Homunculus

Studying neuroanatomy, I have come across pictures like these representing the relative areas of the brain associated with sensory input or motor output. Our mouth and hands require very fine movements to speak and write. They also have a great deal of sensory input. These Homunculi are essentially how our brains view the rest of the body.

From this image one can deduce that, in sensitivity terms, a large mouth ulcer could be more significant than ones tackle - not a pretty thought.
-- From the Mouth Ulcers Homepage

(sensory homunculus on left and motor homunculus on the right)