As has been said, it is one muscle and although different excercises hit the muscles differently, there isnt an isolation excercise that just singles out the inner chest. However, tere are a few excercises that allow greater contraction, for example cable crossovers. With these, you can go a little farther than with flys for example, and that should help you contract your inner chest more fully. To get the full benefit, go past the point where your arms cross and flex and hold at the top.