Lip Characteristics and Appearance

1. The curvature of lips, their size and their eversion (whether they turn out or in) differ depending on gender and according to geographical ancestry. The part of the top lip that creates the dip in the middle and that tends to make the mouth look like a bow is called the philtrum. The area between the upper lip and the nose is the ergotrid.

Vermilion Border

2. The mouth has a colored border between the skin surrounding the mouth and the skin of the lip proper. No other animal has this feature, which is called the vermilion border, or vermilion, for short. The vermilion lip refers to the part of each lip that is visible when your mouth is closed.

Upper, Lower and Mucosal Lip

3. The upper lip is called the Labium superioris, and though the "superioris" implies a larger size, the upper lip is smaller than the bottom lip. That lower lip is called the Labium inferioris. Traveling from the outside skin of your lip to the inside, the lip surface changes, marking the boundary between the vermilion lip and the mucosal lip.

Skin

4. Lip skin is stratified squamous epithelium tissue, which only means that the cells are arranged in layers. The skin is thin, made up of just three to five of these cellular layers. Facial skin, by contrast, is up to 16 layers thick. The tissue of the mucosal lip is translucent and has many capillaries near the surface, making the it red.

Muscles

5. Lips have their own muscles, which are facial expression muscles. Sphincter muscles are the orbicularis oris---the main lip muscle---and buccinator. Several muscles raise the lips: the zygomaticus minor and major, the levator labii superioris and levator anguli oris, and the alaeque nasi. Muscles for lowering the lips are the depressor anuli oris and depressor labii inferioris, the risorious and the mentalis. The modiolus anchors several other muscles.

Nerves

6. The brain is very involved in what goes on in our lips, kept informed and exerting influence through nerves. The infraorbirtal nerve and parts of the trigeminal nerve give the upper lip its sensory ability. These also allow for eating movements: taking a bite, chewing and swallowing. The facial nerve allows us to tighten our lips, curl them and pout. Many nerve endings populate the lips, making kissing enjoyable.

What Lips Don't Possess

7. Lips don't possess certain protective features that other areas of the skin have such as hair, sebaceous glands that would produce oil or wax, or sweat glands. These lacks, among other things, cause lips to dry out easily.