large intestine :
In mammals, the colon consists of four sections: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon(the proximal gut usually refers to the ascending colon and transverse colon, and distal gut refers to the descending colon). The cecum, colon, rectum and anal canal make up the large intestine.
Sections of the colon are:


The parts of the colon are either intraperitoneal or behind it in the retroperitoneum. Retroperitoneal organs in general do not have a complete covering of peritoneum, so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile. Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the caecum, appendix, transverse colon and sigmoid colon are intraperitoneal. This is important as it affects which organs can be easily accessed during surgery, such as a laparotomy.

Cecum and appendix

The cecum is the first section of the colon and involved in the digestion, while the appendix which develops embryologically from it, is a structure of the colon, not involved in digestion and considered to be part of the gut-associated lymphoid tissue. The function of the appendix is uncertain, but some sources believe that the appendix has a role in housing a sample of the colon's microflora, and is able to help to repopulate the colon with bacteria if the microflora has been damaged during the course of an immune reaction.

Ascending colon

The ascending colon is one part of four sections of the large intestine. This first section of the large intestine is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs through the abdominal cavity, upwards toward the transverse colon for approximately eight inches (20 cm).
One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it back into the body. As the waste material exits the small intestine it will move into the cecum and then to the ascending colon where this process of extraction starts. The unwanted waste material is moved upwards toward the transverse section of the colon by the action of peristalsis. The ascending colon is sometimes attached to the appendix via Gerlach's valve. The appendix traditionally seen as a vestigial organ has been shown to have a high concentration of lymphatic cells. In ruminants, the ascending colon is known as the spiral colon. The cecum receives the solid wastes of digestion from the ileum via the ileocecal valve.

Transverse colon

The transverse colon is the part of the colon from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a large fold of peritoneum called the greater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesenteryknown as the transverse mesocolon.
The transverse colon is encased in peritoneum, and is therefore mobile (unlike the parts of the colon immediately before and after it). Cancers form more frequently further along the large intestine as the contents become more solid (water is removed) in order to form feces.
The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of the superior mesenteric artery (SMA), while the latter third is supplied by branches of the inferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the midgut andhindgut, is an area sensitive to ischemia.

Descending colon

The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store faeces that will be emptied into the rectum. It is retroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery. The arterial supply comes via theleft colic artery. The descending colon is also called the distal gut, as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region.

Sigmoid colon

The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped (see sigmoid; cf. sigmoid sinus). The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the stool to move into the rectum.
The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of the sigmoid arteries, a branch of the IMA. The IMA terminates as the superior rectal artery.
Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.

Rectum

Rectum is the last section of the colon.
anus
opening of excretion of fecal matter , undigested food and solid waste .

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