If you don't look where you're going, you could bash a knee, or bark your shin, or bang your elbow. But stubbing your toe will bring tears to your eyes, and may hurt for quite some time afterward. Why is this so painful? At Marquette magazine, the answer comes from Chris Geiser, clinical assistant professor of exercise science and director of Marquette's athletic training program.
"First, our feet and hands are our interface to the world. As such, they are highly innervated with nerve endings that provide sensory feedback to our central nervous system, which uses this information to guide our actions. With our toes and feet, this is as simple as sensing the shape of the ground, the incline, the pressure that our shoes create on our feet, or the slipperiness of the surface we are standing on — all very important information if we are to successfully navigate our world.
That's just the beginning of the explanation, so read the rest at the college's site. Link -via Metafilter
The last part of this explanation comes from an evolutionary perspective. In the not so distant past, infections killed many people. Stubbing a toe can open wounds on the feet, which are constantly in contact with the bacteria-laden environment. It has been suggested that individuals who received lots of sensory information from their toes were less likely to strike them, creating an evolutionary advantage for people blessed with this type of sensory information.
Evolution made stubbing your toe so painful that you're careful not to do it.
That reminded me of how the whiskey distiller Jack Daniel kicked his safe in anger when he couldn't get it open one morning, injured his toe, got an infection and died.