From a draw-over on conceptart.org:

"You are looking at the 10'th rib, which is the landmark for placing the rib cage from the front. It is important to understand the action of the living figure that causes the anatomy to be so prominent. You’ll notice that the tenth rib on the side of the model’s body which is in compression is partially hidden by the fatty tissue surrounding the rectus abdominus. The 10’th rib on the stretching side(the one you’re seeing) is very prominent. The 10’th rib is easily seen here because not only is the oblique on top of it very thin, but the stretch of the pose is actually pushing the rib out so that its influence is felt even more. When I was placing the rib cage in this drawing I noticed that Michelangelo had even animated the rib cage, squashing the model’s left side, and stretching the model’s right, making the action that much more felt.

Since this is a drawing by Michelangelo, the anatomy is completely correct and there is no confusion due to cast shadows. You are not looking at a cast shadow from the pec. You are not looking at the pec either, since the pec is at the same level of the bottom of the sternum.

Learn your anatomy! It’s awesome!"