FIGURE 2.17 Left hemisphere tissue devoted to each body part in the motor cortex and the somatosensory cortex As you can see from this classic though inexact representation, the amount of cortex devoted to a body part is not proportional to that body part’s size. Your brain devotes more tissue to sensitive areas and to areas requiring precise control. So, your fingers have a greater representation in the cortex than does your upper arm.
From Penfield/Rasmussen The Cerebral Cortex of Man © 1950 Gale, A part of Cengage Learning, Inc. Reproduced by permission www.cengage.com/permissions