Vermiform appendix

Vermiform Appendix
Arteries of cecum and vermiform appendix. (Appendix visible at lower right, labeled as "vermiform process").
Normal location of the appendix relative to other organs of the digestive system (frontal view).
Latin	appendix vermiformis
Gray's	subject #249 1178
System	Digestive
Artery	appendicular artery
Vein	appendicular vein
Precursor	Midgut
MeSH	Appendix
Dorlands/Elsevier	Vermiform appendix

In human anatomy, the appendix (or vermiform appendix; also cecal (or caecal) appendix; also vermix) is a blind ended tube connected to the cecum (or caecum), from which it develops embryologically. The cecum is a pouch-like structure of the colon. The appendix is near the junction of the small intestine and the large intestine.

The term "vermiform" comes from Latin and means "worm-like in appearance".

Size and location

The appendix averages 10 cm in length, but can range from 2 to 20 cm. The diameter of the appendix is usually between 7 and 8 mm. The longest appendix ever removed measured 26 cm in Zagreb, Croatia. ^[1] The appendix is located in the lower right quadrant of the abdomen, or more specifically, the right iliac fossa.^[2] Its position within the abdomen corresponds to a point on the surface known as McBurney's point (see below). While the base of the appendix is at a fairly constant location, 2 cm below the ileocaecal valve ^[2], the location of the tip of the appendix can vary from being retrocaecal (74% ^[2]) to being in the pelvis to being extraperitoneal. In rare individuals with situs inversus, the appendix may be located in the lower left side.

Function

Given the appendix's propensity to cause death via infection, and the general good health of people who have had their appendix removed or who have a congenital absence of an appendix, the appendix appears to have no function in the human body. There have also been no reports of impaired immune or gastrointestinal function in people without an appendix.

Vestigiality

The most common explanation is that the appendix is a vestigial structure which does absolutely nothing for the body. In The Story of Evolution, Joseph McCabe argued thus:

The vermiform appendage—in which some recent medical writers have vainly endeavoured to find a utility—is the shrunken remainder of a large and normal intestine of a remote ancestor. This interpretation of it would stand even if it were found to have a certain use in the human body. Vestigial organs are sometimes pressed into a secondary use when their original function has been lost.

One potential ancestral purpose put forth by Darwin^[3] was that the appendix was used for digesting leaves as primates. Over time, we have eaten fewer vegetables and have evolved, over thousands of years, for this organ to be smaller to make room for our stomach. It may be a vestigial organ of ancient man that has degraded down to nearly nothing over the course of evolution. Evidence can be seen in herbivorous animals such as the Koala. The cecum of the koala is attached to the juncture of the small and large intestines-as well as humans are- and is very long, enabling it to host bacteria specific for cellulose breakdown. Early man’s ancestor may have also relied upon this system and lived on a diet rich in foliage. As man began to eat more easily digested foods, they became less reliant on cellulose-rich plants for energy. The cecum became less necessary for digestion and mutations that previously had been deleterious were no longer selected against. These alleles became more frequent and the cecum continued to shrink. After thousands of years, the once-necessary cecum has degraded to what we see today; the appendix. ^[3]

Evolutionary theorists have suggested that natural selection selects for larger appendices because smaller and thinner appendices would be more susceptible to inflammation and disease^[4].

Immune function

Loren G. Martin, a professor of physiology at Oklahoma State University, argues that the appendix has a function in fetuses and adults.^[5] Endocrine cells have been found in the appendix of 11 week old fetuses that contribute to "biological control (homeostatic) mechanisms." In adults, Martin argues that the appendix acts as a lymphatic organ. The appendix is experimentally verified as being rich in infection-fighting lymphoid cells, suggesting that it might play a role in the immune system. Zahid^[6] suggests that it plays a role in both manufacturing hormones in fetal development as well as functioning to 'train' the immune system, exposing the body to antigens in order that it can produce antibodies. He notes that doctors in the last decade have stopped removing the appendix during other surgical procedures as a routine precaution, because it can be successfully transplanted into the urinary tract to rebuild a sphincter muscle and reconstruct a functional bladder.

Apparent function of the human vermiform appendix in the recovery from diarrhea. Although more than 2 million children die each year in developing countries as a result of diarrhea, children living in those countries face an estimated 1.4 billion cases of diarrhea each year ^[7] Thus, the process of recovery from diarrhea is relatively effective in developing countries. This process is expected to be less critical in developed countries, where widespread epidemics resulting in diarrhea are uncommon and thus the loss of beneficial bacteria from the population as a whole is highly unlikely.

Maintaining gut flora

Although it was long accepted that the immune tissue, called gut associated lymphoid tissue, surrounding the appendix and elsewhere in the gut carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of importance as judged by an absence of side-effects following appendectomy. ^[8] William Parker, Randy Bollinger, and colleagues at Duke University proposed that the appendix serves as a haven for useful bacteria when illness flushes those bacteria from the rest of the intestines.^{[9] [10]} This proposal is based on a new understanding of how the immune system supports the growth of beneficial intestinal bacteria ^{[11] [12]}, in combination with many well-known features of the appendix, including its architecture and its association with copious amounts of immune tissue. Such a function is expected to be useful in a culture lacking modern sanitation and healthcare practice, where diarrhea may be prevalent.^[10] Current epidemiological data ^[13] show that diarrhea is one of the leading causes of death in developing countries, indicating that a role of the appendix as an aid in recovering beneficial bacteria following diarrhea may be extremely important in the absence of modern health and sanitation practices.^[10]

Diseases

The most common diseases of the appendix (in humans) are appendicitis and carcinoid tumors. Appendix cancer accounts for about 1 in 200 of all gastrointestinal malignancies. Adenomas also (rarely) present.

Appendicitis (or epityphlitis) 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 localised, visceral pain.

As the inflammation progresses, the pain begins to localise 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 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 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.

Many cases of appendicitis require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, the appendix may rupture, leading to peritonitis, followed by shock, and, if still untreated, death.

The surgical removal of the vermiform appendix is called an appendicectomy (or appendectomy). This procedure is normally performed as an emergency procedure, when the patient is suffering from acute appendicitis. In the absence of surgical facilities, intravenous antibiotics are used to delay or avoid the onset of sepsis; it is now recognized that many cases will resolve when treated non-operatively. In some cases the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relative contraindication to surgery.

Use as efferent urinary conduit

The appendix is used for the construction of an efferent urinary conduit, in an operation known as the Mitrofanoff procedure,^[14] in people with a neurogenic bladder.

Additional image

See also

• Homology (biology)
• McBurney's point
• Noncoding DNA

References

External links

• "The vestigiality of the human vermiform appendix: A Modern Reappraisal" -- evolutionary biology argument that the appendix is vestigial
• Purpose of appendix believed found CNN article
• Appendix May Actually Have a Purpose WebMD article
• SUNY Labs 37:12-0102 - "Abdominal Cavity: The Cecum and the Vermiform Appendix"