The digestive system is composed of an alimentary canal, some accessory glands and organs. The alimentary canal begins from mouth and ends up in anus.
The alimentary canal is 8 – 10 m in length and is also called the digestive canal:
Mouth: The opening of the alimentary canal leads to buccal cavity or mouth which is a bowl-shaped structure. Buccal cavity is lined by soft palate at bottom and hard palate at top. Pharynx: The buccal cavity opens in a funnel-shaped cavity called pharynx. Through the pharynx, the food goes to oesophagus and air goes to the windpipe. Both the pipes open near the lower surface of pharynx, windpipe is in front while food pipe is at back.
Oesophagus: This is a narrow tube which goes from pharynx to stomach. Oesophagus is 25 cm in length. The main function of oesophagus is to carry food from mouth to stomach. There is a cartilaginous flap called epiglottis which ensures that the food goes only to the food pipe and does not go to the windpipe. Secretions of mucus glands in oesophagus make the food slippery. Different parts of oesophagus alternately contract and relax. This movement is called peristalsis and is responsible for forward movement of food.
Stomach: This is a J-shaped structure which lies between oesophagus and duodenum. The stomach lies towards left in the abdominal cavity and just behind the diaphragm. This is a flexible organ which can accommodate up to three litre of food.
Small Intestine: This is a highly coiled tube-like structure which begins from pylorus and ends in large intestine. Small intestine is about 7 m long. This is the most important part of the alimentary canal as major portion of digestion takes place in small intestine. The small intestine can be divided into three portions, viz. duodenum, jejunum and ileum.
Large Intestine: This is much shorter than small intestine and has a larger diameter. Some microbes live in large intestine. These microbes digest the remaining food by fermentation. Moreover, excess water and salt are absorbed by the walls of large intestine.
The significance of Enzymes: Many enzymes are produced in digestive system. These are hydrolytic enzymes. They convert components of food into simpler substances so that they can be easily absorbed.
