Editing Appendix (anatomy)

{{Short description|Tube attached to the intestine}}
{{Other uses|Appendix (disambiguation){{!}}Appendix}}
{{Dist|Appendage}}
{{Infobox anatomy
| Name        = Appendix
| Image       = Tractus intestinalis appendix vermiformis.svg
| Caption     = Appendix with surrounding structures
| Width       =200 
| Precursor   = [[Midgut]]
| System      = [[Gastrointestinal tract|Digestive system]]
| Artery      = [[Appendicular artery]]
| Vein        = [[Appendicular vein]]
| Nerve       = 
| Lymph       = 
|Image2=Appendix locations.svg|Caption2=Variations of the appendix}}

The '''appendix''' ({{plural form}}: '''appendices''' or '''appendixes'''; also '''vermiform appendix''';  '''cecal''' (or '''caecal''', '''cæcal''') '''appendix'''; '''vermix'''; or '''vermiform process''') is a finger-like, blind-ended tube connected to the [[cecum]], from which it [[prenatal development|develops in the embryo]]. 

The cecum is a pouch-like structure of the [[large intestine]], located at the junction of the [[small intestine|small]] and the large intestines. The term "[[vermiform]]" comes from [[Latin]] and means "worm-shaped". The appendix was once considered a [[Human vestigiality|vestigial organ]], but this view has changed since the early 2000s.<ref>{{cite journal|vauthors=Kooij IA, Sahami S, Meijer SL, Buskens CJ, Te Velde AA|date=October 2016|title=The immunology of the vermiform appendix: a review of the literature|journal=[[Clinical and Experimental Immunology]]|volume=186|issue=1|pages=1–9|doi=10.1111/cei.12821|pmc=5011360|pmid=27271818}}</ref><ref>{{Cite journal |last1=Smith |first1=H. F. |last2=Fisher |first2=R. E. |last3=Everett |first3=M. L. |last4=Thomas |first4=A. D. |last5=Randal Bollinger |first5=R. |last6=Parker |first6=W. |date=2009 |title=Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix |journal=Journal of Evolutionary Biology |language=en |volume=22 |issue=10 |pages=1984–1999 |doi=10.1111/j.1420-9101.2009.01809.x|pmid=19678866 |doi-access=free }}</ref> Research suggests that the appendix may serve as a reservoir for beneficial [[gut bacteria]].<nowiki/>

== Structure ==
The human appendix averages {{convert|9|mm|in|abbr=on}} in length, ranging from {{convert|5|to|35|mm|in|abbr=on}}. The diameter of the appendix is {{convert|6|mm|in|abbr=on}}, and more than {{convert|6|mm|in|abbr=on}} is considered a thickened or inflamed appendix. The longest appendix ever removed was {{convert|26|cm|in|abbr=on}} long.<ref name="record">{{cite web |url=https://www.guinnessworldrecords.com/world-records/largest-appendix-removed |title=Largest appendix removed |date=26 August 2006 |publisher=[[Guinness World Records]] |access-date=22 May 2017 |url-status=live |archive-url=https://web.archive.org/web/20201126014839/https://www.guinnessworldrecords.com/world-records/largest-appendix-removed |archive-date=26 November 2020}}</ref> The appendix is usually located in the lower right [[quadrant (anatomy)|quadrant]] of the [[abdomen]], near the right [[iliac fossa|hip bone]]. The base of the appendix is located {{convert|2|cm|in|abbr=on}} beneath the [[ileocecal valve]] that separates the large intestine from the small intestine. Its position within the abdomen corresponds to a point on the surface known as [[McBurney's point]].

The appendix is connected to the [[mesentery]] in the lower region of the [[ileum]], by a short region of the [[mesentery|mesocolon]] known as the [[mesentery#Segments|mesoappendix]].<ref name="auto">{{cite journal |last1=Golalipour |first1=M.J. |last2=Arya |first2=B. |last3=Jahanshahi |first3=M. |last4=Azarhoosh |first4=R. |date=2003 |title=Anatomical Variations Of Vermiform Appendix In South-East Caspian Sea (Gorgan-IRAN) |url=https://medind.nic.in/jae/t03/i2/jaet03i2p141.pdf |journal=J. Anat. Soc. India |access-date=1 October 2014 |url-status=live |archive-url=https://web.archive.org/web/20200711120812/https://medind.nic.in/jae/t03/i2/jaet03i2p141.pdf |archive-date=11 July 2020}}</ref>

===Variation===
Some identical twins—known as [[Twin#Mirror image twins|mirror image twins]]—can have a [[situs inversus|mirror-imaged anatomy]], a [[congenital disorder|congenital condition]] with the appendix located in the lower left quadrant of the abdomen instead of the lower right.<ref>{{cite web|url=https://www.multiplesofamerica.org/research/unusual-types-of-twins/|title=Unusual Types of Twins|publisher=Multiples of America|url-status=dead|archive-url=https://web.archive.org/web/20140502001856/http://www.nomotc.org/index.php?option=com_content&task=view&id=59&Itemid=54|archive-date=2 May 2014|access-date=30 April 2014}}</ref><ref>{{cite journal | vauthors = Gedda L, Sciacca A, Brenci G, Villatico S, Bonanni G, Gueli N, Talone C | title = Situs viscerum specularis in monozygotic twins | journal = Acta Geneticae Medicae et Gemellologiae | volume = 33 | issue = 1 | pages = 81–5 | year = 1984 | pmid = 6540028 | doi = 10.1017/S0001566000007546 | doi-access = free }}</ref> [[Intestinal malrotation]] may also cause displacement of the appendix to the left side.

While the base of the appendix is typically located {{convert|2|cm|in|abbr=on}} below the [[ileocecal valve]], the tip of the appendix can be variably located—in the [[pelvis]], outside the [[peritoneum]] or behind the cecum.<ref name="Surgery">{{Cite book|author=Paterson-Brown, S. |chapter= 15. The acute abdomen and intestinal obstruction |editor1=Parks, Rowan W. |editor2=Garden, O. James |editor3=Carter, David John |editor4=Bradbury, Andrew W. |editor5=Forsythe, John L. R. |title=Principles and practice of surgery |publisher= Churchill Livingstone |location=Edinburgh |year=2007 |isbn=978-0-443-10157-1 |edition=5th}}</ref> The prevalence of the different positions varies amongst populations with the retrocecal position being most common in [[Ghana]] and [[Sudan]], with 67.3% and 58.3% occurrence respectively, in comparison to [[Iran]] and [[Bosnia]] where the pelvic position is most common, with 55.8% and 57.7% occurrence respectively.<ref>{{cite journal | vauthors = Clegg-Lamptey JN, Armah H, Naaeder SB, Adu-Aryee NA | title = Position and susceptibility to inflammation of vermiform appendix in Accra, Ghana | journal = East African Medical Journal | volume = 83 | issue = 12 | pages = 670–3 | date = December 2006 | pmid = 17685212 | doi = 10.4314/eamj.v83i12.9498 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Bakheit MA, Warille AA | title = Anomalies of the vermiform appendix and prevalence of acute appendicitis in Khartoum | journal = East African Medical Journal | volume = 76 | issue = 6 | pages = 338–40 | date = June 1999 | pmid = 10750522 }}</ref><ref>{{cite journal|vauthors=Ghorbani A, Forouzesh M, Kazemifar AM|year=2014|title=Variation in Anatomical Position of Vermiform Appendix among Iranian Population: An Old Issue Which Has Not Lost Its Importance|journal=[[Anatomy Research International]]|volume=2014|pages=313575|doi=10.1155/2014/313575|pmc=4176911|pmid=25295193|doi-access=free}}</ref><ref>{{cite journal | vauthors = Denjalić A, Delić J, Delić-Custendil S, Muminagić S | title = [Variations in position and place of formation of appendix vermiformis found in the course of open appendectomy] | language = bs | journal = Medicinski Arhiv | volume = 63 | issue = 2 | pages = 100–1 | year = 2009 | pmid = 19537667 }}</ref>

In very rare cases, the appendix may not be present at all ([[laparotomy|laparotomies]] for suspected appendicitis have given a frequency of 1 in 100,000).<ref>{{cite journal | vauthors = Zetina-Mejía CA, Alvarez-Cosío JE, Quillo-Olvera J | title = Congenital absence of the cecal appendix. Case report | journal = Cirugia y Cirujanos | volume = 77 | issue = 5 | pages = 407–10 | year = 2009 | pmid = 19944032 }}</ref>

Sometimes there is a semi-circular fold of [[mucous membrane]] at the opening of the appendix. This ''valve of the vermiform appendix'' is also called [[Joseph von Gerlach|Gerlach's]] valve.<ref name="auto"/>

==Functions==

===Maintaining gut flora===
[[File:Appendix function diagram.svg|left|thumb|A possible function of the human appendix is a "safe house" for beneficial bacteria in the recovery from [[diarrhea]]]]

Although it has been long accepted that the immune tissue surrounding the appendix and elsewhere in the gut—called [[gut-associated lymphoid tissue]]—carries out several important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of specific importance and function as judged by an absence of side effects following [[appendectomy|its removal]].<ref name="PBD4thEd">{{Cite book |author1=Kumar, Vinay |author2=Robbins, Stanley L. |author3=Cotran, Ramzi S. |title=Robbins' pathologic basis of disease |publisher=Saunders |location=Philadelphia |year=1989 |pages=[https://archive.org/details/robbinspathologi00robb/page/902 902–3] |isbn=978-0-7216-2302-3 |edition=4th |url-access=registration |url=https://archive.org/details/robbinspathologi00robb/page/902 }}</ref> Therefore, the notion that the appendix is only [[vestigiality|vestigial]] became widely held.

William Parker, Randy Bollinger, and colleagues at [[Duke University]] proposed in 2007 that the appendix serves as a haven for useful [[bacteria]] when illness flushes the bacteria from the rest of the intestines.<ref name=health>{{cite news |agency=[[Associated Press]] |date=5 October 2007 |title=Scientists may have found appendix's purpose |url=http://www.nbcnews.com/id/21153898/ |url-status=dead |work=[[NBC News]] |archive-url=https://web.archive.org/web/20200204135004/http://www.nbcnews.com/id/21153898 |archive-date=4 February 2020 |access-date=24 August 2019}}</ref><ref name="JTBpaper">{{cite journal | vauthors = Randal Bollinger R, Barbas AS, Bush EL, Lin SS, Parker W | title = Biofilms in the large bowel suggest an apparent function of the human vermiform appendix | journal = Journal of Theoretical Biology | volume = 249 | issue = 4 | pages = 826–31 | date = December 2007 | pmid = 17936308 | doi = 10.1016/j.jtbi.2007.08.032 | bibcode = 2007JThBi.249..826R }}</ref> This proposition is based on an understanding that emerged by the early 2000s of how the immune system supports the growth of beneficial [[gut flora|intestinal bacteria]],<ref>{{cite journal | vauthors = Sonnenburg JL, Angenent LT, Gordon JI | title = Getting a grip on things: how do communities of bacterial symbionts become established in our intestine? | journal = [[Nature Immunology]] | volume = 5 | issue = 6 | pages = 569–73 | date = June 2004 | pmid = 15164016 | doi = 10.1038/ni1079 | s2cid = 25672527 }}</ref><ref>{{cite journal |author1=Everett M.L. |author2=Palestrant D. |author3=Miller S.E. |author4=Bollinger R.R. |author5=Parker W. |title=Immune exclusion and immune inclusion: a new model of host-bacterial interactions in the gut |journal=Clinical and Applied Immunology Reviews |volume=4 |pages=321–32 |year=2004|doi=10.1016/j.cair.2004.03.001|issue=5}}</ref> in combination with many well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue. 

Research performed at [[Winthrop–University Hospital]] showed that individuals without an appendix were four times as likely to have a recurrence of [[Clostridioides difficile infection|''Clostridioides difficile'' colitis]].<ref>{{cite web |url=https://blogs.scientificamerican.com/guest-blog/2012/01/02/your-appendix-could-save-your-life/ |title=Your Appendix Could Save Your Life |last=Dunn |first=Rob |date=January 2, 2012 |publisher=[[Scientific American]] |access-date=22 December 2016 |url-status=live |archive-url=https://web.archive.org/web/20201111234503/https://blogs.scientificamerican.com/guest-blog/your-appendix-could-save-your-life/ |archive-date=11 November 2020}}</ref> The appendix, therefore, may act as a "safe house" for beneficial bacteria.<ref name="health" /> This reservoir of bacteria could then serve to repopulate the [[gut flora]] in the [[human digestive system|digestive system]] following a bout of [[dysentery]] or [[cholera]] or to boost it following a milder gastrointestinal illness.<ref name="JTBpaper"/>

===Immune and lymphatic systems===
The appendix has been identified as an important component of [[mammal]]ian [[mucosal immunology|mucosal immune function]], particularly [[B cell]]-mediated immune responses and [[thymus|extrathymically]] derived [[T cell]]s. This structure helps in the proper movement and removal of waste matter in the digestive system, contains lymphatic vessels that regulate pathogens, and lastly, might even produce early defences that prevent deadly diseases. Additionally, it is thought that this may provide more immune defences from invading pathogens and getting the lymphatic system's B and T cells to fight the viruses and bacteria that infect that portion of the bowel and training them so that immune responses are targeted and more able to reliably and less dangerously fight off pathogens.<ref>{{cite journal | vauthors = Zahid A | title = The vermiform appendix: not a useless organ | journal = Journal of the College of Physicians and Surgeons--Pakistan | volume = 14 | issue = 4 | pages = 256–8 | date = April 2004 | pmid = 15228837 }}</ref> In addition, there are different immune cells called [[innate lymphoid cell]]s that function in the gut to help the appendix maintain digestive health.<ref>{{cite journal|display-authors=6|vauthors=Rankin LC, Girard-Madoux MJ, Seillet C, Mielke LA, Kerdiles Y, Fenis A, Wieduwild E, Putoczki T, Mondot S, Lantz O, Demon D, Papenfuss AT, Smyth GK, Lamkanfi M, Carotta S, Renauld JC, Shi W, Carpentier S, Soos T, Arendt C, Ugolini S, Huntington ND, Belz GT, Vivier E|date=February 2016|title=Complementarity and redundancy of IL-22-producing innate lymphoid cells|journal=[[Nature Immunology]]|volume=17|issue=2|pages=179–86|doi=10.1038/ni.3332|pmc=4720992|pmid=26595889}}</ref> 

Research also shows a positive correlation between the existence of the appendix and the concentration of cecal lymphoid tissue, which supports the suggestion that not only does the appendix evolve as a complex with the cecum but also has major immune benefits.<ref>{{cite journal | vauthors = Smith H, Parker W, Kotze S, Laurin M| title = Morphological evolution of the mammalian cecum and cecal appendix | journal =  Comptes Rendus Palevol| volume = 16 | issue = 1 | pages = 39–57 | date = September 2016 |doi=10.1016/j.crpv.2016.06.001| url = https://zenodo.org/record/894682 | doi-access = free }}</ref>

==Clinical significance==
[[File:Appendiceal carcinoid 1.JPG|thumb|An appendiceal carcinoid tumor]]

Common [[disease]]s of the appendix (in humans) are [[appendicitis]] and [[carcinoid]] tumors (appendiceal carcinoid).<ref>{{cite journal |url=https://pubmed.ncbi.nlm.nih.gov/15807474/ |title=Miscellaneous conditions of the appendix |author=<!--Not stated--> |journal=Seminars in Diagnostic Pathology |year=2004 |volume=21 |issue=2 |pages=151–63 |publisher=[[National Institutes of Health]] |access-date=30 December 2020 |doi=10.1053/j.semdp.2004.11.006|pmid=15807474 }}</ref> [[Appendix cancer]] accounts for about 1 in 200 of all gastrointestinal malignancies. In rare cases, [[adenomas]] are also present.<ref>{{cite web|access-date=2020-12-30 |url=https://www.rightdiagnosis.com/a/appendix_disorders/stats.htm |title=Statistics about Appendix disorder |work=rightdiagnosis.com |url-status=dead |archive-url=https://web.archive.org/web/20191016210240/https://www.rightdiagnosis.com/a/appendix_disorders/stats.htm |archive-date=2019-10-16}}</ref>

===Appendicitis===
{{Main|Appendicitis}}
[[Appendicitis]] is a condition characterized by [[inflammation]] of the appendix. Pain often begins in the center of the abdomen, corresponding to the appendix's development as part of the embryonic [[midgut]]. This pain is typically a dull, poorly localized, [[viscus|visceral]] pain.<ref name="Biology 92-98">{{Cite book|author=Miller R., Kenneth |author2=Levine, Joseph  |title=Biology |publisher=[[Prentice Hall]] |year=2002 |isbn=978-0-13-050730-3 |pages=92–98}}</ref>

As the inflammation progresses, the pain begins to localize more clearly to the right lower quadrant, as the [[peritoneum]] becomes inflamed. This peritoneal inflammation, or [[peritonitis]], results in [[rebound tenderness]] (pain upon removal of pressure rather than the application of pressure). In particular, it presents at [[McBurney's point]], 1/3 of the way along a line drawn from the [[anterior superior iliac spine]] to the [[navel|umbilicus]]. Typically, point (skin) pain is not present until the [[parietal peritoneum]] is inflamed, as well. Fever and an immune system response are also characteristic of appendicitis.<ref name="Biology 92-98"/> Other signs and symptoms may include [[nausea]] and vomiting, low-grade fever that may get worse, [[constipation]] or [[diarrhea]], abdominal bloating, or [[flatulence]].<ref name=Mayo-Appendicitis>{{cite web |url=https://www.mayoclinic.org/diseases-conditions/appendicitis/symptoms-causes/syc-20369543 |title=Appendicitis - Symptoms and causes - Mayo Clinic |author=<!--Not stated--> |website=mayoclinic.org |publisher=[[Mayo Clinic]] |access-date=29 December 2020 |url-status=live |archive-url=https://web.archive.org/web/20201125012725/https://www.mayoclinic.org/diseases-conditions/appendicitis/symptoms-causes/syc-20369543 |archive-date=25 November 2020 }}</ref>

Appendicitis usually requires the removal of the inflamed appendix, in an [[appendectomy]] either by [[laparotomy]] or [[laparoscopy]]. Untreated, the appendix may rupture, leading to [[peritonitis]], followed by [[septic shock|shock]], and, if still untreated, death.<ref name="Biology 92-98"/>

===Surgery===

{{Main|Appendectomy}}

The surgical removal of the appendix is called an [[appendectomy]]. This removal is normally performed as an emergency procedure when the patient is suffering from [[acute (medical)|acute]] appendicitis. In the absence of surgical facilities, [[intravenous]] [[antibiotics]] are used to delay or avoid the onset of [[sepsis]]. In some cases, the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relative [[contraindication]] to surgery.

The appendix is also used for the construction of an efferent urinary conduit, in an operation known as the [[Mitrofanoff procedure]],<ref name=mingin>{{cite journal | vauthors = Mingin GC, Baskin LS | title = Surgical management of the neurogenic bladder and bowel | journal = International Braz J Urol | volume = 29 | issue = 1 | pages = 53–61 | year = 2003 | pmid = 15745470 | doi = 10.1590/S1677-55382003000100012 | url = http://www.brazjurol.com.br/january_february_2003/Baskin_ing_53_61.htm | doi-access = free }}</ref> in people with a [[neurogenic bladder]].

The appendix is also used as a means to access the colon in children with paralysed bowels or major rectal sphincter problems. The appendix is brought out to the skin surface and the child/parent can then attach a catheter and easily wash out the colon (via normal defaecation) using an appropriate solution.<ref>{{cite web|url=http://www.healthpoint.co.nz/default,169125.sm;jsessionid=1CD66A058B10550C51041E477C8E7075|title=Wellington Children's Hospital: Caring for an ACE or Chait Tube: Healthpoint|access-date=22 December 2016|archive-date=16 October 2019|archive-url=https://web.archive.org/web/20191016171448/https://www.healthpoint.co.nz/public/paediatrics/wellington-childrens-hospital-paediatric/caring-for-an-ace-or-chait-tube/|url-status=dead}}</ref>

==History==
[[Charles Darwin]] suggested that the appendix was mainly used by earlier hominids for digesting fibrous vegetation, then evolved to take on a new purpose over time. The very long cecum of some herbivorous animals, such as in the [[horse]] or the [[koala]], appears to support this hypothesis. The koala's cecum enables it to host bacteria that specifically help to break down cellulose. Human ancestors may have also relied upon this system when they lived on a diet rich in foliage. 

As people began to eat more easily digested foods, they may have become less reliant on cellulose-rich plants for energy. As the cecum became less necessary for digestion, mutations that were previously deleterious (and would have hindered evolutionary progress) were no longer important, so the mutations survived. It is suggested that these [[allele]]s became more frequent and the cecum continued to shrink. After millions of years, the once-necessary cecum degraded to be the appendix of modern humans.<ref name="DescentOfMan">{{Cite book |last=Darwin |first=Charles |author-link=Charles Darwin |title=The Descent of Man, and Selection in Relation to Sex |title-link=The Descent of Man, and Selection in Relation to Sex |publisher=John Murray |year=1871 |location=London |chapter=Jim's Jesus}}</ref>

Dr. Heather F. Smith of [[Midwestern University]] and colleagues explained:

<blockquote>Recently ... improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix.
Three morphotypes of cecal-appendices can be described among mammals based primarily on the shape of the cecum: a distinct appendix branching from a rounded or sac-like cecum (as in many primate species), an appendix located at the apex of a long and voluminous cecum (as in the rabbit, greater glider and Cape dune mole rat), and an appendix in the absence of a pronounced cecum (as in the wombat). In addition, long narrow appendix-like structures are found in mammals that either lack an apparent cecum (as in monotremes) or lack a distinct junction between the cecum and appendix-like structure (as in the koala). A cecal appendix has evolved independently at least twice and represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world. Although the appendix has been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade.<ref name=SEAL09>{{cite journal | vauthors = Smith HF, Fisher RE, Everett ML, Thomas AD, Bollinger RR, Parker W | title = Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix | journal = Journal of Evolutionary Biology | volume = 22 | issue = 10 | pages = 1984–99 | date = October 2009 | pmid = 19678866 | doi = 10.1111/j.1420-9101.2009.01809.x | s2cid = 6112969 | doi-access = free }}</ref></blockquote>

In a 2013 paper, the appendix was found to have independently evolved in different animals at least 32 times (and perhaps as many as 38 times) and to have been lost no more than six times throughout history.<ref name = SEAL13>{{cite journal | author1 = Smith H. F. |author2=Parker W. |author3=Kotzé, S. H. |author4=Laurin, M. | year = 2013 | title = Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors | journal = Comptes Rendus Palevol | volume =  12| issue = 6| pages =  339–354| doi = 10.1016/j.crpv.2012.12.001 |bibcode=2013CRPal..12..339S | doi-access = free }}</ref> A more recent study using similar methods on an updated database yielded similar, though less spectacular results, with at least 29 gains and at the most 12 losses (all of which were ambiguous), and this is still significantly asymmetrical.<ref name = SEAL17>{{cite journal | author1 = Smith H. F. |author2=Parker W. |author3=Kotzé, S. H. |author4=Laurin, M. | year = 2017 | title = Morphological evolution of the mammalian cecum and cecal appendix | journal = Comptes Rendus Palevol | volume =  11| issue = 1| pages =  39–57| doi = 10.1016/j.crpv.2016.06.001 |bibcode=2017CRPal..16...39S | doi-access = free }}</ref> 

This suggests that the cecal appendix has a selective advantage in many situations and argues strongly against its vestigial nature. Given that this organ may have a selective advantage in numerous situations, it appears to be associated with greater maximal longevity, for a given body mass.<ref name="Collard et al. 2021">{{cite journal |last1=Collard |first1=Maxime K. |last2=Bardin |first2=Jérémie |last3=Laurin |first3=Michel |last4=Ogier‐Denis |first4=Eric |title=The cecal appendix is correlated with greater maximal longevity in mammals |journal=Journal of Anatomy |date=November 2021 |volume=239 |issue=5 |pages=1157–1169 |doi=10.1111/joa.13501 |url=https://doi.org/10.1111/joa.13501 |language=en |issn=0021-8782|pmc=8546507 }}</ref> For example, in a 2023 study, the protective functions conferred against diarrhea were observed in young primates.<ref>{{Cite journal |last1=Collard |first1=Maxime K. |last2=Bardin |first2=Jérémie |last3=Marquet |first3=Bertille |last4=Laurin |first4=Michel |last5=Ogier-Denis |first5=Éric |date=2023-09-23 |title=Correlation between the presence of a cecal appendix and reduced diarrhea severity in primates: new insights into the presumed function of the appendix |journal=Scientific Reports |language=en |volume=13 |issue=1 |pages=15897 |doi=10.1038/s41598-023-43070-5 |issn=2045-2322 |pmc=10517977 |pmid=37741857|bibcode=2023NatSR..1315897C }}</ref> This complex evolutionary history of the appendix, along with a great heterogeneity in its evolutionary rate in various taxa, suggests that it is a recurrent trait.<ref name = LEAL11>{{cite journal | vauthors = Laurin M, Everett ML, Parker W | title = The cecal appendix: one more immune component with a function disturbed by post-industrial culture | journal = Anatomical Record | volume = 294 | issue = 4 | pages = 567–79 | date = April 2011 | pmid = 21370495 | doi = 10.1002/ar.21357 | s2cid = 3237168 }}</ref>

Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where [[diarrhea]] may be prevalent. Current [[epidemiology|epidemiological]] data on the cause of death in developing countries collected by the [[World Health Organization]] in 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by the [[Bill & Melinda Gates Foundation|Bill and Melinda Gates Foundation]]). Two of the other leading causes of death are expected to have exerted limited or no selection pressure.<ref>{{Cite web |date=21 August 2009 |orig-date=20 August 2009 |title=Evolution of the Appendix: A Biological 'Remnant' No More |url=http://www.dukehealth.org/health_library/news/evolution_of_the_appendix_a_biological_remnant_no_more |url-status=dead |archive-url=https://web.archive.org/web/20120726030646/http://www.dukehealth.org/health_library/news/evolution_of_the_appendix_a_biological_remnant_no_more |archive-date=2012-07-26 |access-date= |website=Duke Medicine News and Communications}}</ref>

==Additional images==
<gallery>
File:Ultrasonography of a normal appendix, annotated.jpg|[[Abdominal ultrasound]] showing a normal appendix between the [[external iliac artery]] and the abdominal wall
File:Blausen 0043 Appendix Child.png|Illustration depicting the location of the appendix in a child
File:Stomach colon rectum diagram-en.svg|Normal location of the appendix relative to other organs of the digestive system (frontal view)
File:Appendix vermiformis.jpg|Vermiform appendix
File:Appendixcancer.jpg|Mucinous [[adenocarcinoma]] of the appendix tip
File:Enterobius - very low mag.jpg|[[Cross section (geometry)|Cross section]] of the appendix with ''[[Enterobius]]'' with [[H&E stain]]
File:Micrograph of entry point of appendicular arteries.jpg|Micrograph of entry point of [[appendicular arteries]] (arrows at level of inner muscular layer), not to be confused with a perforation.
</gallery>

== See also ==
* [[Meckel's diverticulum]]
* [[Appendix of the epididymis]], a detached efferent duct of the epididymis
* [[Appendix testis]], a vestigial remnant of the Müllerian duct
* [[Epiploic appendix]], one of several small pouches of fat on the peritoneum along the colon and rectum
** [[Appendix of the laryngeal ventricle]], a sac that extends from the laryngeal ventricle
* [[Mesoappendix]], the portion of the mesentery that connects the ileum to the vermiform appendix

== References ==
{{Reflist|2}}

== Further reading ==
* [https://web.archive.org/web/20071013113818/http://www.webmd.com/digestive-disorders/news/20071012/appendix-may-have-purpose Appendix May Actually Have a Purpose]—2007 [[WebMD]] article
* {{SUNYAnatomyLabs|37|12|01|02}}—"Abdominal Cavity: The Cecum and the Vermiform Appendix"
* [http://www.talkorigins.org/faqs/vestiges/appendix.html "The vestigiality of the human vermiform appendix: A Modern Reappraisal"]—[[evolutionary biology]] argument that the appendix is vestigial
* {{cite journal|vauthors=Smith HF, Fisher RE, Everett ML, Thomas AD, Bollinger RR, Parker W|title=Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix|journal=[[Journal of Evolutionary Biology]]|volume=22|issue=10|pages=1984–99|date=October 2009|pmid=19678866|doi=10.1111/j.1420-9101.2009.01809.x|s2cid=6112969|doi-access=free}}
* Cho, Jinny (August 27, 2009). [https://web.archive.org/web/20141227153301/http://www.dukechronicle.com/articles/2009/08/27/scientists-refute-darwins-theory-appendix "Scientists refute Darwin's theory on appendix"]. ''[[The Chronicle (Duke University)|The Chronicle]]'' ([[Duke University]]). (News article on the above journal article.)

== External links ==
* {{Commons category inline|Vermiform appendix|Appendix (anatomy)}}

{{Digestive tract}}
{{Authority control}}

{{DEFAULTSORT:Vermiform Appendix}}
[[Category:Digestive system]]
[[Category:Vestigial organs]]