This is from St. Thomas Aquinas:
On the contrary, It is written (De Consecr., dist. 12): “It has come to our
knowledge that some
priests deliver the Lord’s body to a
layman or to a
woman to carry it to the sick: The synod therefore forbids such presumption to continue; and let the
priest himself communicate the sick.”
I answer that, The dispensing of
Christ’s body belongs to the
priest for three reasons. First, because, as was said above (
Article 1), he consecrates as in the
person of
Christ. But as Christ
consecrated His body at the supper, so also He gave it to others to be partaken of by them. Accordingly, as the
consecration of
Christ’s body belongs to the
priest, so likewise does the dispensing belong to him. Secondly, because the
priest is the appointed intermediary between
God and the people; hence as it belongs to him to offer the people’s
gifts to
God, so it belongs to him to deliver
consecrated gifts to the people. Thirdly, because out of reverence towards this
sacrament, nothing touches it, but what is
consecrated…
Source:
SUMMA THEOLOGIAE: The minister of this sacrament (Tertia Pars, Q. 82). (This is the online version of the Summa Theologica).