Appendix (anatomy)

Appendix
Appendix with surrounding structures
Variations of the appendix
Details
Precursor	Midgut
System	Digestive system
Artery	Appendicular artery
Vein	Appendicular vein
Identifiers
MeSH	D001065
TA98	A05.7.02.007
TA2	2976
FMA	14542
Anatomical terminology [edit on Wikidata]

The appendix (pl.: 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

Structure

The human appendix averages 9 cm (3.5 in) in length, ranging from 5 to 35 cm (2.0 to 13.8 in). The diameter of the appendix is 6 mm (0.24 in), and more than 6 mm (0.24 in) is considered a thickened or inflamed appendix. The longest appendix ever removed was 26 cm (10 in) long.

The appendix is connected to the mesentery in the lower region of the ileum, by a short region of the mesocolon known as the mesoappendix.^[4]

Variation

Some identical twins—known as

In very rare cases, the appendix may not be present at all (laparotomies for suspected appendicitis have given a frequency of 1 in 100,000).^[12]

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 Gerlach's valve.^[4]

Functions

Maintaining gut flora

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 a number of 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 its removal.^[13] Therefore, the notion that the appendix is only vestigial became widely held.

William Parker, Randy Bollinger, and colleagues at

The appendix has been identified as an important component of mammalian mucosal immune function, particularly B cell-mediated immune responses and extrathymically derived T cells. 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.^[19] In addition, there are different immune cells called innate lymphoid cells that function in the gut in order to help the appendix maintain digestive health.^[20] 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.^[21]

Clinical significance

Common

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 shock, and, if still untreated, death.^[24]

Surgery

The surgical removal of the appendix is called an

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.[27]

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 alleles 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.^[28]

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

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 apparently represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world. Although the appendix has apparently been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade.^[29]

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 over the course of history. ^[30] 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.^[31] 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 have effects that are unique to certain species. For example, in a 2023 study, the protective functions conferred against diarrhea were only observed in humans.^[32] 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.^[33]

Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where

• 
  Abdominal ultrasound showing a normal appendix between the external iliac artery

  and the abdominal wall
• 
  Illustration depicting the location of the appendix in a child
• 
  Normal location of the appendix relative to other organs of the digestive system (frontal view)
• 
  Vermiform appendix
• 
  Mucinous adenocarcinoma of the appendix tip
• 
  Enterobius with H&E stain
• 
  Micrograph of entry point of

  appendicular arteries

  (arrows at level of inner muscular layer), not to be confused with a perforation.

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

Further reading

Wikimedia Commons has media related to Appendix (anatomy).

• Appendix May Actually Have a Purpose—2007 WebMD article
• Anatomy photo:37:12-0102 at the SUNY Downstate Medical Center—"Abdominal Cavity: The Cecum and the Vermiform Appendix"
• "The vestigiality of the human vermiform appendix: A Modern Reappraisal"—evolutionary biology argument that the appendix is vestigial
• Smith HF, Fisher RE, Everett ML, Thomas AD, Bollinger RR, Parker W (October 2009). "Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix".
  S2CID 6112969
  .
• Cho, Jinny. "Scientists refute Darwin's theory on appendix".
  The Chronicle (Duke University
  ), August 27, 2009. (News article on the above journal article.)