Job 41:24

His heart is as firm as a stone; yea, as hard as a piece of the nether [millstone].

Ver. 24. His heart is as firm as a stone] He is corpore et corde validissimus. Of the sword fish, Plutarch saith, that he hath a sword, but not a heart to use it, μαχαιραν μεν εχει, εχει, καρδιαν δε μη εχει. But the whale hath courage to his bulk: his heart is as firm as a stone, as his head (saith Scaliger) is as hard as a flint. In the hearts of some creatures, saith Aristotle, is found a bony or grisly hardness; but the whale's heart is all as it were a bone; and this bone as a stone.

As a piece of the nether millstone] Metae, upon which the whole weight lieth. The Greek call it ονος μυλικος, Matthew 18:6, the mill ass; because it is the bigger and harder of the two. The Vulgate here for the nether millstone hath the smith's anvil, which, by hammering, is made harder.