But it is true that there is no particular inner pec muscle separate from the pectoralis major. In fact, working the inside (in order to get a noticeable crease) would only make one less balanced
There isn't an inner pec muscle, but there is
an inner chest:
It is important to hit different areas of the chest, as you do not want a lopsided or under-developed chest. For any upper chest development, you should do incline movements, such as incline press or incline Flyes. For lower development, it is said to do decline movements, but flat bench works my lower chest just fine. For outer development, wide grip bench presses and dumbbell flyes. Just the opposite for inner chest development, you should do close grip bench presses.
I thought that I might be able to kill two birds with one stone, and not have to do a separate exercise for the inner chest. But if not, I'll still do them, since I do full-body workouts and want to keep them under 1 - 1.5 hours