[hiisukun]

In addition to the appendix, my go to example of this is my friend's favourite organ: the Thymus [1]! If you've heard of "T-Cells" you indirectly know about it.

The interesting thing is that it is in a human when they are born, grows until puberty, then gets smaller and smaller until it can be quite small and difficult to detect in a grown adult.

I can imagine medical explorers cutting open dead 40 year olds in the year 1900, probably not finding any obvious organ there -- while perhaps cutting open dead children may have been a lot less common (and perhaps distasteful). If you did find something there, you would not assume an important organ present in a child and essential for their immune system would shrink and almost go away.

It would be more likely to be labelled nothing, an abnormal growth, or even a cause for death or illness (pressure on the heart/lungs!).

[dekhn]

The thymus is amazing... I ignored it for a long time then saw an amazing seminar. The thymus plays an important role in training the immune system: it expresses cells similar to cells all over the body, and then "educates" T-cells to avoid attacking those. Failures of the thymus often lead to autoimmune disorders.

[bregma]

I dunno. Comparative anatomy was a thing then and the anatomist may even have been fond of sweetbreads. They were still popular on menus in Paris when I was there a few months ago. It's possible they were more notable for their absence in adult humans.

[twic]

Although, as it happens, the thymus was known at least as far back as the ancient Greeks:

https://pubmed.ncbi.nlm.nih.gov/29858845/

[dennis_jeeves2]

>my friend's favourite organ

Woody Allen's second favorite organ is the brain : https://www.youtube.com/watch?v=ngizj5FIcjo