This drawing shows a view of the human male in which the sizes of the body parts have been scaled to the size of their representation in the brain. In particular, the brain region that first processes touch sensation, called the primary somatosensory cortex. Most people who look at this image (which is called a sensory homunculus) long enough will eventually stammer out something like “Dude, given how sensitive the genitals are, shouldn’t they be, like, larger?” This is a good question and there is not an absolutely definitive answer. We know that the genitals are sensitive to touch and that there are particular nerves which carry sensory information from the genitals into the spinal cord and up to the brain. One potential explanation for the size issue is that we need to be more precise when we say “sensitive to touch”. The parts of the homunculus which have huge representations (such as hands, lips and tongue) are not merely able to detect faint sensations but can also discriminate the location of these sensations very precisely. You might imagine that these two abilities always go hand-in-hand but they do not. In fact, the ability to do the finest discrimination, required for tactile form perception (as in reading Braille) requires a special type of nerve ending in the skin (called a Merkel ending) which is abundant in the fingers, lips and tongue but almost completely absent in either the penis or the clitoris. The genitals, while they can easily detect faint sensations, cannot accomplish tactile form perception. In the spirit of old-fashioned natural philosophy, you can experiment with this at home (unless you live in Georgia where it’s illegal). In this way, they are kind of like the cornea of the eye: quite sensitive to faint sensations like a grain of sand, but without an ability to precisely locate those sensations. The medical term for this ability to detect faint stimuli without precise localization is called “protopathic sensation.” This difference in the exact type of touch sensation is likely to explain why neither the cornea nor the genitals (male or female) are particularly large in the sensory homunculus.