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| Human Digestive System |
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| Man is omnivorous - feeding on different kinds of food. Thus, the structure of the digestive system should be adapted to this diet. The human digestive system consists of an alimentary canal, which is approximately 8metres long. The alimentary canal also carries out absorption of the digested food in addition to digestion. |
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| The digestive system is an organ system. An organ system is a collection of organs which carry out a life process. The structures that form the digestive system are discussed below: |
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| It is the anterior opening of the digestive system which ingests food. It has a muscular tongue on which are arranged the taste buds. There are two rows of teeth - upper and lower. |
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| Teeth are structures present on the jaw bones extending into the buccal cavity which help in mastication of food. |
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| Vertical section through a molar tooth showing internal structure |
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| A tooth consists of two main parts: |
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 crown |
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root |
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| The crown is the portion extending above the gum and root is the portion extending below the gum. It is further divided into the following layers: |
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| Enamel |
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| It is white. It is the hardest substance in the human body and covers the outer portion of the crown. It is made up of mineral salts (of calcium and magnesium) and keratin. It can withstand high pressure. |
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| Cement |
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| It is the layer present covering the root portion of the tooth. It is made up of mineral salts and water and is almost as hard as bone. |
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| Periodontal membrane or ligament |
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| It consists of fibres which extend across the cement and anchor the tooth in the bony socket. They also allow a certain degree of movement while chewing thereby acting as shock absorbers. |
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| Dentine |
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| It is yellow and is a bone-like material which is present along the full height of the tooth. It is enclosed by the enamel in the crown portion and cement in the root portion. Dentine can also be composed of living cells which show divisions with new cells being added to it regularly. |
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| Pulp cavity |
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| It is the innermost region of the teeth and shows the presence of blood vessels and nerve endings. The blood vessels serve to nourish the tooth and the nerves transmit messages of heat, cold and pain to the brain and back. The pulp cavity in the root portion is also referred to as the root canal. The number of root canals may range from 1 to 3 depending on the type of tooth. The blood vessels and the nerves enter the root canal through a small hole at the tip of the root canal. |
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| In a set of adult human teeth, the following four types of teeth can be found: |
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Incisors |
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Canines |
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Pre-molars |
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Molars |
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| Incisors |
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| Incisors are teeth which have chisel-like ends and are meant for cutting and biting. They have a single root canal. |
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| Canines |
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| Canines are teeth which are pointed and are meant for tearing the food. They are well-developed in carnivores. They too have single root canal. |
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| Pre-molars |
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| Pre-molars are bigger than incisors and canines and are flattened on top. There are four pre-molars each on upper and lower jaws, two on either side. The first pre-molar on the upper jaw has two root canals. All others have one. The surface has small humps which facilitates grinding and mastication of food. These humps are called the cusps. The pre-molars are normally bi-cuspid, having two cusps. |
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| Molars |
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| Molars are also flattened on top. However, the number of cusps are more ranging from three to five. They are bigger teeth and appear only in the permanent set of teeth. In fact, the last molar in each side, appears only much later in the adulthood. Hence, they are commonly called the wisdom tooth. |
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| During the life span of man, he grows two sets of teeth. The teeth which appear during the infancy are only 20 in number and are temporary. They are thus called deciduous teeth (also called milk teeth). In each jaw there are 10 (4 incisors, 2 canines and 4 pre-molars). |
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| The permanent teeth take the place of the milk teeth after they fall off. The types of teeth remain the same. In addition to these 20 teeth, the permanent set has 12 molars which are present 3 on each side. Thus, a total of 32 teeth are present in the normal human adult. |
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| Complete Complement of Teeth on one Jaw |
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| Dentition can also be represented by dental formula which gives the number of teeth in one half of a jaw. |
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| Thus, the dental formula for adult man is |
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| I:C:Pm:M = 2:1:2:3 or 2,1,2,3 |
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