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It does not seem to have much digestive function, if any, and is not present in all herbivores, even those with large caeca. [13] As shown in the accompanying pictures, the human appendix typically is about comparable to that of the rabbit's in size, though the caecum is reduced to a single bulge where the ileum empties into the colon. [7]
The human appendix averages 9 cm (3.5 in) in length, ranging from 5 to 35 cm (2.0 to 13.8 in). The diameter of the appendix is 6 mm (0.24 in), and more than 6 mm (0.24 in) is considered a thickened or inflamed appendix. The longest appendix ever removed was 26 cm (10 in) long. [3]
The human caecum is vestigial, as often is the case in omnivores, being reduced to a single chamber receiving the content of the ileum into the colon. The ancestral caecum would have been a large, blind diverticulum in which resistant plant material such as cellulose would have been fermented in preparation for absorption in the colon.
The appendix, long misunderstood as a vestigial organ, is now recognized as a key MALT structure, playing an essential role in B-lymphocyte-mediated immune responses, hosting extrathymically derived T-lymphocytes, regulating pathogens through its lymphatic vessels, and potentially producing early defenses against diseases. [1]
If a structure was once called vestigial, but is now known to be functional, then calling it "Functioning Vestigial Structures" violates the definition of vestigial, because by definition, it is not vestigial. It was previously called vestigial, but that was a mistake. It is not correct to call the vermiform appendix vestigial.
Vestigial structures are anatomical structures of organisms in a species which are considered to have lost much or all of their original function through evolution. [8] These body parts can be classed as additional to the required functioning of the body. In human anatomy, the vermiform appendix is sometimes classed as a vestigial remnant.
Presumably that consensus is still evolving and both views should be represented, but the current state of the field in 2015 is that the appendix is not vestigial. The human horripilation response (goose bumps) is another example that is frequently repeated in the media, but is actually rather controversial, and you don't need research to ...
Although structures called vestigial often appear functionless, a vestigial structure may retain lesser functions or develop minor new ones." I think the connection made between appendix and vestigial organs is therefore wrong, because vestigial organs can have function and still remain vestigial.