teeth

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Teeth Teeth or dentes are accessory digestive organs located in sockets of the alveolar processes of the mandible and maxillae. The job of the teeth is to start clamping, slashing, piercing, grinding, tearing and crushing the food down into small enough pieces so that it can fit the throat. Teeth also give shape to the face and aid in the process of speaking clearly. Each tooth consists of a crown, neck, and one or more roots. The crown is the functional part of the tooth that is visible above the gum. The neck is the constricted junction of the crown and root near the gum line. The root is the unseen portion that supports and fastens the tooth in the jawbone. The shapes of the crowns and the roots vary in different parts of the mouth. The teeth on one side of the jaw are essentially a mirror image of those located on the opposite side. The upper teeth differ from the lower and are complementary to them.[4][3]

Figure 6. Section of a Mandibular Molar [1] Internally, dentin forms the majority of the tooth. Dentin consists of a calcified connective tissue that gives the tooth its basic shape and rigidity. It is harder than bone because of its higher content of calcium salts (70% of dry weight). The dentin of the crown is covered by enamel, which serves to protect the tooth from the wear and tear of chewing. It also protects against acids that can easily dissolve dentin. It is the hardest substance in the body (calcium salts about 95% of dry weight). The dentin of the root is covered by cementum, another bonelike substance, which attaches the root to the periodontal ligament.

The pulp cavity lies within the crown and is filled with pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels. Narrow extensions of the pulp cavity, called root canals, run through the root of the tooth. Each root canal has an opening at its base, the apical foramen, through which blood vessels, lymphatic vessels, and nerves extend. The blood vessels bring nourishment, the lymphatic vessels offer protection, and the nerves provide sensation.[1] Humans normally have two dentitions or sets of teeth during their lifetime. The first set, known as the deciduous, milk, or primary dentition, is acquired gradually between the ages of six months and two years. As the jaws grow and expand, these teeth are replaced one by one by the teeth of the secondary set, the permanent teeth. There are five deciduous teeth and eight permanent teeth in each quarter of the mouth, resulting in a total of 32 permanent teeth to succeed the 20 deciduous ones.[4]

Figure 7. (a) Deciduous (Primary) Dentition and (b) Permanent (Secondary) Dentition [1] There are four types of teeth in human mouth[4]:  A total of eight incisors, or two sets of four on the upper and lower jaw, are located in the front and center of the mouth. The middle pair on the top and bottom are called central incisors, while the teeth on each side of the pair are named lateral incisors. The flat, broad and chisel-like narrow edges of the incisors allow for a "chopping" motion and cutting of food into chewable pieces.  Humans have four canines, one in each quadrant of the jaw if the layout of teeth were divided into two sections on both the upper and lower jaw. Canine teeth, also known as cuspids, are located on each side of the incisors. These sharp and pointed teeth are





anchored in the jaw by a long root which gives the canines stability needed for ripping, tearing and piercing motions. The pointed surface is necessary for consumption of tough foods like meat. Right behind the canine teeth are two premolars on each side of the jaw. Also known as bicuspids, premolars have cusps on top of broader surfaces that allow for grinding and chewing. Premolars act as a combination of a canine and a molar because in addition to a broad chewing surface, they also include sharp points on the surface for piercing and ripping food. After biting off a piece of food with the incisors or the canines, most of the chewing occurs in the back of the jaw by way of the molar teeth. Molars are the largest teeth, and have a flattened surface and several cusps to chew, crush and grind food to the smallest particle possible before swallowing. Humans have three molars in each quadrant of the jaw. Molars receive numbered names according to their location. First molars are adjacent to second premolars towards the back of the mouth, with second and third molars following consecutively. Third molars, often called wisdom teeth, are the last to erupt and sometimes do not come through at all.

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