Anatomical basics of the human lip

One differentiates between the Upper and lower lip. The lower lip is usually somewhat larger. The border between the lips and the surrounding skin is referred to as the vermilion border, or simply the vermilion. The vertical groove on the upper lip is known as the philtrum.
The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, having up to 16 layers. With light skin color, the lip skin contains no melanocyte. Because of this, the blood vessels appear through the skin of the lips, which leads to their notable red coloring. With darker skin color this effect is less prominent, as in this case the skin of the lips contains more melanin and thus is visually thicker.
The lip skin is not hairy and does not have sweat glands or sebaceous glands. Therefore, it does not have the usual protection layer of sweat and body oils which keep the skin smooth, kill pathogens, and regulate warmth. For these reasons, the lips dry out faster and become chapped more easily.

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