Read the following passage and answer the questions that follow.
We use them every day, but have you ever stopped to consider that your lips do not look, feel or act like other parts of your body? Why are they so red, so sensitive and so prone to dryness? And why have humans evolved to have lips, when other creatures — birds and turtles, for example — get on just fine without them.
Lips are fundamentally important for eating, respiration and speech. They are sensitive. They contain around 1 million nerve endings, which is why they are so affected by touch, and changes in temperature and moisture levels. The skin of the lip forms the border between the exterior skin of the face, and the interior mucous membrane of the inside of the mouth. The mucous membrane is represented by a large area in the sensory cortex of the brain and is therefore highly sensitive. Because of this, lips are capable of fine and gross muscular movement. This capacity for delicate and precise movement — provided by five muscles for lip elevation (upward movement) and four for lip depression (downward movement) — enables humans to communicate in the manner we do.
Lips are vital for bilabial and labiodental consonant sounds, as well as vowel rounding. Bilabial sounds can only be created via the use of both lips (the letter 'p' in picnic, for example), while labiodental sounds necessitate the use of lips and teeth (the letter 'f' in fructose).
Without using or moving one's lips, it is incredibly difficult to create some sounds or to pronounce certain letters: Try vocalizing the letters M, W or B without using your lips, for example, to get an idea of the difficulties faced by ventriloquists.
Let's return to our original discussion. Why do our lips look the way they do? Why are they so red, especially when compared with other parts of the face? The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, which has up to 16 layers. "With light skin color, the lip skin contains fewer melanocytes (cells that produce melanin pigment, which give skin its color). Because of this, the blood vessels appear through the skin of the lips, which leads to their notable red coloring.
Lip skin is very thin, not hairy, and does not have sweat glands. Therefore, it is relatively fragile, is dry to the touch, and splits easily. It does not have the usual protection layer of sweat and body oils, which keep the skin smooth, inhibit pathogens and regulate warmth. Therefore, the lips dry out faster and become chapped more easily.