We all know what bodies look like from the outside. This cortical homunculus is how your brain sees your body from the inside.
In the 1930s, Wilder Penfield performed surgeries on patients with epilepsy. While he had a live brain on the table, he figured he might as well poke around a bit. The doctor gathered data, finding out which parts of the cerebral cortex control which voluntary body functions and feeling. What he discovered was a vastly distorted view of the human body: the cortical homunculus.
The cortical homunculus represents the importance of various parts of your body as seen by your brain.
There is little need for the brain to know what's going on in the arms and legs. All these limbs need to do is stay out of an open flame and get your hands and feet to the right places. The hands, the tongue, the genitals, and the facial features are extremely important, and give people a ton of sensory information. As a result, they take up a lot of brain space.
Although the cortical homunculus is a curiosity, Penfield's work in mapping the brain's relationship to the body was invaluable.
Via JNNP, World Lingo, and Psychweb.