Explanation The human adult has 32 teeth in the mouth. They exhibit different shapes. They are of four types-- incisors, canines, premolars and molars. Incisors The four front teeth in the centre of each jaw are called incisors. They are chisel shaped, used for biting and cutting and have sharp cutting edges. Canine One canine tooth is present on either side of the incisors in each jaw. They are used for tearing the food and are sharp and pointed. Premolars There are two pre-molars on each side of the canines on each jaw. Premolars help in grinding and crushing the food. The premolar tooth has two small projections on its surface called cusps - so it is often referred to as bicuspid tooth. Molars The last three teeth on each side of the premolars are the molars. They grind and crush the food. The last molar of each side of each jaw is called the wisdom tooth, as they develop when the person is about 17 - 20 years of age and it is assumed that he has attained maturity and wisdom Structure of a Tooth A tooth has three parts, the crown, neck and root. The crown is the exposed part of the tooth above the gum. The neck is the constricted region between the crown and the root. The neck region of the tooth is at the gum level. The root is part of the tooth that is embedded in a cup-like depression of the jawbone. The root consists of one process as in the case of incisors and canines, two processes as in the case of premolars and three processes in the case of molars. A longitudinal section of the tooth exhibits the following parts-enamel, dentine, pulp cavity, pulp and cement. Enamel is the outermost part of the tooth and covers the crown. It is the hardest substance in the body and is the non-living part of the tooth. Dentine is the portion below the enamel and forms the bulk of the tooth. It is not as hard as enamel but is harder than bone. It has small canals in which strands of cytoplasm are present. It surrounds the pulp cavity. Pulp is the soft connective tissue located in pulp cavity. It has blood capillaries, lymph vessels and nerve fibres, which are continuous with the base of the root. Cement is a bone-like structure that fixes the root in position in the gum. Dental Formula The number of teeth in mammals (including man) is indicated in a formula called the dental formula. The dental formula is written as i ; c ; pm ; m ____________ = x i ; c ; pm ; m where i represents the number of incisors c represents the number of canines pm represents the number of premolars m represents the number of molars x represents the total number of teeth present in the mouth. The numbers are given in the same order for half of each jaw. The numerator is for the half of the upper jaw and the denominator is the half of the lower jaw. The dental formula for adult and baby human beings is as follows: Adult man = 2 ; 1 ; 2 ; 3 __________ = 32 2 ; 1 ; 2 ; 3 Human baby = 2 ; 1 ; 2 ; 0 __________ = 20 2 ; 1 ; 2 ; 0 Dental Caries Dental caries is a common disease in which there is progressive decay and crumbling of the teeth due to demineralisation of the enamel. This is caused by dental bacteria, which break down sugars to form acids that eat away the enamel of the teeth. Regular usage of toothpaste and mouthwashes enriched with fluoride salts may promote the resistance of the enamel to acid attack.