Vestigiality of the Human Appendix

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The caecum is specially structured like a blind-ended pouch to increase the efficiency of cellulose fermentation by giving the bacteria ample time to act. As a side-path, from the colon, caecum has the capacity to house a large number of colonies of specialized symbiotic bacteria. The purpose of the caecum being structured like a blind-ended pouch at the beginning of the large intestine is to give the digesting food more time to stay in and to ferment completely before flowing through to the large intestine where the broken down nutrients are absorbed into the blood stream. However, even though humans consume plants, the vermiform appendix does not house significant quantities of the cellulase secreting bacteria, and thus we cannot digest cellulose in the vegetables and fruits we take (Luskan, 2012) .

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Figure 4; Appendix/caecum for each animal is highlighted in magenta color

For birds, caecum takes many shapes and sizes but the striking difference is that they occur in pairs. The size, shape, and structure of avian caeca varies according to the species and its diet, and in some cases it may even be absent in certain species. In birds caeca handle many roles such as fermentation of plant matter, absorption of water and salts, and production of immunoglobulin and antibodies (Clench & Mathias, 1995) .

Structural evolution of the appendix and caecum in primate phylogeny

Examining the tree branches of order primate downward to the family, genus, and species provides evidence of intermediate states of caecum and large appendix or both. Many primates have both while those that are less evolved have a caecum and those that are more evolved have an appendix (Scott, 1980). It was confirmed that the size of the caecum reduced as one ascended the primate scale of evolution. Hominoid apes have vermiform appendix while non-anthropoid primates (those that don’t resemble humans) have caeca or both. Recent study of primates’ caeca and appendix revealed that it is quite difficult to draw a clear line separating the point where the caecum ends and the point where the appendix begins in different species (Rebecca, 2000).

Using systematic analysis to compare these two related organs shows that a species which bears a caecum but no appendix is more primitive to our ancestors (Clarke, 1971). As one traverses the phylogenetic tree from monkeys to humans, it can be seen that the caecum decreases in size in relation to the colon while the size of the appendix increases (Fiennes, 1972).

Difference in structure of Appendix and Caecum

Appendix is made up of an internal layer of mucosa and sub-mucosa surrounded by muscularis and serosa layers, similar to the layers of any other digestive track. However, the submucosa layer contains masses of lymphoid tissue suggesting that the appendix may play a role in the immune system. It forms a tubular shape with thickened lymphatic tissue that almost closes the existing blind hole.

On the other hand, the caecum is a blind pouch located at the start of the large intestine/colon and it houses a large number of microbes and receives vegetation matter for a longer duration before it proceeds to the colon. In carnivorous animals, caecum is tiny and useless in terms of it aiding digestion. It is also made up of mucosa and sub-mucosa surrounded by muscularis and serosa layers just like the appendix. There is a mucosal layer that separates the food being digested by the bacteria from the submucosa to protect the caecum from being acted upon by the same bacteria it houses. Therefore, both the appendix and caecum are quite similar in construction, and they differ mainly in terms of volume and size - caecum is open and voluminous while the appendix is tubular and closed (Yapp, 1955). However, even though both have rich lymphatic tissues and house symbiotic microbes, the caecum plays an additional role of absorbing fluids and salts (Clench & Mathias, 1995).

Possible Functions of the appendix

Leonardo da Vinci was the first to postulate that one of the functions of the appendix is to dilate so that excessive wind does not rapture the caecum. The most accepted role, however, is that the appendix houses the right type of enzyme secreting symbiotic bacteria which aids in digestion. It regulates the population of these bacteria in the colon and replenishes them when their population levels drop (Martin, 1999).

Still, for a long time in medical and biological studies, many other possible functions for the appendix have been suggested, examined, and even refuted. These include contribution to the immune system among others (Martin, 1999). Due to its extensive lymphoid tissue, most medical experts agree that human appendix may have a direct effect on the body's immunity. Experts propose that the appendix is involved in the production of antibodies called immunoglobulins and the maturation of B lymphocytes (Martin, 1999). Scientists argue that by allowing B lymphocytes to mature in the appendix, they get exposed to gut bacteria and therefore suppress destructive antibody reactions against these gastrointestinal bacteria. This promotes local immunity by informing the body of potential foreign antigens in the digestive track. In addition, the appendix produces thick mucus that flows to the colon, and this mucus serves to protect the walls of the colon as well as aid the movement of bowels.

Complications of Appendix and effects of appendectomy

The human appendix has for long being credited for life-threatening complications that it can cause. Risk of the appendix being infected at a younger age stands at about 10% (Samaha, Tawfik, Abbas, & Abdelhamid, 2011). But the most prevalent age group is eight to thirty years, when acute appendicitis is more common. When an appendix inflames and gets infected, the most logical treatment is surgical removal. However, before the onset of 21st century medical practices, surgeries were unheard of and any acute appendicitis led to death.

The culprit to appendix complications is its minute hole which is prone to physical blockage, ultimately causing appendicitis (McFadden & Liu, 1997). Causes of blockage include thickening of the mucus that the appendix produces, faecal matter from the colon, or entry of a foreign body. The problem is worsened by the swelling of the lymphatic tissue as it reacts to the inflammation which causes more occlusion and holding in of bacteria (McFadden & Liu, 1997). However, if the inside of the appendix was not so confined and compacted, then there would