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 embryo logically. The cecum is a pouchlike structure of the colon. The appendix is located near the junction of the small intestine and the large intestine.
The term "vermiform" comes from Latin and means"worm-shaped".
It is widely present in the Euarchontoglires and has also evolved independently in the diprotodons marsupials and is highly diverse in size and shape.
Size and location in humans
The appendix averages 11 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 from a patient in Zagreb,Croatia. The appendix is located in the right lower quadrant of the abdomen, or, more specifically, the right iliac fossa. 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 ileocecal valve, the location of the tip of the appendix can vary from being retrocecal(behind the cecum) (74%) to being in the pelvis to being extra peritoneal. In rare individuals with situs inversus, the appendix may be located in the lower left side.
The most common explanation for the appendix's existence in humans is that it is a vestigial structure which has lost its original function.
One potential ancestral purpose put forth byCharles Darwin was that the appendix was used for digesting leaves as primates. It may be a vestigial organ, evolutionary baggage, of ancient humans that has degraded down to nearly nothing over the course of evolution. The very long cecum of some herbivorous animals, such as found in the horse or the koala, supports this theory. The koala's cecum enables it to host bacteria that specifically helpto 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 became 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 longerimportant, so the mutations have survived. These alleles became more frequent and the cecum continued to shrink. After thousands of years, the once-necessary cecum has degraded to be the appendix of today. On the other hand, evolutionary theorists have suggested that natural selection selects for larger appendices because smaller and thinner appendices would be more susceptible to inflammation anddisease.
New studies propose that the appendix may harbor and protect bacteria that are beneficial in the function of the human colon.
Loren G. Martin, a professor of physiology at Oklahoma State University, argues that the appendix has a function in fetuses and adults. 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 suggests that it plays a role in both manufacturing hormones in fetal development as well as functioning to "train" the immunesystem, exposing the body to antigens so 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.
Maintaining gut flora
Although it was long...