Vermiform appendix

In human anatomy, the appendix (or vermiform appendix; also cecal appendix) is a blind ended tube connected to the cecum, 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 "wormlike in appearance".

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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. ^[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 seeming perfect health of those who have had their appendix removed, the biological purpose of the appendix has mystified scientists for some time. There have been cases of people who have been found, usually on laparoscopy or laparotomy, to have a congenital absence of an appendix. There have been no reports of impaired immune or gastrointestinal function in these people.

Historical Interpretation: Vestigiality

The most common explanation is that the appendix is a vestigial structure with no absolute purpose. 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] that the appendix was used for digesting leaves as primates. Over time, we have eaten fewer vegetables and have evolved, over millions of years, for this organ to be smaller to make room for our stomach.

Few mammals other than humans have an appendix, and evidence based on comparative primate anatomy is nowadays seen to contradict the explanation of the appendix as a vestige of evolutionary development.^[4]  

Recent Interpretation: Immune Use

Loren G. Martin, a professor of physiology at Oklahoma State University, argues that the appendix has a function in fetuses and adults.^[6] 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. A. Zahid^[7] 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.

Latest Interpretation: 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. ^[9] William Parker, Randy Bollinger, and colleagues at Duke University proposed that the appendix serves as a safe haven for useful bacteria when illness flushes those bacteria from the rest of the intestines.^{[10] [4]} 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.^[4] This function would explain the apparent health of those individuals without an appendix in developed countries, and provides an explanation for the worm-like shape of the appendix. ^[4] 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.^[4]

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. Virtually all cases of Appendicitis require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, the appendix may rupture, leading to peritonitis, then shock, and, if continued untreated, death. Pain often begins in the center of the abdomen where the lining of the stomach is irritated then moves lower right as the condition develops, it is important to note that this makes diagnoses difficult in the early stages because an MRI or CT scan must be used to detect it. Appendicitis presents as pain in the right lower quadrant with 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 immune system response are also characteristic of appendicitis.

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.

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See also

• Homology (biology)
• McBurney's point

References