The bulge created by your thumbs represents the cerebellum, the part of
your brain that coordinates all of your body movements. The cerebellum performs
many functions, including storing learned body movements. For example, if you
haven't ridden a bike in a long time and get back on one, your body "remembers"
what it needs to do to keep the bike stable. This is why you "practice" sports,
so that your cerebellum automatically can remember all the right moves.