Albert Barnes Commentary Job 40:16

"Lo now, his strength is in his loins, And his force is in the muscles of his belly." — Job 40:16 (ASV)

Lo now, his strength is in his loins - The inspection of the figure of the hippopotamus will show the accuracy of this. The strength of the elephant is in its neck; of the lion in its paw; of the horse and ox in their shoulders; but the principal power of the river-horse is in its loins . This passage is one that proves that the elephant cannot be referred to.

And his force is in the navel of his belly - The word which is here rendered “navel” (שׁריר shârı̂yr) properly means “firm, hard, tough,” and in the plural form, which occurs here, means the “firm,” or “tough” parts of the belly. It is not used to denote the “navel” in any place in the Bible, and should not have been so rendered here. The reference is to the muscles and tendons of this part of the body, and perhaps particularly to the fact that the hippopotamus, by crawling so much on its belly among the stones of the stream or on land, acquires special hardness or strength in those parts of its body.

This clearly proves that the elephant is not intended. In that animal, this is the most tender part of the body. Pliny and Solinus both remark that the elephant has a thick, hard skin on its back, but that the skin of its belly is soft and tender. Pliny says (Historia Naturalis, Book 8, Chapter 20) that the rhinoceros, when about to attack an elephant, “seeks its belly, as if it knew that that was the most tender part.” So Aelian (Historia Animalium, Book 17, Chapter 44); see Bochart, as above.