Objection 1. It seems that the defects occurring during the celebration of this sacrament cannot be sufficiently met by observing the statutes of the Church. For it sometimes happens that before or after the consecration the priest dies or goes mad, or is hindered by some other infirmity from receiving the sacrament and completing the mass. Consequently it seems impossible to observe the Church's statute, whereby the priest consecrating must communicate of his own sacrifice.
Objection 2. Further, it sometimes happens that, before the consecration, the priest remembers that he has eaten or drunk something, or that he is in mortal sin, or under excommunication, which he did not remember previously. Therefore, in such a dilemma a man must necessarily commit mortal sin by acting against the Church's statute, whether he receives or not.
Objection 3. Further, it sometimes happens that a fly or a spider, or some other poisonous creature falls into the chalice after the consecration. Or even that the priest comes to know that poison has been put in by some evilly disposed person in order to kill him. Now in this instance, if he takes it, he appears to sin by killing himself, or by tempting God: also in like manner if he does not take it, he sins by acting against the Church's statute. Consequently, he seems to be perplexed, and under necessity of sinning, which is not becoming.
Objection 4. Further, it sometimes happens from the server's want of heed that water is not added to the chalice, or even the wine overlooked, and that the priest discovers this. Therefore he seems to be perplexed likewise in this case, whether he receives the body without the blood, thus making the sacrifice to be incomplete, or whether he receives neither the body nor the blood.
Objection 5. Further, it sometimes happens that the priest cannot remember having said the words of consecration, or other words which are uttered in the celebration of this sacrament. In this case he seems to sin, whether he repeats the words over the same matter, which words possibly he has said before, or whether he uses bread and wine which are not consecrated, as if they were consecrated.
Objection 6. Further, it sometimes comes to pass owing to the cold that the host will slip from the priest's hands into the chalice, either before or after the breaking. In this case then the priest will not be able to comply with the Church's rite, either as to the breaking, or else as to this, that only a third part is put into the chalice.
Objection 7. Further, sometimes, too, it happens, owing to the priest's want of care, that Christ's blood is spilled, or that he vomits the sacrament received, or that the consecrated hosts are kept so long that they become corrupt, or that they are nibbled by mice, or lost in any manner whatsoever; in which cases it does not seem possible for due reverence to be shown towards this sacrament, as the Church's ordinances require. It does not seem then that such defects or dangers can be met by keeping to the Church's statutes.
On the contrary, Just as God does not command an impossibility, so neither does the Church.
I answer that, Dangers or defects happening to this sacrament can be met in two ways: first, by preventing any such mishaps from occurring: secondly, by dealing with them in such a way, that what may have happened amiss is put right, either by employing a remedy, or at least by repentance on his part who has acted negligently regarding this sacrament.
Reply to Objection 1. If the priest be stricken by death or grave sickness before the consecration of our Lord's body and blood, there is no need for it to be completed by another. But if this happens after the consecration is begun, for instance, when the body has been consecrated and before the consecration of the blood, or even after both have been consecrated, then the celebration of the mass ought to be finished by someone else. Hence, as is laid down (Decretal vii, q. 1), we read the following decree of the (Seventh) Council of Toledo: "We consider it to be fitting that when the sacred mysteries are consecrated by priests during the time of mass, if any sickness supervenes, in consequence of which they cannot finish the mystery begun, let it be free for the bishop or another priest to finish the consecration of the office thus begun. For nothing else is suitable for completing the mysteries commenced, unless the consecration be completed either by the priest who began it, or by the one who follows him: because they cannot be completed except they be performed in perfect order. For since we are all one in Christ, the change of persons makes no difference, since unity of faith insures the happy issue of the mystery. Yet let not the course we propose for cases of natural debility, be presumptuously abused: and let no minister or priest presume ever to leave the Divine offices unfinished, unless he be absolutely prevented from continuing. If anyone shall have rashly presumed to do so, he will incur sentence of excommunication."
Reply to Objection 2. Where difficulty arises, the less dangerous course should always be followed. But the greatest danger regarding this sacrament lies in whatever may prevent its completion, because this is a heinous sacrilege; while that danger is of less account which regards the condition of the receiver. Consequently, if after the consecration has been begun the priest remembers that he has eaten or drunk anything, he ought nevertheless to complete the sacrifice and receive the sacrament. Likewise, if he recalls a sin committed, he ought to make an act of contrition, with the firm purpose of confessing and making satisfaction for it: and thus he will not receive the sacrament unworthily, but with profit. The same applies if he calls to mind that he is under some excommunication; for he ought to make the resolution of humbly seeking absolution; and so he will receive absolution from the invisible High Priest Jesus Christ for his act of completing the Divine mysteries.
But if he calls to mind any of the above facts previous to the consecration, I should deem it safer for him to interrupt the mass begun, especially if he has broken his fast, or is under excommunication, unless grave scandal were to be feared.
Reply to Objection 3. If a fly or a spider falls into the chalice before consecration, or if it be discovered that the wine is poisoned, it ought to be poured out, and after purifying the chalice, fresh wine should be served for consecration. But if anything of the sort happen after the consecration, the insect should be caught carefully and washed thoroughly, then burned, and the "ablution," together with the ashes, thrown into the sacrarium. If it be discovered that the wine has been poisoned, the priest should neither receive it nor administer it to others on any account, lest the life-giving chalice become one of death, but it ought to be kept in a suitable vessel with the relics: and in order that the sacrament may not remain incomplete, he ought to put other wine into the chalice, resume the mass from the consecration of the blood, and complete the sacrifice.
Reply to Objection 4. If before the consecration of the blood, and after the consecration of the body the priest detect that either the wine or the water is absent, then he ought at once to add them and consecrate. But if after the words of consecration he discover that the water is absent, he ought notwithstanding to proceed straight on, because the addition of the water is not necessary for the sacrament, as stated above (Question 74, Article 7): nevertheless the person responsible for the neglect ought to be punished. And on no account should water be mixed with the consecrated wine, because corruption of the sacrament would ensue in part, as was said above (Question 77, Article 8). But if after the words of consecration the priest perceive that no wine has been put in the chalice, and if he detect it before receiving the body, then rejecting the water, he ought to pour in wine with water, and begin over again the consecrating words of the blood. But if he notice it after receiving the body, he ought to procure another host which must be consecrated together with the blood; and I say so for this reason, because if he were to say only the words of consecration of the blood, the proper order of consecrating would not be observed; and, as is laid down by the Council of Toledo, quoted above (ad 1), sacrifices cannot be perfect, except they be performed in perfect order. But if he were to begin from the consecration of the blood, and were to repeat all the words which follow, it would not suffice, unless there was a consecrated host present, since in those words there are things to be said and done not only regarding the blood, but also regarding the body; and at the close he ought once more to receive the consecrated host and blood, even if he had already taken the water which was in the chalice, because the precept of the completing this sacrament is of greater weight than the precept of receiving the sacrament while fasting, as stated above (Question 80, Article 8).
Reply to Objection 5. Although the priest may not recollect having said some of the words he ought to say, he ought not to be disturbed mentally on that account; for a man who utters many words cannot recall to mind all that he has said; unless perchance in uttering them he adverts to something connected with the consecration; for so it is impressed on the memory. Hence, if a man pays attention to what he is saying, but without adverting to the fact that he is saying these particular words, he remembers soon after that he has said them; for, a thing is presented to the memory under the formality of the past (De Mem. et Remin. i).
But if it seem to the priest that he has probably omitted some of the words that are not necessary for the sacrament, I think that he ought not to repeat them on that account, changing the order of the sacrifice, but that he ought to proceed: but if he is certain that he has left out any of those that are necessary for the sacrament, namely, the form of the consecration, since the form of the consecration is necessary for the sacrament, just as the matter is, it seems that the same thing ought to be done as was stated above (ad 4) with regard to defect in the matter, namely, that he should begin again with the form of the consecration, and repeat the other things in order, lest the order of the sacrifice be altered.
Reply to Objection 6. The breaking of the consecrated host, and the putting of only one part into the chalice, regards the mystical body, just as the mixing with water signifies the people, and therefore the omission of either of them causes no such imperfection in the sacrifice, as calls for repetition regarding the celebration of this sacrament.
Reply to Objection 7. According to the decree, De Consecr., dist. ii, quoting a decree of Pope Pius I, "If from neglect any of the blood falls upon a board which is fixed to the ground, let it be taken up with the tongue, and let the board be scraped. But if it be not a board, let the ground be scraped, and the scrapings burned, and the ashes buried inside the altar and let the priest do penance for forty days. But if a drop fall from the chalice on to the altar, let the minister suck up the drop, and do penance during three days; if it falls upon the altar cloth and penetrates to the second altar cloth, let him do four days' penance; if it penetrates to the third, let him do nine days' penance; if to the fourth, let him do twenty days' penance; and let the altar linens which the drop touched be washed three times by the priest, holding the chalice below, then let the water be taken and put away nigh to the altar." It might even be drunk by the minister, unless it might be rejected from nausea. Some persons go further, and cut out that part of the linen, which they burn, putting the ashes in the altar or down the sacrarium. And the Decretal continues with a quotation from the Penitential of Bede the Priest: "If, owing to drunkenness or gluttony, anyone vomits up the Eucharist, let him do forty days' penance, if he be a layman; but let clerics or monks, deacons and priests, do seventy days' penance; and let a bishop do ninety days'. But if they vomit from sickness, let them do penance for seven days." And in the same distinction, we read a decree of the (Fourth) Council of Arles: "They who do not keep proper custody over the sacrament, if a mouse or other animal consume it, must do forty days' penance: he who loses it in a church, or if a part fall and be not found, shall do thirty days' penance." And the priest seems to deserve the same penance, who from neglect allows the hosts to putrefy. And on those days the one doing penance ought to fast, and abstain from Communion. However, after weighing the circumstances of the fact and of the person, the said penances may be lessened or increased. But it must be observed that wherever the species are found to be entire, they must be preserved reverently, or consumed; because Christ's body is there so long as the species last, as stated above (77, 4,5). But if it can be done conveniently, the things in which they are found are to be burned, and the ashes put in the sacrarium, as was said of the scrapings of the altar-table, here above.
Source: Summa Theologica (New Advent)