The Middle Matzah
The middle of something is often represented by the word heart: the heart of the heavens, the heart of the earth, the heart of the sea. Seforim hakedoshim tell us that "there is nothing so perfect as a broken heart," a heart broken by feelings of remorse and humility. We break the middle matzah, our hearts, subjugating our hearts and deeds to Hashem.
(Haggada Dvar Tzvi)