A lip is one of two fleshy folds around the entrance to the mouth. Externally the lips are covered with skin and internally with mucous membrane, the relative transparency of which allows the red-pink of the underlying capillaries to show through.
The main substructure of the lips is a ring of muscle, whose functions including keeping food in the mouth, helping to produce speech and other sounds (whistling, for example), and kissing. Smaller muscles at the corners of the lips are responsible for facial expression.
Labial and buccal glands
The labial salivary glands are small, closely set mucus glands that lie in the submucous coat of the lips, and are palpable as little nodules when the tongue is pressed against the lip. Their ducts piece the mucus membrane and open into the vestibule of the mouth. he buccal salivary glands are similar mucus glands that lie in the submucous coat of the cheeks.