The Appendix.

The appendix is a sac-like structure lying along our digestive tract (specifically located at the junction between the small and large intestines). Although it's function is debatable and largely unknown, it presumed to have once had a role in our immune system because it is full of lymphoid tissue (which is found in all other immune system-related organs/structures in the body). It is thought that the appendix used to contain bacteria to help humans digest plant matter (remember: we can't eat grass because of their cellulose (beta 1-4 linkages) content). Perhaps the appendix was a more frequently used organ when we were primates, feasting on leaves and such.

Image source here.

Often people have their appendix removed (called an appendectomy) when they are diagnosed with acute appendicitis. This is when the appendix is inflamed and the patient experiences a great deal of abdominal pain.  The pain starts vaguely/diffusely around the belly button, and then moves to a particular point between the belly button and the right hip bone (this halfway point is called McBurney's Point). One of the key questions to ask a patient with hypothesized appendicitis is whether or not the pain moved. Appendicitis is considered a medical emergency because without treatment it can lead to shock if the appendix were to rupture.