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plural thymi
(anatomy, immunology) A ductless gland, consisting mainly of lymphatic tissue, located behind the top of the breastbone. It is most active during puberty, after which it shrinks in size. It plays an important role in the development of the immune system and produces lymphocytes. quotations
For a very long time the role of the thymus in the body was a complete mystery.
2019, Bill Bryson, The Body: A Guide for Occupants, Black Swan (2020), page 234