The lips are the gateway to the soul
The skin of our lips is very different from the skin that covers the rest of our bodies, consisting of only three to five layers of cells in comparison to the around 16 layers that make up the skin of the rest of the face. The outer skin layer, or stratum corneum, is only a twentieth of a millimetre thick and has no protective hydrolipid film or hair. Because the skin of the lips is so thin, the underlying blood vessels show through, which gives the lips their red colour. The skin of the lips also does not contain any sweat or sebaceous glands, nor does it produce any protective melanin – the substance that causes our normal skin to tan. This is why, for example, cold sores can develop as a result of exposure to the sun's rays.
Instead, the skin of the lips contains numerous nerve endings and blood vessels, which we use to sense the slightest touch and differences in temperature. Being thin-skinned is the price for this high degree of sensitivity that even small children are aware of – which is why they explore new objects by putting them in their mouths.