At the same instant Buck peered out where the spruce-bough lodge had been and saw what made his hair leap straight up on his neck and shoulders.
A gust of overpowering rage swept over him.

He did not know that he growled, but he growled aloud with a terrible ferocity.

For the last time in his life he allowed passion to usurp cunning and reason, and it was because of his great love for John Thornton that he lost his head.
The Yeehats were dancing about the wreckage of the spruce-bough lodge when they heard a fearful roaring and saw rushing upon them an animal the like of which they had never seen before.

It was Buck, a live hurricane of fury, hurling himself upon them in a frenzy to destroy.

He sprang at the foremost man (it was the chief of the Yeehats), ripping the throat wide open till the rent jugular spouted a fountain of blood.
He did not pause to worry the victim, but ripped in passing, with the next bound tearing wide the throat of a second man.