MOUTH & TEETH OF HUMAN DIGESTIVE SYSTEM: Structural arrangement and function of different regions of mouth and teeth.

The mouth and teeth admit food into the gastrointestinal tract. They cut and break large pieces, chop, grind and moisten what can be chewed, and prepare a smooth, round bolus that can be swallowed and passed on to the rest of the system. Of course, the lips and mouth also serve other functions.

Structure 

The sensitive, flexible, muscular lips that form the anterior border of the mouth can assess food by palpation, and their flexibility enables them to seal off the oral cavity and form variously a funnel, suction tube or shallow ladle to ingest fluids and food of varying consistency. The main muscles of the lips are orbicularis ori.
The maxilla and mandible support the roof and floor of the mouth, respectively. The arch of the mandible supports a sling of muscles that forms the floor, including the tongue. The maxilla is continuous with the rest of the skull and forms the roof of the mouth anteriorly and, simultaneously, the floor of the nasal cavity and paranasal maxillary sinus. Posteriorly, the roof is formed by the soft palate, composed of cartilage and connective tissue.
The sides of the mouth comprise the cheek muscles, chiefly buccinator, and supporting connective tissue. Posteriorly, the oral cavity opens into the oropharynx and the tonsils are situated between the fauces laterally, marking the posterior limit of the oral cavity.
The entire mouth, including the gingivae or gums, is lined with a tough, non-cornified stratified squamous epithelium,which changes to skin (cornified stratified squamous epithelium) at the vermillion border of the lips.
Teeth arise in the alveolar bone of the mandible and maxilla. Infants are born without external teeth and with precursors within the jaw. A transient set of 20 ‘milk’ teeth erupts through the surface of the bone between 6 months and 2 years of age. They are shed between 6 and 13 years of age and permanent teeth take their place. There are 32permanent teeth and the posterior molars, also known as wisdom teeth, may only erupt in young adulthood.
Teeth are living structures with a vascular and nerve supply(derived from the trigeminal, or IIIrd cranial, nerve) in the center of each tooth, which is termed the pulp. Surrounding the pulp is a bony layer called dentine and surrounding this is an extremely hard, calcified layer called enamel. Teeth lie in sockets within the alveolar bone and the joint is filled with a layer of tough fibrous tissue (the periodontal membrane) allowing a small amount of flexibility. The margins of the tooth joint are surrounded by gingivae, which are a continuation of the mucosal lining of the mouth.

Function 

The lips, cheeks and tongue help to keep food moving and place it in the optimal position for effective chewing. The main muscles of chewing or mastication are the masseter and temporalis, which powerfully bring the lower jaw up against the upper jaw, and the pterygoids, which open the jaws, keep them aligned, and moves them sideways, and backwards, and forwards for grinding. The trigeminal (Vth cranial) nerve controls the muscles of mastication.

Teeth are specialized for different tasks as follows:

• Incisorshave flat, sharp edges for cutting tough foods, such as meat and hard fruits.
• Canines have pointed, sharp ends for gripping food, particularly meat, and tearing away pieces.
• Premolarsand molarshave flattened, complex surfaces that capture tiny bits of food, such as grains, and allow them to be crushed between the surfaces of two opposed teeth. As people get older, the grinding surfaces of the molars are gradually worn down.
Certain drugs can be absorbed across the oral mucosa and may be prescribed sublingually(under the tongue). In this way, the need to swallow is avoided and the absorbed drug bypasses the liver and avoids hepatic first-pass metabolism. Glyceryl trinitrate is one of the most common drugs administered in this way.

Common disorders 

Herpes simplex infection of the mouth is very common, causing cold sores, which often erupt on the lips when people have other illnesses. Serious oral infections, usually caused by a mixture of anaerobic bacteria, are less common. The corners of the mouth may be ulcerated or fissured in patients who cannot take care of their mouths, for example after a stroke, so careful oral hygiene is important in these cases. Nutritional deficiency, particularly of B complex vitamins and iron, is also associated with fissures at the edge of the mouth, known as angular stomatitis. Shallow ‘apthous’ ulcers in the mouth are common and are usually not associated with a more serious condition. Rarely squamous cell carcinoma can develop. Risk factors include smoking and chewing tobacco or betel nut, which is particularly common on the Indian subcontinent. Dental caries is the commonest disorder of teeth, resulting in tooth loss with advancing age. It is caused by chronic bacterial infection of the gums and periodontal membrane, encouraged by carbohydrate and sugar-rich food residues left in the mouth. Bacteria grow in the gap between the tooth enamel and gums, forming a hard, impenetrable layer called plaque, within which they multiply. Their metabolic products, including organic acids, damage tooth enamel. Gradual erosion of enamel and retraction of the gingivae weakens the tooth joint. Infection can penetrate the pulp causing an abscess, and chronic infection can destroy and devitalize the pulp. Dental hygiene, including brushing and flossing and having fluoride in drinking water, which strengthens tooth enamel, reduces the incidence of caries.

STRUCTURAL ARRANGEMENTS OF HUMAN MOUTH & TEETH