Our mouth and hands are closely linked in the brain:
[I]t actually takes work not to gesture when we’re having a tough time getting our point across. When researchers have prevented people from gesturing while trying to think of words—inferno, say, or criminal negligence—they are less likely to successfully retrieve them. There are cultural differences in the size and frequency of our gestures (falling, more often than not, along stereotypical lines—want to hazard a guess about whether the Italians or the Japanese gesture more boisterously?—but we all gesture. The blind even gesture to the blind. As both artifact and facilitator, gestures may prove to be as much a part of speaking as speech itself.
The Dish 