Most of us take the three thousand square inches of skin that sheaths our body for granted. Throughout this sensitive wrapping, there are hundreds of thousands of sensory nerves. Huge numbers of them are clustered at the base of every hair, so that the slightest breath stirs some of them, while others signal whether the air is warm or cool, moist or dry.
Different parts of the skin respond to different stimuli. Some spots are especially sensitive to warmth and pain, while others respond to cold, touch or pressure. The nerves carrying these sensations of warmth, cold, pressure, pain or texture, although distributed all over the body, do not respond uniformly.
Many more cold sensitive spots occur on the back of the body, and more warm sensitive ones on the front. This makes sense, since the back is broad and more exposed, and is the ideal spot to warn us of chilliness in our environment. The abdominal surface on the other hand is a cozy shelter for infants, so the heat receptors here are an important signal of warmth.
The sensitivity of human touch depends on the thickness of the skin and its supply of hairs. We feel the lightest touch on the skin of the forehead, but it takes two and a half times as much pressure to perceive things using the finger tip.
So next time you name your senses, don’t forget to include the skin, which makes you aware of everything that’s going on around you.