Appendix
From Memory Alpha, the free Star Trek reference
The vermiform appendix was a dead-end tube on the large intestine of Humans. Largely useless, many people have them removed in operations called appendectomies, usually in treatment of appendicitis. Elias Sandoval had such an operation, though his appendix grew back when he was exposed to the Omicron spores in 2264. (TOS: "This Side of Paradise")
