As I was processing my bear this past fall, I was sorta thrown off by the shape of the humerus bone. It just seemed... wonky. So I kept it to boil and clean with the skull. I'm glad I did! Check it out - my guess is I wasn't the first one to shoot this bear. There are bone fragments inside the obvious cavity, which was filled with cartilage.
(I don't know if the pics will show up, they're not in the preview)