A lip is a soft, movable tissue structure that forms the opening of the mouth. Humans have an upper lip and a lower lip, both made up of skin, muscle, connective tissue, and a mucous membrane lining. Lips serve essential roles in eating, speaking, facial expression, and sensory perception.
The Four Zones of the Lip
What looks like a simple body part is actually made up of four distinct surface zones, each with different tissue types. Starting from the outside and moving inward, these are: the hairy skin surrounding the mouth, the vermilion border, the vermilion, and the oral mucosa.
The vermilion is the red or pink part most people think of as “the lip.” It’s covered with a thin, specialized layer of tissue that’s continuous with the lining inside your mouth. Unlike regular skin, the vermilion lacks sweat glands, oil glands, and hair follicles. This is why your lips dry out and chap so easily compared to the rest of your face: there’s no built-in system to keep them moisturized.
The vermilion border is the pale rim that outlines the red part and separates it from surrounding facial skin. This border gives lips their defined shape and is one of the features cosmetic procedures often aim to enhance. On the upper lip, the border dips into a shape sometimes called Cupid’s bow, a contour formed during fetal development when the two sides of the face grow independently and fuse together at the midline. The vertical groove running from the nose to the center of the upper lip, called the philtrum, marks where this fusion happened. When the fusion doesn’t complete properly (roughly 1 in 750 births), the result is a cleft lip or palate.
The Muscle That Controls Your Lips
The primary muscle of the lips is the orbicularis oris, a complex, multi-layered ring of muscle fibers encircling the mouth. It also serves as an anchor point for many other facial muscles in the area. Its fibers work in two distinct ways depending on their depth.
The deeper fibers act as a constrictor, tightening the mouth like a drawstring. This sphincter action is what lets you hold food and liquid in your mouth, and it plays a role in swallowing and sucking. The more superficial fibers handle finer, more precise movements: the subtle shaping of lips needed for speech, facial expressions, and activities like whistling or kissing (which is why the orbicularis oris is sometimes called the “kissing muscle”).
Together with the cheek muscles and muscles of the throat, the orbicularis oris forms a functional unit that coordinates swallowing, chewing, sucking, and the pronunciation of vowels and consonants. Damage to this muscle or the nerve that controls it can significantly affect a person’s ability to eat, drink, and speak clearly.
Why Lips Are So Sensitive
Your lips are among the most sensitive parts of your body. They’re packed with nerve endings, and the brain dedicates a disproportionately large area of its sensory processing region to lip input. In brain mapping studies, researchers have found that a large portion of the primary sensory cortex is reserved for the lips alone, far more than you’d expect given their small physical size. This is similar to the way fingertips are over-represented in the brain’s sensory map.
This density of nerve endings makes lips highly responsive to touch, temperature, pressure, and pain. It’s why a tiny cut on the lip feels much more painful than the same cut on your forearm, and why lips are so effective at evaluating the texture and temperature of food before you commit to swallowing it.
How Lips Shape Speech
Many of the sounds in spoken language depend on precise lip positioning. Linguists categorize these as labial consonants, meaning sounds where one or both lips are the active part doing the work. In English, there are two main types.
Bilabial sounds are made by pressing both lips together. These include the sounds “m,” “p,” and “b.” Labiodental sounds are made by pressing the lower lip against the upper teeth, producing “f” and “v.” Vowel sounds also require lip shaping: rounding the lips for “oo,” spreading them for “ee.” Without precise lip control, intelligible speech becomes difficult, which is why conditions affecting the orbicularis oris or the facial nerve often cause noticeable changes in how a person talks.
Internal Attachments
If you pull your upper or lower lip away from your teeth and look in a mirror, you’ll see a small fold of tissue in the center connecting the lip to the gum. This is the labial frenulum, a band of connective tissue present on both the upper and lower lips. Under normal circumstances, it simply anchors the lip without restricting movement.
When a frenulum is unusually thick, tight, or positioned too close to the teeth, it can pull on the gum tissue, create a gap between the front teeth, or limit how far the lip can move. In some cases, particularly in infants having difficulty breastfeeding, a tight frenulum may be released with a simple procedure.
How Lips Change With Age
Lips undergo measurable changes over a lifetime. Research from the American Society of Plastic Surgeons found that the upper lip lengthens significantly in older adults: about 19 percent longer in women and 18 percent longer in men compared to younger people. At the same time, the soft tissue of the upper lip thins dramatically, losing roughly 41 percent of its thickness in women and 33 percent in men.
This combination of lengthening, thinning, and volume loss explains why lips appear to flatten and lose definition with age. The process is driven by the same collagen breakdown that affects skin elsewhere on the body, but the lip’s lack of oil glands and its constant movement may accelerate the effect. The vermilion border also becomes less distinct over time, which is why the sharp outline of younger lips gradually softens.
Lips in Other Animals
Fleshy, mobile lips are a defining feature of mammals. They evolved alongside the ability to suckle milk, which requires a soft, flexible seal around the nipple. Reptiles, birds, fish, and amphibians don’t have true lips in this sense. Some animals, like horses and elephants, have highly mobile and muscular lips adapted for grasping food. Others, like whales, have rigid lip structures that serve very different functions, such as filtering water. The soft, expressive lips that humans use for communication and emotional display are relatively unusual even among primates.