nothing actually contains the sternum the sternum is the bone that connects the ribs together also helping to protect the ventral side of the ribs it also holds at the bottom the xyphoid process
I guess one could think of the sternum as the "backbone" that holds the ribs together, keeps them in place. The sternum and ribs are a cage like system that protects your heart and lungs from injury....
Obviously you're supposed to put contains when "something contains something else" such as "That jar contains food." then contain in a present tense sentence like "Can you contain your dog, please?"