Lip

From ArticleWorld

The lips are a pair of features of the mammalian mouth that consist of both an upper lip and a lower lip. The lips are the visible component of the mouth and have a different cellular structure from the surrounding skin. The lips have both sensory and muscular functions.

Anatomy

The surface of the lip is considerably thinner that the surrounding skin. In fair-skinned individuals, the lips appear red because one can see the blood flow beneath its thin covering. In light-skinned people, the lips contain no melanocytes and, therefore, cannot become tanned. Dark-skinned individuals have some melanocytes in the outer layer of their lips. Because there are no sweat glands or oil glands on the lips, the outer layer can easily become dry.

The lips actually have two different surfaces. The external epithelial surface is similar to the surrounding skin. The inner mucosal membrane surface is, instead, similar to the surface on the inside of the mouth.

The muscles that act on the lips are numerous. Many of them are among the muscles of facial expression that also control muscles affecting the eyes and the nose. Inside the lips themselves are two muscles that function to allow the mouth to open and close. The buccinator muscle and the orbicularis oris muscle are the two muscles that work together to perform this function. External to the lips, a total of five separate muscles are necessary to elevate the lips (to smile) and four different muscles help the lips depress (or frown).

Function

The lips have both muscular and sensory function. As muscles, the lips can get food into the mouth, suck up liquids and are necessary to keep food in the mouth while chewing. The suctioning ability of the lips is essential to helping babies who breastfeed.

As a sensory organ, the lips are sensitive to touch and temperature variations. They are useful when testing the texture or temperature of food. Because lips are so filled with nerve endings, they are considered erogenous zones. Kissing is provokes erotic feelings because the lips are so sensitive to touch.

The muscles around and inside the lips give the lips much importance in the articulation of sounds and words. The position and tension on the lips control the actual sound that can be produced when air is forced through them. Facial expressions, too, rely on the lips to relay the facial message, along with other parts of the face. Muscles that collectively place the lips in certain positions can convey everything from extreme happiness to extreme sadness.