What did Huck find that made his ''heart jump amongst his lungs''?

Huck found the smoking ashes or a recently doused campfire.

I clipped along, and all of a sudden I bounded right on to the ashes of a camp fire that was still smoking. My heart jumped up amongst my lungs. I never waited for to look further, but uncocked my gun and went sneaking back on my tiptoes as fast as ever I could.