It was Dr. Wilder Penfield at the Montreal Neurological Institute in the 1940’s and 50’s who was able, during surgical procedures to control seizures, to show that movement of body parts was mapped out on the surface of his patients’ brains in orderly, predictable ways. (This brain surface region, called the cortex reminds me of a head of cauliflower). He showed that the face and fingers had much larger representation on the cortex than the abdomen and the legs. He proposed that larger cortical representation was due to the finer control (more neurons, more brain) required to move these body parts. In other words, movement requiring greater precision had disproportionally larger representations on the brain surface that larger, less “skilled” body parts. Think of the control required to speak; the precise coordination of breath, vocal cords, tongue and lips to make understandable sounds. The tongue and lips take up a lot of geography on the brain’s cortex.
We must give kudos to Dr. Penfield and the MNI (check his bio on Wikipedia) for this ground-breaking work. I have visions of Dr. Penfield standing over a person’s splayed open skull and tickling the brain with a needle. (I have no idea if that is what he actually did). Now that we have sophisticated imaging techniques, the researcher does not have to lift off your skull to map out your brain (yippee!).