The equivalent of the true appendix in most animals is known as the cecum and is at the junction of the small and large intestines.
In general, carnivorous mammals have a small cecum that serves the same purpose as it does in humans. However, in many herbivorous mammals the cecum is greatly enlarged to create all sorts of wonderful anatomical arrangements.
The function of the cecum in these mammals is to ferment the complex carbohydrates from the herbivorous diet into volatile fatty acids and then to absorb these as a source of energy. A functioning cecum is also vital for providing the energy needs of hindgut fermenters like horses, rabbits, rats, guinea pigs, and swine.
The ruminant stomach of cattle and sheep performs a similar function in these animals and so they are less dependent on their ceca.
An additional function of the cecum is to reabsorb water from the gastrointestinal tract, this job is performed by the colon in humans and carnivorous mammals.
Although it used to be believed that the appendix had no function and was an evolutionary relic, this is no longer thought to be true. Its greatest importance is the immunological function it provides in the developing embryo, but it continues to function even in the adult, although it’s not so important and we can live without it.
The function of the appendix appears to be to expose circulating immune cells to antigens from the bacteria and other organisms living in your gut. That helps your immune system to tell friend from foe and stops it from launching damaging attacks on bacteria that happily coexist with you.
There are other parts of the body that appear to do the same thing.
Peyer’s patches in the intestine help to expose your immune system to the usual contents of the intestine. By the time you are an adult, it seems your immune system has already learned to cope with the foreign substances in the gastrointestinal tract, so your appendix is no longer important.
But defects in these immune sampling areas may be involved in autoimmune diseases and intestine inflammation.
Interestingly, the appendix has been used as a personal “spare part” in surgery. It can be removed and its tissue used in reconstructive surgery of the bladder without risking the immune reaction that would be triggered by using tissue from another individual.
The true appendix is a worm-like, narrow extension beginning abruptly at the apex of the cecum.
It is present only in anthropoid apes, gibbons, orangutans, chimpanzees, and gorillas, a few rodents, rabbits and rats, and a few marsupials such as the wombat and the South American opossum. However, in many herbivorous mammals, the large appendix-like pouch of the cecum is an alternative site for fermentation of food. It contains microorganisms that break down cellulose in plant cell walls.
In humans, the appendix was thought to have no physiological function. However, it is now known to play a role in fetal immunity and in young adults.
During the early years of development, the appendix functions as a “lymphoid organ,” assisting with the maturation of B lymphocytes (a type of white blood cell) and in the production of immunoglobulin A antibodies. In addition, at around the eleventh week of fetal development, endocrine (hormone-producing) cells appear in the appendix.
These cells produce peptide hormones that control various biological mechanisms.