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An appendix is, anatomically, a section at the end of an organ. However, the vermiform appendix of hominoid in particular is often simply called "the appendix".
Hominoid vermiform appendix
In the digestive system of humans and other apes, there exists a vestigial extension of the cecum called the vermiform appendix ('vermiform' is Latin for 'worm-like'). It is generally only a few centimetres long, but like many vestiges the size and orientation is quite variable. Other primates also have cecal appendices of some sort.
The vermiform appendix is particularly vulnerable to inflammation (appendicitis). Appendicitis occurs when the entrance to the appendix becomes blocked, for various reasons, causing the appendix to swell with mucous and, ultimately, burst. As appendicitis is a particularly dangerous condition, due to the risk of peritonitis (infection of the abdominal cavity with gut bacteria) leading to sepsis (infection of the entire body), appendicitis may often warrant the removal of the appendix in an appendectomy.
It is sometimes claimed that the appendix plays a role in the immune system, since its walls contain aggregated lymphoid tissue, which are involved in the gut immune system. However, this lymphoid tissue is not confined to the appendix but is found throughout the gut, forming the gut-associated lymphoid tissue. It is therefore not the reason for the appendix: it is clearly not a specialised organ for housing lymphoid tissue, but rather a vestigial part of the colon that keeps its immune function, perhaps just to defend against bacterial infection of the appendix itself.
In addition, one of the common causes of appendicitis is lymphoid hyperplasia — a growth of the lymphoid tissue around the appendix causing it to be occluded. A part of the immune system which, by its very 'design', frequently leads to infection!
The purpose of the hominoid appendix still remains something of a physiological mystery; however, several possible answers have been suggested. One hypothesis states that the human appendix may have once served the same purpose it does in present-day herbivores, such as rabbits — to harbor colonies of bacteria that help in the digestion of cellulose in plant material. Therefore, the hominoid appendix would be a reduced vestige, as the primate diet contains fruit and meat, which contain far richer sources of energy than cellulose.
Another hypothesis suggests that the human appendix — as well as the tonsils, the spleen, lymph nodes, and bone marrow — manufacture the antibody-producing white blood cells called B lymphocytes.
A third hypothesis is that the appendix may "attract" infections within the body. In that way, infections become localized in one organ that is not critical to body function. If this was shown to be the case, then it could be argued that the appendix is a derived organ rather than a vestige.
Regardless of what the function may be, there is no evidence that a person with a removed or congenitally absent appendix has any noticeable consequences due their lack of the structure. Therefore, any function it may have is clearly very minor indeed.
New information published in 2007 indicate that the appendix is essentially a safe-house for normal flora (i.e. good bacteria) during episodes of diarrhea, a function more useful in developing countries or in historical times.
Appendices in other animals
Some other animals, such as the wombat, oppossum and the rabbit (and some rodents) have a vermiform appendix also, however phylogenic analyses have shown that these are not homologous to the appendix of apes. That is, they were developed multiple times — examples of convergent evolution.
- Fisher, Rebecca. 2000. "The Primate Appendix: A Reassessment" Anat. Rec. 261(6):228-236, 18 Dec 2000. [PubMed]
- Bergman, Jerry. 2000. Do any vestigial organs exist in humans? Technical Journal 14(2):95-98, August 2000
- Appendix Isn't Useless At All: It's A Safe House For Good Bacteria October 8, 2007 ScienceDaily.
- Shoshani, Jeheskel & McKenna, Malcolm (June 1998). ""Higher Taxonomic Relationships among Extant Mammals Based on Morphology, with Selected Comparisons of Results from Molecular Data"". Mol. Phylogenet. Evol.: 572-584.
- The Cecal Appendix: One More Immune Component With a Function Disturbed By Post-Industrial Culture by Michel Laurin et al. Volume 294, Issue 4, pages 567–579, April 2011 DOI: 10.1002/ar.21357 The Anatomical Record.