Editing Large intestine (section)

===Sections===
[[File:Diameters of the large intestine.svg|thumb|Inner diameters of colon sections]]
In [[mammal]]s, the large intestine consists of the [[cecum]] (including the [[appendix (anatomy)|appendix]]), colon (the longest part), [[rectum]], and [[anal canal]].<ref name='NCILargeIntestineDef'/>

The four sections of the colon are: the [[ascending colon]], [[transverse colon]], [[descending colon]], and [[sigmoid colon]]. These sections turn at the [[colic flexures]].

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.<ref>{{cite web |url=http://www.mananatomy.com/digestive-system/peritoneum |title=Peritoneum |publisher=Mananatomy.com |date=2013-01-18 |access-date=2013-02-07 |archive-date=2018-10-08 |archive-url=https://web.archive.org/web/20181008195347/http://www.mananatomy.com/digestive-system/peritoneum |url-status=dead }}</ref> Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal.<ref>{{cite web|url=http://www.ucd.ie/vetanat/ga-subject/abdomen/ab13.html|title=Untitled}}</ref> This is important as it affects which organs can be easily accessed during surgery, such as a [[laparotomy]].

In terms of diameter, the cecum is the widest, averaging slightly less than 9&nbsp;cm in healthy individuals, and the transverse colon averages less than 6&nbsp;cm in diameter.<ref name=":0">{{Cite journal|last1=Horton|first1=K. M.|last2=Corl|first2=F. M.|last3=Fishman|first3=E. K.|date=March 2000|title=CT evaluation of the colon: inflammatory disease|journal=Radiographics |volume=20|issue=2|pages=399–418|doi=10.1148/radiographics.20.2.g00mc15399|issn=0271-5333|pmid=10715339|doi-access=}}</ref> The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging {{convert|4–5|cm|in|abbr=on}} in diameter.<ref name=":0" /><ref>{{Citation|last=Rossini|first=Francesco Paolo|chapter=The normal colon|date=1975|pages=46–55|editor-last=Rossini|editor-first=Francesco Paolo|publisher=Springer New York|doi=10.1007/978-1-4615-9650-9_12|isbn=9781461596509|title=Atlas of coloscopy}}</ref> Diameters larger than certain thresholds for each colonic section can be diagnostic for [[megacolon]].
[[File:3DPX-002736 Large intestine Nevit Dilmen.stl|thumb|3D file generated from [[computed tomography]] of large intestine]]

====Cecum and appendix====
{{Main|Cecum|Appendix (anatomy)}}
The [[cecum]] is the first section of the large intestine and is involved in digestion, while the [[appendix (anatomy)|appendix]] which develops embryologically from it, is not involved in digestion and is considered to be part of the [[gut-associated lymphoid tissue]]. The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of the [[gut microbiota]], and is able to help to repopulate the colon with microbiota if depleted during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells.

====Ascending colon====
{{Main|Ascending colon}}
The ascending colon is the first of four main sections of the large intestine. It is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity toward the transverse colon for approximately eight inches (20&nbsp;cm).

One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it. As the waste material exits the small intestine through the [[ileocecal valve]], it will move into the cecum and then to the ascending colon where this process of extraction starts. The waste material is pumped upwards toward the transverse colon by [[peristalsis]]. The ascending colon is sometimes attached to the [[appendix (anatomy)|appendix]] via [[Joseph von Gerlach|Gerlach's valve]]. In [[ruminants]], the ascending colon is known as the '''spiral colon'''.<ref>{{citation
| url = http://medical-dictionary.thefreedictionary.com/spiral+colon
| title = Medical dictionary}}</ref><ref>{{citation
| url = http://faculty.ucc.edu/biology-potter/Pig_Digestive_System/sld009.htm
| title = Spiral colon and caecum
| access-date = 2014-04-02
| archive-url = https://web.archive.org/web/20160304022928/http://faculty.ucc.edu/biology-potter/Pig_Digestive_System/sld009.htm
| archive-date = 2016-03-04
| url-status = dead
}}</ref><ref>{{cite web|url=http://www.answers.com/topic/colon-9|title=Answers – The Most Trusted Place for Answering Life's Questions|website=[[Answers.com]]}}</ref>
Taking into account all ages and sexes, colon cancer occurs here most often (41%).<ref name="Colorectal cancer statistics, 2017">{{cite journal|vauthors=Siegel RL, Miller KD, Fedewa SA, Ahnen DJ, Meester RG, Barzi A, Jemal A |title=Colorectal cancer statistics|journal=CA Cancer J. Clin.|volume=67|issue=3|pages=177–193|date=March 1, 2017|doi=10.3322/caac.21395|pmid=28248415|doi-access=free}}</ref>

====Transverse colon====
{{Main|Transverse colon}}
The transverse colon is the part of the colon from the [[Colic flexures|hepatic flexure]], also known as the right colic, (the turn of the colon by the [[liver]]) to the [[Colic flexures|splenic flexure]] also known as the left colic, (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 [[mesentery]] known 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).

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]] and [[hindgut]], is an area sensitive to [[ischemia]].

====Descending colon====
{{Main|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 feces 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.<ref>{{Cite journal|last1=Smithivas|first1=T.|last2=Hyams|first2=P. J.|last3=Rahal|first3=J. J.|date=1971-12-01|title=Gentamicin and ampicillin in human bile|journal=The Journal of Infectious Diseases|volume=124 Suppl|pages=S106–108|issn=0022-1899|pmid=5126238|doi=10.1093/infdis/124.supplement_1.s106}}</ref> The arterial supply comes via the [[left 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====
{{Main|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 [[wiktionary:sigmoid|sigmoid]]; cf. [[sigmoid sinus]]). The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing the [[feces|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====
{{Main|Rectum}}
The [[rectum]] is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation.
It is about 12&nbsp;cm long.<ref>{{Cite web|title=Anatomy of Colon and Rectum {{!}} SEER Training|url=https://training.seer.cancer.gov/colorectal/anatomy/|access-date=2021-04-14|website=training.seer.cancer.gov}}</ref>