In humans, the mouth is an integral part of digestion, speech, and breathing. Food enters the mouth to be broken down both by the teeth and by enzymes secreted by three salivary glands—the sublingual gland, the submandibular gland, and the parotid gland. The tongue pushes food down the pharynx. The tongue and nasal cavity modify sound waves to produce the sounds of speech, while the tongue and teeth work together to form words.
Mouth, opening in an animal's body used for taking in food. Mouths are also typically used for making sounds, such as barks, chirps, howls, and in humans, speech. In most animals, the mouth is found on the face, near the eyes and nose.
Lips, which form the mouth's muscular opening, are an especially familiar part of the body for humans. Lips help hold food in the mouth and are used to form words during speech. They also help form facial expressions, such as smiling and frowning. Lips open wide during a yawn and squeeze together during a whistle. Lips are darker than the surrounding skin because of the many extremely small blood vessels, called capillaries, that show through the skin.
The cheeks form the sides of the mouth. They are composed of muscle tissue that is covered on the outside by skin. Like the lips, the cheeks help hold food and they also play a role in speech.
Inside the mouth is the large, muscular tongue. This extremely flexible muscle is used for eating and swallowing and also for talking. It is attached to the floor, or bottom, of the mouth. Its upper surface is covered with tiny projections, called papillae, that give the tongue a somewhat rough texture. The papillae contain tiny pores that are the site of taste buds, the receptor cells responsible for our sense of taste. There are four kinds of taste buds that are grouped together on certain areas of the tongue’s surface—those that are sensitive to sweet, salty, sour, and bitter flavors.
The roof, or top, of the mouth is called the palate. It separates the mouth from the nasal passages above it. The front part of the palate—the part closer to the lips—is made of bone covered with moist tissue, called mucous membrane. This part of the mouth is known as the hard palate. Behind the hard palate is the soft palate, a small area composed mainly of muscle tissue. During swallowing, the soft palate presses against the back of the throat, preventing food or liquid from moving upward into the nasal passages.
Teeth are used for biting into and chewing food. Their interaction with the lips and tongue helps a person speak clearly. Children have 20 primary teeth, which begin to erupt, or break through the gums, at about six months of age. At six years of age, the primary teeth start to fall out, as permanent teeth replace them. The number of permanent teeth is 32. The crown, or top, of each tooth is covered with enamel, the hardest substance in the human body.
The mouth also contains three pairs of salivary glands. These glands secrete a watery fluid called saliva, which moistens food and the tissues of the mouth. Saliva contains amylase, a digestive enzyme that starts to break down carbohydrates in food even before it is swallowed. Saliva also contains a specialized protein, or enzyme, called lysozyme, which fights bacteria.
Despite the presence of saliva, many kinds of bacteria live in the warm, moist environment of the mouth. Caring for the mouth, called oral hygiene, helps keep these bacteria from multiplying and causing illness. Daily brushing of the teeth and tongue, flossing between the teeth, and regular checkups with a dentist help keep the mouth clean and the teeth and gums healthy (see Dentistry).
The most common ailment of the mouth is tooth decay. Other disorders affecting the mouth include gingivitis, a condition marked by inflamed, infected gums; trench mouth, a severe form of gingivitis that causes bleeding ulcers in the mouth; and thrush, a fungal infection characterized by white sores in the mouth. Oral cancer is a risk for individuals who smoke or chew tobacco or who drink alcohol excessively. A small lump or thickened tissue in the mouth may indicate cancer. It should be checked by a doctor or dentist without delay, as many oral cancers can be cured if treated early.