Describe the structure of intestine of vertebrates.

Structure of intestine of vertebrates:

Introduction to the vertebrate intestine:

• The part of the digestive tract following stomach is intestine.In the intestine digestion and absorption of food takes place.

• Hence it is the most important part of the digestive tract and undergoes several modification in the vertebrates.In cyclostomes(lamprey), elasmobranchs(dogfish), some primitive bony fishes(sturgeon, polyodon),and occasional teleost intestine is short, straight and wide tube.
• Its lumen contains a typhlosole or spiral valve which conpensates for the short absorptive area.

Structure of intestine of vertebrates:

The valvular or spiral intestine is equivalent to the small intestine of higher vertebrates.

In case of Teleost and Tetrapod it is seen that spiral valve, intestine is not present.

And the intestine is greatly elongated, coiled,and further differentiated into an anterior small intestine and posterior large intestine.

Small intestine of vertebrates:

For digestion and absorption small intestine play the most important role.

➡️ In the surface of the small intestine ,present a large number of small finger like projections, which are regarded as villi.

➡️Villi increases the surface area of the small intestine. Because of the increase in the surface area of the small intestine, the area for the absorption of the food products also increases.

➡️A large number of digestive gland are also present in the small intestine ,they are tubular in structure, called crypts of liberkhun, found all along the length of the intestine.

These glands helps in the secretion of mucus and group of enzymes,which are known as succus entericus .

⏺️Parts of small intestine:

1. First part of the small intestine is known as duodenum.It is short, starts at the pyloric valve and terminates beyond the entrance of ducts,from pancreas and liver into it .In the submucosa of deodenum of small intestine, present characteristics Brunner's gland and also secretes hormones for stimulating pancreas and gall bladder to release their juices.
2. After the duodenum there is present another part of small intestine which is known as ileum. It is narrow greatly elongated and much coiled.
3. Only in mammals small intestine beyond duodenum is divided into anterior two-fifth jejunum and posterior three-fiifth ileum.Nodules of lymphoid tissues called Peyer's patches are found in ileum.

➡️ Large intestine of vertebrates:

• In most fishes and amphibians large intestine is relatively wider than small intestine.
• The characteristics of large intestine is that it is short, straight and leads into a posterior terminal chamber, the cloaca.
• Urinary and genital ducts joins with cloaca, which then pass through the cloacal apperture, and opens to the outside.In reptiles, birds and mammals large intestine is longer and divided into a proximal colon and a distal rectum the latter ending into cloaca.
• Cloaca is present in all mammals, but the exceptions are many bony fishes and monotremes .By partitioning of embryonic cloaca rectum of mammals is derived , because of this reason rectum of mammals is not homologous with the rectum of other vertebrates.

• At the joining of small and large intestine ,ileocaecal valve or ileocolic sphincter is present in case of tetrapods but in case of fishes it is absent.At the ileocolic junction,in case of amniotes, there is present an ileocolic caecum,in case of birds this is usually two in number.Man, monkeys, apes have a small caceum.The rectal gland,  present in elasmobranchs ,is a caceum that helps in the secretion of Nacl.