He calls Himself the bread, because He is the origin of His own body. And lest it should be thought that the virtue and nature of the Word had given way to the flesh, He calls the bread His flesh, that, inasmuch as the bread came down from heaven, it might be seen that His body was not of human conception, but a heavenly body. To say that the bread is His own, is to declare that the Word assumed His body Himself.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Trin. x. c. 18.