I think it's your heart because its LHL (Lung, Heart, Lung) and then the ribs protect those organs.
Heart
A horse's heart is located in the same place as a human's. It is between the lungs and ribs and above the diaphragm.
your lungs and your heart
Superficial
Several organs are located between the ribcage and the back. These organs include the kidneys, heart, and both of the lungs.
the small potential space between the parietal and visceral layers of the pleura.
The inter coastal muscles in between the ribs make the ribs expand. the pleural membrane is like a slippery skin that sticks to the ribs and when they expand the lungs expand aswell.
Intercostal musles are present between the ribs.
the flesh and padding muscles in between each
Lungs
Your lungs are in the thoracic cavity, which includes the chest down to the diaphragm. The lungs are located on both sides of the heart in pleural cavities.Source:http://www.dummies.com/how-to/content/what-is-in-the-thoracic-cavities.html
the lungs are anterior to the ribs