Ante agnus offerebatur, offerebatur et vitulus, nunc Christus offertur...et offert se ipse quasi sacerdos, ut peccata nostra dimittat. Hic in imagine, ibi in veritate, ubi apud Patrem pro nobis quasi advocatus intervenit.
Formerly a lamb was offered, a calf was offered. Christ is offered today...and he offers himself as priest in order that he may remit our sins: here in image, there in truth where, as our advocate, he intercedes for us before the Father.
De officiis ministrorum ("On the Offices of Ministers" or, "On the Duties of the Clergy"), Book I, ch. 48.
In, The Eucharist in the West: History and Theology, Edward J. Kilmartin, SJ, Robert J. Daly, SJ, Editor, 1998, The Liturgical Press, ISBN 0814662048 ISBN 9780814662045, p. 19 
Alternate translation: In old times a lamb, a Calf was offered; now Christ is offered. But He is offered as man and as enduring suffering. And He offers Himself as a priest to take away our sins, here in an image, there in truth, where with the Father He intercedes for us as our Advocate.