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thymus

(Science: anatomy) The lymphoid organ in which t lymphocytes are educated, mature and multiply. It is composed of stroma (thymic epithelium) and lymphocytes, almost entirely of the t-cell lineage. In mammals the thymus is just anterior to the heart within the rib cage, in other vertebrates in rather undefined regions of the neck or within the gill chamber in teleost fish. The thymus regresses as the animal matures.