The process of converting food into energy giving substances is carried out by this system.
The digestive system comprises the alimentary canal and the associated digestive glands like liver and pancreas.
1. Buccal Cavity
In the mouth salivary glands secrete saliva which contains the enzyme Ptyalin (or salivary amylase)
Here the food is called bolus.
Also contains Lysozyme which kills bacteria.
Teeth : They are produced in two sets – Temporary and Permanent.
The teeth are of four types : Incisors, Canines, Premolars and Molars.
Structure of a Tooth : A tooth consists of 3 Regions :
Crown, Neck and Root. The crown is the exposed part. It is covered by a shining material called Enamel.
Enamel is the hardest substance in the human body
Dental Formula : 2123 / 2123. This means that there are 2 incisors, 1 canine, 2 Premolars and 3 molars in half of the upper jaw and exactly the same arrangement in half of the lower jaw.
This figure is doubled to get the full number, i.e. 32.
In elephants, incisors of the upper jaw grow into huge tusks.
Canines are very large in predators such as cat, dog and lion.
Rabbit and squirrel lack canines. In walrus, the upper canines are enlarged into projecting tusks.
The premolars and the molars are called the grinding teeth. The last molars are called the wisdom teeth.
Linked with pharynx.
Links Buccal Cavity to Oesophagus.
Food is carried by peristaltic movement [Peristalsis is a series of wave of contraction that passes from end to other and is meant for pushing the food].
Trachea opens into Pharynx through an aperture called Glottis.
It also receives the opening of the Eustachian Tubes from the middle ear through an Aperture called Gullet.
25 cm Long Tube. Longitudinal folds keep its cavity almost closed, except during swallowing of food.
This checks die air going into it during breathing.
Leads to stomach.
Oesophagus opens into it. The opening is guarded by a valve which checks the regurgitation of food.
In stomach, the food is called chyme. Has 3 Parts :
It is a warehouse where food can be stored to await the main process of digestion.
Gastric juices produced in the stomach help in digestion of food.
Also contains HCI which kills bacteria and provides acidic medium.
Besides it activates pepsinogen to pepsin and prorennin to rennin.
Ruminents (Cattle, Buffaloes, Goats, Sheep, Deer, Camel) have their stomach consisting of 4 Chambers :
Rumen, Reticulum, Omasum and Abomasum.
This helps the ruminants to take food in large quantities in short time without chewing much.
5. Small Intestine (Duodenum + Jejenum + Ileum)
Here the food from the stomach is mixed with bile (from liver) and pancreatic juice (from pancreas) and moves forward by peristaltic movement.
Duodenum receives the bile – pancreatic duct formed by the union of bile duct and pancreatic duct.
Both jejunum and ileum have numerous figure like projections called the villi.
The villi contain blood vessels to receive the products of digestion after they have been absorbed.
Bile doesn’t take direct part in digestion of fat; it just makes the food alkaline. (It is stored in gall bladder)
Pancreatic juice contains several enzymes, which act on proteins, carbohydrates and fats.
6. Large Intestine (Caecum + Colon + Rectum)
It receives undigested material from the small intestine and absorbs water.
Caecum is the diverted part of intestine. Cellulose digestion takes place in it. It is very large in herbivores.
Vermiform appendix is a part of caecum, which produces antibodies. Both caecum and appendix are vestigial in humans.
The remaining waste material is sent to rectum and discharged through anus.