The vermilion border of the lips (lip for short) is the only part on the face where the oral mucosa is persistently exposed to the outside. Despite its prominent presence on the face, constituting not only the target of cosmetics but also the site for various skin diseases, its functional properties remain almost unknown.

Source :British Journal of Dermatology (paywalled)Volume 150, Issue 3, 2004

Human lips have considerably less keratin than the rest of the face, and have no sweat glands, sebaceous glans, or hair. Blood vessels are closer to the surface, producing the so called 'vermilion border' which can be very distinct in lighter-skinned individuals.

The 'vermilion border' has (or is presumed to have) an intense psychological impact which is ubiquitous, but as yet unexplained.