Digestive and Associative Glands
The method of taking in and synthesis of various types of foods by different organisms is called nutrition.
Digestion involves splitting of food molecules by hydrolysis into smaller molecules that can be absorbed through the epithelium of the gastrointestinal tract.
There are two types of the digestive system:
The process of digestion which takes place inside the cell is known as intercellular digestion. This type of digestion is found in protozoans and in some multicellular animals like a hydra, echinoderms, etc.
The process of digestion which takes place outside the cell is known as extracellular digestion. This type of digestion occurs in most multicellular animals.
The digestive system differs widely in different animals though it has basic similarities in structure and functions. The digestive system of a mammal consists of the alimentary canal and associated glands
The alimentary canal is a long tube of about 8-10 metres in length. It has the following structure :
Mouth and Buccal Cavity
- It is a transverse slit-like aperture.
- Covered by lips.
- Muscular tongue and teeth are present inside the buccal cavity.
- The tongue is attached to the buccal cavity with the help of ligamentous folds known as frenulum.
- On the surface of the tongue different papilla which is responsible for the identification of different taste.
- An adult human being has 16 teeth on each jaw embedded on jaw sockets.
- Human beings are thecodont.
- They have four types of teeth or they are heterodont.
- The teeth appear twice in life so they are known as diphyodont.
- There are four kinds of teeth incisors, canine, premolar, and molar.
Structure of Teeth
- The upper part of the tooth is called a crown.
- The embedded part is called roots.
- The junction of the crown and root is known as the neck.
- The tooth is formed by a material called dentine.
- The dentine is covered by a harder material known as enamel.
- The dentine has a thin layer of cement over the root.
- It has a pulp cavity which has a mass of cells, blood cells, and nerves.
The mouth leads to a funnel-shaped pharynx which opens into a long muscular tube-like structure called the oesophagus.
- It is 20-25 cm long narrow, muscular and tubular structure.
- The food is propelled towards the stomach through the oesophagus.
- It is a J-shaped sac.
- The upper part is called cardiac or fundus.
- The middle part is the dome-shaped called the body.
- The distal part is called pyrrolic or antrum.
- The stomach has greater curvature at the left and has lesser curvature at the right.
- The mucus membrane of the stomach is thick having longitudinal folds called rugae.
- It has the presence of gastric juices.
- It is the longest part of the alimentary canal.
- It is about 6m in length.
It has three regions:
- It is a C-shaped structure.
- It is helpful in the secretion of pancreatic, bile and intestinal juice.
- Duodenum opens into jejunum.
- It is about 2.5m long.
- The ileum is the highly coiled structure having minute finger-like projections known as microvilli.
- These villi are helpful in increasing the surface area for the digestion and absorption of food.
- Ileum opens into the large intestine.
- It is the shorter and wider than the small intestine.
It has three parts:
- Small pouch-like structure.
- Having a tubular part called the vermiform appendix.
- Both these form are vestigial organs.
- It is a long speculated structure.
- Differentiated onto ascending colon, transverse colon and pelvic colon.
- It is S-shaped continues in the rectum.
- Slightly dilated.
- About 13cm long.
- Opens outside my anus, which is guarded by two anal sphincters
Digestive glands include salivary glands, liver and the pancreas.
There are three pairs of salivary glands parotid, submaxillary/submandibular, sublingual. These glands situated just outside the buccal cavity and secrete salivary juice into the buccal cavity.
The liver is the largest gland of our body. It is situated in the abdominal cavity. The hepatic lobules are the structural and functional units of the liver containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.
The pancreas is a compound elongated organ situated between the limbs of the U-shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.