The GIRM assumes it. “Facing the people, the priest says this.
Facing the altar, the priest does that.” is all over the GIRM.
JMJ
Thank you, TheMc, for your response!
With all due respect, I think “all over” is a rather gross hyperbole. In the instruction for “The Different Forms of Celebrating Mass” (chapter IV), under the section “Mass without a Deacon” (section A), nos. 120-170, there are only five mentions of “facing the people” and one mention of “facing the altar”. Having only one mentioning of “facing the altar” does not presume that the priest is facing the altar the rest of the time, but rather implies he is not (and that in this instance he should). Then again, there are the five mentions of “facing the people” about which the same could be said. I won’t go into much detail here as this is aside from the topic of the thread (which is about the bell), and I am sure there are many other threads on the ad orientem/versus populum topic. But here’s my brief analysis:
Two of the references for “facing the people” are from the chair (cf. nos 124 and 165), before the Collect and Prayer after Communion. Note that in the concluding rite–including the dialog preceding the final blessing, and the blessing itself–no instruction is given to “face the people”. Thus, there is the implication that the posture is retained from its previous mention (that is, as the priest is facing the people already from the Prayer after Communion). These remarks are interesting, as no. 310 indicates that the chair is most properly placed at the head of the sanctuary, facing the people (although it should not look throne-like). However, a number of exceptions are allowed, including if the tabernacle is behind the altar. More commonly today the celebrant is sitting to the side of the altar facing inward (profile to the congregation). Therefore, I would argue that at least these two mentions of “facing the people” are for the case when the priest is not already facing the people.
In no. 146, the priest “faces the people” for the
“Orate, fratres”, after which he is not told to “face the altar”. He is then not instructed to face the people again until no. 154, before saying “The peace of the Lord be with you always.” My understanding of this, which will apply to the other instances, is that it is designated whenever there is a change of position of the priest (with one exception, which I will mention). Prior to 146, the priest is preparing the altar and the gifts, blessing them, and incensing them; his actions prior to 146 are oriented toward the gifts and the altar and God. Therefore, the priest is instructed, in 146, to, literally, “face” the people, to direct his words to the congregation.
No. 154 I think is particularly important. The instruction to face the people follows the prayer “Lord Jesus Christ, who said to your Apostles…”. I don’t know how often I’ve seen priests looking around the congregation while praying this. Who are they speaking to? The prayer is directed to Jesus Christ (it’s the only prayer directed to Jesus in the whole Eucharistic Canon), and Jesus Christ, while in a spiritual way is manifest in each individual and in the corporate body of the faithful gathered, is
substantially present directly in front of him, on the altar. (If only they would have included "Facing the Host, the Priest says aloud the prayer
Domine Iesu Christe, qui dixisti.)
No. 157 follows the priest’s internal preparation before receiving the Eucharist. Again, his disposition is changed, as he is oriented toward the Sacrament on the altar, which is in front of him (regardless of which way he’s facing with respect to the people). So when he proclaims
Ecce Agnus Dei, although he should preferably (according to my opinion–not explicit in the text) be looking with adoration at the Eucharist he is elevating, he is never the less directing his words to the people (in a literal way: his words are facing the people). He is not speaking to God or to the Eucharist, but to the people.
Lastly, no. 158 is the ONLY case where the priest is directly instructed to “face the altar”, which is when he says quietly
Corpus Christi custodiat me in vitam aeternam. I think this is also to emphasize that this prayer is to prepare the priest for receiving the Eucharist: some priests have taken to saying this prayer out loud, and even modifying it so as to say “May the Body and Blood of our Lord, Jesus Christ, bring all of us into eternal life” (or something of the sort), to which the people tend to respond “Amen”. When the priest is oriented to the altar, he is not speaking to the people, but praying for himself before consuming the Host (and likewise with the chalice).
NB: The section I refer to is just one form of celebrating Mass; I did not take to all the forms.
This section went longer than I intended, so I’ll make some very short comments in a separate post (since I already wrote these and it ended up being too long…)