Although teeth are of different sizes and shapes, they each perform different functions. However, their structure consists of three parts: the crown, the root and the neck of the tooth. The crown is the part of the tooth that is exposed in the mouth and is mainly responsible for ingestion, processing food, supporting the face and assisting in pronunciation. The shape of the crown can be broadly categorized into three types: spatula-shaped, wedge-shaped, and molar-shaped. According to the shape of the crown, the milk teeth are divided into three categories (milk incisors, milk cusps and milk molars) and the permanent teeth are divided into four categories (incisors, cusps, premolars and molars). The incisors and cuspids are located in the front of the mouth and have the role of assisting pronunciation, collectively known as the front teeth, which are also commonly referred to as incisors and tigers. The incisors, or front teeth, are shovel-like in shape and are mainly used for ingesting and cutting food. The tiger teeth, or cuspids, are located in the corners of the mouth to support the face, and their shape is similar to a wedge, with long, strong roots, mainly used for tearing food. The anterior molars and molars are located in the back of the mouth, so they are called posterior teeth, or what we usually call the alveolar teeth, whose main task is to grind food into fine pieces until they are blended with saliva to form a food mass for easy digestion and absorption. The root is the part of the tooth that is buried in the alveolar bone and cannot be seen by the naked eye. The roots of the anterior and posterior teeth differ greatly in form and number. Incisors, cuspids, and premolars generally have single roots, mandibular molars tend to have double roots, and maxillary molars usually have three roots. The roots play the role of stabilizing the teeth and conducting chewing forces. The cervical part of the tooth is the part where the crown and the root meet, mostly in a curved shape, which is a weak area of the tooth tissue. The hard tissues of the crown and the hard tissues of the root meet here in three ways, one is exactly, another is overlapping each other, and another is a certain distance between the two. Dissecting the tooth longitudinally reveals the internal structure of the tooth. In the crown, looking from the outside in, the enamel, dentin and pulp are seen in that order. At the root of the tooth, the dentin, dentin, and pulp are seen in order from the outside in. The enamel is located on the surface of the crown, is white, translucent, highly calcified and is the hardest tissue in the body. Dentin is located in the inner layer of enamel and bone, yellowish, opaque, highly calcified but not as hard as enamel. It forms the bulk of the hard tissue of the tooth. Dentin contains many small canals that connect to the pulp chamber. The dentin tubules contain fluid from the pulp, nerve fibers and cell protrusions. The pulp, located in the innermost layer of the tooth, is soft tissue, rich in blood vessels, lymphatic vessels and nerve fibers that fill the pulp cavity and act as a nutrient to the hard tissues of the tooth. The nerves of the pulp are connected to the nerves of the brain mainly through the maxillary and mandibular branches of the facial trigeminal nerve. Some scholars have found that neurons within the ganglia of the trigeminal nerve innervate both the maxillary and mandibular and cephalic senses. One neuron can control the sensation of two or more teeth. The nerves of adjacent teeth or upper and lower teeth on the same side at the same time also cross in the ganglion. The nerve enters the pulp cavity from the alveolar bone through the apical foramen of the tooth, and after traveling around it for a week still exits the pulp cavity through the apical foramen. The part of the nerve inside the pulp cavity is the pulp nerve. The bone is located in the most superficial layer of the root and forms the superficial, yellowish hard tissue of the root.