Tongue: The tongue is a muscular organ. Tongue helps to mix saliva in the food. It also helps to push the food down the food-pipe or oesophagus. Taste receptors are present on tongue and give us the sense of taste.
Oesophagus: It is a tube-like structure connecting the mouth and the stomach. It is about 30 cm. long. Oesophagus has powerful muscles which gently push the food down to the stomach. The oseophagus contracts and relaxes in a rhythmic fashion to facilitate the forward movement of food. This movement happens in other parts of the alimentary canal as well and is called peristalsis. No digestion takes place in oesophagus.
Stomach: It is a muscular J- shaped thick walled bag. Stomach is the widest part of alimentary canal. It receives food at one end from food pipe and opens into the small intestine on the other end.
Stomach churns the food to mix digestive juices. The food in the stomach is churned into semi solid. The churned semi-solid food is called chime. Gastric juice is secreted from the wall of stomach and mixed with food. Gastric juice contains some enzymes and hydrochloric acid. The enzymes present in the gastric juices break down protein from food. The hydrochloric acid kills the harmful bacteria (if any) present in the food and helps the gastric enzymes to work.
Small intestine: The food leaves the stomach at certain intervals of time and enters into the small intestine.
The small intestine is the longest part of the digestive system. It is about 20 feet or seven meters long in an adult human. Small intestine is a highly coiled tube. It consists of three parts: duodenum, jejunum and Ileum.
In the duodenum, the liver and pancreas pour their secretions. Liver secretes bile juice and pancreas secretes pancreatic juice. The bile juice contains bile which carries out emulsification of fat. In this process, the fat is broken into tiny droplets. The pancreatic juice contains several enzymes. The enzymes of the pancreatic juice break down starch into simple sugars and proteins into amino acids.
Minerals and vitamins do not need to be changed because cells are able to absorb them easily.
From duodenum the food goes to the lower part of the intestine. There are numerous finger-like projections on the wall of the small intestine. These projections are called villi. They have fine blood capillaries to absorb the food. After absorption, food mixes in the blood stream and is carried to all the cells of the body. The cells utilize this food to release energy.
Large intestine: The digested food enters into large intestine after small intestine. The large intestine is wider and shorter than small intestine. It is about 1.5 metre in length.
In large intestine, excess of water from the materials is absorbed. The semi solid residue is stored in the last part of the large intestine called rectum and finally throw out of the body through the anus from time to time. The throwing out of waste of digested food from rectum is called egestion. Egestion is also known as defecation.
Digestion in Grass Eating Animals
Ruminants: None of the animals can digest cellulose which is a major component of the food eaten by herbivores. The plant eating animals digest their food in two steps. Their stomach is divided into four chamber, viz. rumen, reticulum, omasum and abomasum.
First of all, half chewed food is swallowed and it then goes from mouth to the rumen, the first chamber of the stomach. Here, it is acted upon by bacteria. These microorganisms digest the cellulose. This half digested food goes to the second muscular chamber, the reticulum. From the reticulum the food is sent back to the mouth (as cud) to be chewed again. Chewing of the cud is called rumination and such animals are called ruminating animals or ruminants. Cow, goat, buffaloes, sheep, bison, etc. are good example of ruminating animals. The re-chewed food is swallowed for the second time. After passing the first two chambers it enters the third chamber, the omasum. Here the food is further broken down into smaller pieces and finally enters the fourth chamber, the abomasum. Here, all enzymes act upon the food and the digestion is completed.
After digestion and absorption, nutrients from food are taken to the cells in all parts of the body. The cells oxidize the food to release energy.