Nor can it be said that this power was given to the disciples of Christ not to be transmitted to others. It was given unto edification (2 Cor. xiii, 10), to the building up of the Church, and must be perpetuated so long as the Church needs building up, that is, to the end of the world (Matt. xxviii, 20). And since spiritual effects are transmitted to us from Christ under sensible signs, this power had to be delivered to men under some such signs, -- certain forms of words, definite acts, as imposition of hands, anointing, the delivery of a book or chalice, and the like.* Whenever anything spiritual is delivered under a corporeal sign, that is called a Sacrament.* Thus in the conferring of spiritual power a Sacrament is wrought, which is called the Sacrament of Order. Now it is a point of divine liberality that the bestowal of power should be accompanied with the means of duly exercising that power. But the spiritual power of administering the Sacraments requires divine grace for its convenient exercise: therefore in this Sacrament, as in other Sacraments, grace is bestowed.
Among Sacraments the noblest, and that which sets the crown on the rest, is the Sacrament of the Eucharist. Therefore the power of Order must be considered chiefly in relation to this Sacrament: for everything is ruled by the end for which it is made. Now the power that gives perfection, also prepares the matter to receive it.* Since then the power of Order extends to the consecration of the Body of Christ and the administration of the same to the faithful, it must further extend to the rendering of the faithful fit and worthy for the reception of that Sacrament. But the believer is rendered fit and worthy by being free from sin: otherwise he cannot be united with Christ spiritually, with whom he is sacramentally united in the reception of this Sacrament.* The power of Order therefore must extend to the remission of sins by the administration of those Sacraments which are directed to that purpose, Baptism and Penance.*
4.73 : Of the Sacrament of Extreme Unction
4.75 : Of the Distinction of Orders