Job 41:24

His heart is as firm as a stone - As hard; as solid. Bochart remarks that the word “heart” here is not to be regarded as denoting the “courage” of the animal, as it sometimes does, but the heart literally. The statement occurs in the description of the various parts of the animal, and the object is to show that there was special firmness or solidity in every one of his members. There is special firmness or strength needed in the “hearts” of all animals, to enable them to propel the blood through the arteries of the body; and in an animal of the size of the crocodile, it is easy to see that the heart must be made capable of exerting vast force. But there is no reason to suppose that the affirmation here is made on the supposition that there is need of extraordinary strength in the heart to propel the blood. The doctrine of the circulation of the blood was not then known to mankind, and it is to be presumed that the argument here would be based on what “was” known, or what might be easily observed. The presumption therefore is, that the statement here is based on what had been “seen” of the remarkable compactness and firmness of the heart of the animal here referred to. Probably there was nothing so unique in the heart of the crocodile that this description would be applicable to that animal alone, but it is such doubtless as would apply to the heart of any animal of extraordinary size and strength.

Yea, as hard as a piece of the nether millstone - The mills commonly used in ancient times were hand-mills; see a description of them in the notes at Matthew 24:41. Why the lower stone was the hardest, is not quite apparent. Perhaps a more solid stone might have been chosen for this, because it was supposed that there was more wear on the lower than the upper stone, or because its weight would make the machine more solid and steady.