Find the original on Google images.
Look closely. It turns out that the figures of God, His angels and even the soon-to-be-created Eve under His arm form a nearly perfect cross-section of the human brain.

Along with drawing, painting, sculpting, St. Peter's Basilica building and generally being among the universe's top bananas, Michelangelo counted cadaver dissecting as a favorite way to pass the time. He was so mad about corpse-cutting, in fact, that a friend once presented him with a perfectly formed dead Moor as a gift.
Seems as if Michelangelo feels that Almighty God is the ultimate 'Brain' behind the reason and purpose of creation and wanted to do an 'easter egg' of this fact in his painting of God.
So kewl!