Good advice above. What angle did you hit him? A rib will not stop an arrow's penetration like that. The arrow breaks the ribs and keeps on going. My biggest buck had a similiar situation with about half the arrow sticking out as he ran off. Due to the height of the shot I caught the spine as the arrow entered and that is why mine did not pass through. It did its job though and I watched him fall 60 yards from my stand.

Good luck!