Charles Ellicott Commentary Lamentations 2:18

"Their heart cried unto the Lord: O wall of the daughter of Zion, let tears run down like a river day and night; Give thyself no respite; let not the apple of thine eye cease." — Lamentations 2:18 (ASV)

Their heart. —The possessive pronoun does not refer to any immediate antecedent, but points, with a wild abruptness, to the mourners of Zion. Yet more boldly their cry is an appeal to the “wall” of Zion (Compare to Lamentations 2:8, and Isaiah 14:31), to take up its lamentation, as though it were a human mourner.

Like a river. —Better, like a torrent.

The apple of thine eye.— Literally, “the daughter,” as in the English phrase, the “pupil” of the eye.