The rib cage isn’t just made of bone. In between each rib there are two kinds of muscle. The internal and external intercostals.

The internal intercostal muscles are inside the rib cage. They extend from the front of the ribs and go all they way back.

During force full exhalation the internal intercostal muscles contract.

The external intercostal muscles are outside the rib cage. They extend from the back of the ribs and go all they way to the front.

During inhalation the external intercostal muscles contract.