Here’s what’s happening. Your lips are pretty delicate things – this one at the top is your Labium superius oris, and the one at the bottom is your Labium inferius oris.
Collectively they form an enormously sensitive, incredibly flexible part of your body.
However, they also have some vulnerabilities. For instance, the skin of your lips is different from the rest of your face – let’s take a closer look.
The outer layer is called the epidermis, and it has a protective covering called the stratum corneum. Underneath your epidermis is another layer of skin, the dermis.
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Fuente: Youtube / BrainStuff - HowStuffWorks