Digestion is the process in which the insoluble complex food material is broken down by physical and chemical methods into simple soluble food materials. In this process
    (I) Carbohydrates are converted into glucose.
    (II) Proteins are converted into Amino Acids.
    (III) Lipids are converted into fatty acid and glycerol.

Types of Digestion
(A)    Intracellular digestion:- When digestion takes place inside the cells. e.g. Amoeba, Paramecium, Euglena, Sponges.
(B)    Extracellular digestion:- When digestion takes place outside the cells. e.g. Earthworm, Starfish and Human beings etc. Human beings are omnivorous feed on different parts of the plant like root, shoot, fruits, seeds and also on animal products like                   eggs, meat and milk. 
•    The Human digestive system divided into two parts.
(A)    Alimentary Canal:- It has following parts

4. Oesophagus or food pipe
5. Stomach.

(B)    Digestive Glands:-
       1. Liver
       2. Pancreas
       3. Salivary gland
       4. Gall bladder - Absent in rat
       5. Gastric gland in stomach
       6. Intestinal gland in intestine

In human the alimentary canal is about 9 m long tube. The small intestine is about 6 m and large intestine about 1.5 m long.
Mouth and Buccal Cavity:-
Food is taken into the alimentary canal through mouth. It is called ingestion. Mouth is surrounded by lips. The food is chewed with the help of teeth, tasted with the help of tongue and moistened with saliva.

Tongue:- It is a muscular organ found in buccal cavity. It is free in front but posterior region is attached with a structure known as Frenulum.
 It contain two important structures
 1. Papillae            2. Taste buds

Papillae has taste buds for identification of taste. Sweet, Sour, Salty, Bitter.

        Function of the tongue:-
1.    Essential for swallowing and mastication.
2.    Essential for talking & speech
3.    Essential for taste the food.
4.    For mixing of saliva with food.

     Teeth:- The arrangement of teeths on upper and lower jaw is known as dentition. Teeth are rooted in sockets in the gums. Human has Heterodont dentition. There are four types of teeths.
    1. Incisors - For cutting
    2. Canines - For piercing and tearing


Dental formula:-

So Total No. of teeth in Human being is 32 in a adult.

  •     The Ist set of teeth grows during infancy and they fall off at the age between 6 - 8 years. These are called milk teeth. Their number is about 20.
  •     The second set of teeth that replaces them are called permanent teeth, which last through out life or fall off during old age. The number of permanent teeth are 28 including 8 molars.

    Structure of tooth:- A typical tooth is made up of three parts.

    (A) Crown
    (B) Neck
    (C) Root.
    Crown is externally covered by enamel whereas neck & root is covered by cement. Enamel is formed by Ameloblast cells.

The main part of tooth is dentine formed by odontoblast cells and have a pulp cavity. Pulp cavity is living part of tooth because it has blood supply and nerve supply.

Tooth Decay:-    If we do not clean our teeth after eating, they turn yellowish because a yellowish and sticky film formed from food particles, saliva and bacteria. This is called plaque.

The bacteria in the plaque convert sugar and starch of food into acids. This acid dissolve the enamel and damage the teeth. This is called tooth decay. This leads to the formation of cavity in the tooth & than tooth loss. Chocolates, sweets and cold drinks are responsible for tooth decay and tooth loss.