Consider the lines where Tolkien shows us Grendel eating a knight. The monster seizes the man, “biting the bone-joints, drinking blood from veins, great gobbets gorging down. Quickly he took all of that lifeless thing to be his food, even hands and feet.” In Heaney’s translation, the monster, picking up the knight,
Bit into his bone-lappings, bolted down his blood
And gorged on him in lumps, leaving the body
Utterly lifeless, eaten up
Hand and foot.
Here, for the sake of alliteration and rhythm, we lose, among other things, the great gobbets (what a phrase!), the idea of using a man as food, and, most unfortunately, the picture of Grendel eating the feet and the hands……