The cells in each primary sensory cortex is arranged according to the plan of the body surface. Each region is a map of the body in which the relative areas devoted to body regions reflect the density of body innervation. Furthermore, parts of the body where we are especially sensitive to touch (like our hands and fingers) send information to a larger area of S1 than do less sensitive body regions (like our shoulder). The attached picture shows the two homunculi (little men), which represent how we would appear if our body proportions matched cortical representations. As you can see, the hands and face far outweigh other body parts in representation due to innervation.