In anatomical terms, the lips consist of two walls that lie over the teeth, connect at the lateral sides, and meet the nose, cheeks, and chin. The rosy portions commonly referred to as the lips are called the red margins of the lips. Many of the terms for the lips are based on the Latin word, labia (LAY-bee-uh).
The upper lip is separated from the cheeks by the nasolabial furrow on each side. The center is split by a ridged depression called the philtrum, which is narrower toward the septum than the lip. The red margin of the upper lip is divided into threes: a tubercle (a prominence, just like one on a bone) and two wings.
The lower lip is separated from the chin by the mentolabial furrow (referring to the mental bone of the chin). This is another important depression, similar to the one under the glabella (see p. 0), which is often in shadow and is useful for forming the masses of the lips and the chin. The red margin of the lower lip is also separated into threes: the groove, an indentation that is shaped to meet with the tubercle of the upper lip, and two lobes, which press upward into the points formed between the tubercle and the wings.
The corners of the mouth, becuase of their many attachments to the facial muscles, each has an appreciable mass that asserts itself on the shape of the lips in the form of a diagonal wall. This wall truncates the lateral edges of the upper lip, and the lower lip seems to be tucked in behind this wall at its corners.