The mouth is the first portion of the alimentary canal that receives food and saliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth.
In addition to its primary role as the beginning of the digestive system, in humans the mouth also plays a significant role in communication. While primary aspects of the voice are produced in the throat, the tongue, lips, and jaw are also needed to produce the range of sounds included in human language.
The mouth, normally moist, is lined with a mucous membrane, and contains the teeth. The lips mark the transition from mucous membrane to skin, which covers most of the body.
Illustration of the inside of a human mouth. Cheeks have been cut and lips pulled back for an unobstructed view. The first space of the mouth is the mouth cavity, bounded laterally and in front by the alveolar arches (containing the teeth), and posteriorily by the isthmus of the fauces. The oral cavity is also known as the mouth which swallows food that then goes down the esophagus and into the stomach.
While shut, the orifice of the mouth forms a line between the upper and lower lip. In facial expression, this mouth line is iconically shaped like a up-open parabola in a smile, and like a down-open parabola in a frown. A down-turned mouth means a mouth line forming a down-turned parabola, and when permanent can be normal. Also, a down-turned mouth can be part of the presentation of Prader-Willi syndrome.
The mouth plays an important role in eating, drinking and breathing. Infants are born with a sucking reflex, by which they instinctively know to suck for nourishment using their lips and jaw.
Lips can be adorned with lipstick or lip gloss, although in most cultures, this is typically only practiced by females. Both men and women, however, apply lip balm in order to soothe chapped or dry lips.
Piercings in or around the mouth have been made popular by younger generations, including those on the lip or tongue. The uvula piercing, while increasing in popularity, remains relatively rare.
The philtrum is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryo development. When these processes fail to fuse fully, a hare lip and/or cleft palate can result. This can result in the encouraging process of the release of metabolic wastes.
The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.
In non-human animals
Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostome and deuterostome. The first space of the mouth is the mouth cavity, bounded laterally and in front by the alveolar arches (containing the teeth), and posteriorly by the isthmus of the fauces.
Alligators are known for their large mouths and sharp teeth