Objection 1. It would seem that the foregoing prayer is not a suitable form for this sacrament. For in the forms of the other sacraments mention is made of the matter, for instance in Confirmation, whereas this is not done in the aforesaid words. Therefore it is not a suitable form.
Objection 2. Further, just as the effect of this sacrament is bestowed on us by the mercy of God, so are the effects of the other sacraments. But mention is made in the forms of the other sacraments, not of the Divine mercy, but rather of the Trinity and of the Passion. Therefore the same should be done here.
Objection 3. Further, this sacrament is stated in the text (Sent. iv, D, 23) to have a twofold effect. But in the foregoing words mention is made of only one effect, viz. the remission of sins, and not of the healing of the body to which end James directs the prayer of faith to be made (James 5:15): "The prayer of faith shall save the sick man." Therefore the above form is unsuitable.
I answer that, The prayer given above (Article 8) is a suitable form for this sacrament, for it includes the sacrament by the words: "By this holy unction," and that which works in the sacrament, viz. "the mercy of God," and the effect, viz. "remission of sins."
Reply to Objection 1. The matter of this sacrament may be understood in the act of anointing, whereas the matter of Confirmation cannot be implied by the act expressed in the form. Hence there is no parity.
Reply to Objection 2. The object of mercy is misery: and because this sacrament is given when we are in a state of misery, i.e. of sickness, mention of mercy is made in this rather than in other sacraments.
Reply to Objection 3. The form should contain mention of the principal effect, and of that which always ensues in virtue of the sacrament, unless there be something lacking on the part of the recipient. Now bodily health is not an effect of this kind, as we shall state further on (30, 1,2), though it does ensue at times, for which reason James ascribes this effect to the prayer which is the form of this sacrament.
Source: Summa Theologica (New Advent)