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The human teeth function in mechanically breaking down items of food by cutting and crushing them in preparation for swallowing and digestion. There are four different types of teeth, namely incisors, canines, molars and premolars. Each type of tooth has a different job. The incisors cut the food, the canines tear the food and the molars and premolars crush the food. The roots of teeth are embedded in the maxilla (upper jaw) or the mandible (lower jaw) and are covered by gums. Teeth are made of multiple tissues of varying density and hardness. Teeth are among the most distinctive (and long-lasting) features of mammal species. Humans, like other mammals, are diphyodont, meaning that they develop two sets of teeth. The first set (called the "baby", "milk", "primary", or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful. Dental trauma refers to trauma to the face, mouth, and especially the teeth, lips and periodontium. The study of dental trauma is called dental traumatology. ==Anatomy== (詳細はDental anatomy is a field of anatomy dedicated to the study of tooth structure. The development, appearance, and classification of teeth fall within its field of study, though dental occlusion, or contact among teeth, does not. Dental anatomy is also a taxonomic science as it is concerned with the naming of teeth and their structures. This information serves a practical purpose for dentists, enabling them to easily identify teeth and structures during treatment. The anatomic crown of a tooth is the area covered in enamel above the cementoenamel junction (CEJ) or "neck" of the tooth. Most of the crown is composed of dentin (dentine in British English) with the pulp chamber inside. The crown is within bone before eruption. After eruption, it is almost always visible. The anatomic root is found below the CEJ and is covered with cementum. As with the crown, dentin composes most of the root, which normally has a pulp canals. A tooth may have multiple roots or just one root (single-rooted teeth). Canines and most premolars, except for maxillary first premolars, usually have one root. Maxillary first premolars and mandibular molars usually have two roots. Maxillary molars usually have three roots. Additional roots are referred to as supernumerary roots. Humans usually have 20 primary (deciduous, "baby" or "milk") teeth and 32 permanent (adult) teeth. Teeth are classified as incisors, canines, premolars (also called ''bicuspids''),〔("Bicuspid teeth" ). The Medical Dictionary. Retrieved on 2013-02-21.〕 and molars. Incisors are primarily used for biting pieces from foods such as raw carrots or apples and peeled but uncut bananas, while molars are used primarily for grinding foods after they are already in bite size pieces inside the mouth. Most teeth have identifiable features that distinguish them from others. There are several different notation systems to refer to a specific tooth. The three most common systems are the FDI World Dental Federation notation, the universal numbering system, and Palmer notation method. The FDI system is used worldwide, and the universal is used widely in the United States. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「human tooth」の詳細全文を読む スポンサード リンク
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