Here's a Medieval oddity which I don't think anybody has noticed.
In the Statuta Antiqua, when a bishop is consecrated, "two bishops place and hold a book of the Gospels over his neck, and as the Ordainer pours the Blessing over him, the bishops who are present touch his head with their hands".
Now to the Spanish, Mozarabic, rite for the ordination of a presbyter. "The presbyters lay hands on him, and he is blessed by the bishop as follows ...".
Spot it? Well, in neither of these very different sources does the rubric actually say that the Ordainer himself lays hands on the ordinand.
I know what you're going to say. The imposition of hands by the ordainer is taken-for-granted. The rubrician doesn't bother to specify the blindingly obvious. And you might very well be right. But I'm not totally sure.
In each of these cases, I am convinced that what we have is a collegial act. The new bishop is being incorporated into the world-wide (and, as E L Mascall would insist, time-wide) college of bishops. The new presbyter is being incorporated by the corporate, collegiate presbyterium, into the priesthood of the local church (and since the local church is the manifestation of the Church Universal, this simultaneously incorporates him into the whole priestly body of Christ's whole Church).
I am quite certain that those presbyters could not so incorporate a new member if they acted on their own without the presidency of their head, the bishop. An attempt to do so would be, in the still appropriate language of the old manuals, 'invalid'. But with him they truly can do what they could not do without him. Just like the coconsecrators in the episcopal rite, they truly confer the sacrament.
And I feel pretty sure that in the Mozarabic rite, it was thought appropriate for the form to be uttered by the Bishop, the matter supplied by his presbyterium. See I Timothy 4:14.