physical and psychological accidents, limbs, eyes, hair, measurements, etc, mind ( intellect and will ), all alive and united in the Second Person of the Trinity. It is the real living Christ just as you will see Him in Heaven.
How is this possible? First of all most of it is attributable to the Miracle itself. However a " glorified " body is not bound by the physical laws and limitations it had on earth. Firstly, it can apparantly assume any form desired and apparantly assume complete invisibility, and escape all detection, and can assume any size.
Several other Miracles occur along with the transubstantion. The Whole Christ is in every crumb of the host that is broken off, and every drop of the wine which might fall. So theoretically a hundred people or more could receive communion from one host or one thimble full of the wine.
Besides this Miracle, the Whole Christ is present in every host that has been consecrated anywhere in the world - at the same time!!!
Another Miracle is that the Accidents of the bread and Wine adhere in no substance.** Nor do adhere in the Substance of Christ. Nor does he dwell in the substance of the bread and wine ( they no longer exist), only the accidents remain - he dwells " behind " them, so to speak. They provide a " veil " like the Veil of the Temple which hid the Holy of Holies.
- When we speak of the Divine Nature, that is really by metaphore only, because, properly speaking, God ( including each of the three Persons ) has no nature. God is PURE EXISTENCE, PURE ACT, utterly Simple, utterly One, PURE INTELLECT. :thumbsup
** The substantial forms of the bread and wine have vanished, they are gone after the consecration because they cannot exist without their substances and the substances of the bread and wine have been changed into the Body and Blood of Christ. S.T. Part lll,Ques 75, art 6.
So the accidents remain without substances and without forms.