The Mouth

Generalities.

    The mouth is the gateway, for food and liquid to enter our bodies. Of course, as we breath, the air can flow through our mouth, and we can also use it to speak or to make sounds (vocalizations), but these functions are primarily those of the respiratory system. In this page, we will discuss the mouth, considering only its relation to the functions of the digestive system.

    The mouth is mostly involved in chewing. Rhythmic movements, opening and closing of the mouth (chewing), are used to grind food. Chewing results from movement of the mobile lower jaw, the mandible, on the fixed upper jaw. Indeed, only the lower jaw could move, the upper jaw is part of the skull.

    The main muscles, that makes the mandible move, is the masseter muscle. This 'joint' is very strong: it can generate up to 25 Kg of compression at the front teeth and up to 85 Kg at the rear teeth. If this pressure force is applied to a very small surface area, like a seed, the generated pressure could be equivalent to several thousand "pounds per square inch (psi)" or tens of thousand of KPa.

Teeth.

    Chewing is used to cut food into tiny pieces. This is particularly important for substances which are hard to digest, like plant cellulose. Digestion per se, occurs when the nutrients enter in contact with digestive enzymes. So, the smaller the particles will the easier they will be to digested.

    It is the work of the teeth to cut and tear down the food. The incisors are used primarily to cut foods, particularly vegetables. The canines are used to tear tough membranes found in the muscles (meat) and around the bones. And, the molars do the final grinding.

     The figure on the left shows the upper teeth (from above) of a child's mouth (top) and the mouth of an adult (bottom).

     The figure below shows the lower dentition (mandible) of an adult.

The mandible, and teeth.

    Adult teeth (permanent) are already present in the skull of the child (figure bellow). They will grow slowly and push away the deciduous teeth, the children's teeth, which will fall one by one and be replaced.

    
Frontal and lateral view of a 5y old skull.

     The body of the tooth is made of dentin, which is essentially a calcified tissue. At the surface, outside the gum, the crown is covered with a very hard layer of calcified epithelium which forms the tooth enamel. In the gum, the roots are anchored to the jawbone. These are the periodontal ligaments that ensure adherence between the cement of the tooth's root, and the jawbone.

    Inside the tooth we find the pulp which extends to form the root canal. This is where the small blood vessels and nerve fibers will enter the tooth. The blood vessels serve to feed the cells, the odontoblasts, that line inside the tooth and secrete dentin.

The Tongue.

     The main role of the tongue is to ensure mixing of the food during mastication. The tongue is also very important for tasting, but this will be discussed with the nervous system. Another function of the tongue is to help control the upper airway and blowing for vocalization.

    The tongue is a mass of striated muscle. During mastication, the tongue constantly replace food between the teeth and mix it with saliva. This form a mass of chopped food and saliva, called a bolus, which is then swallowed.

     The arrangement, in all directions, of the muscle fibers of the tongue allows it to change shape, making it thicker, thinner, longer or shorter. Conversely, muscle fibers that connect the tongue to the base of the skull are used to modify the position of the tongue. The tongue is divided in two, by a connective tissue, the lingual septum.

     The surface of the tongue is covered with papillae, small skin protrusions, which contain nerve endings and participate to taste. There are mainly three types of papillae: 1) the filiform papillae (tapered) are the most numerous. They are made of keratin and make the tongue rough, helping to better grind the food; 2) the fungiform papillae (mushroom shaped) are found scattered on the surface of the tongue. In their center, there is a taste bud with its vasculature and innervation; and 3) the vallate papillae, numbered eight to twelve, are located at the base of the tongue, in the back.

     As the name says, the salivary glands produce and secrete saliva. Distributed across the oral mucosa, there are small intrinsic glands. But it is mostly the three pairs of extrinsic glands, located outside of the mouth cavity per se, which secrete, via small channels, most of the saliva.

     There are the parotid glands in front of the ears on each side of the head. These glands, which have their conduits flowing at the level of the 1st or 2nd molars, provide most of the saliva. Located in the mandible, attached to the base of the tongue, there are the submandibular glands, which have their conduits flowing near the lingual frenulum. Finally, there are the sublingual glands that secrete saliva, just under the tongue.

     Although saliva is not essential for digestion, it has a multitude of functions that could help. Saliva is mainly composed of water (99.5%) and this water allows tasting by dissolving the molecules that stimulate the taste buds. The water also facilitates phonation; it's hard to talk when your mouth is dry.

    The continuous flow of saliva helps to eliminate alimentary particles and dead cells. And, saliva also serves to neutralize the acidity in the mouth. But, saliva serves three main functions: 1) it contains the amylase enzyme, which initiates the digestion of carbohydrates (sugars); 2) saliva also contains a mucus that, by lubricating food, facilitates swallowing; and 3) saliva helps the immune system, it contains lysozyme, an enzyme which can destroy some germs and bacteria.