thymus, pl. thymi thymuses (thI´mus, thI´mI)

1. [NA] A primary lymphoid organ, located in the superior mediastinum and lower part of the neck, that is necessary in early life for the normal development of immunological function. It reaches its greatest relative weight shortly after birth and its greatest absolute weight at puberty; it then begins to involute, and much of the lymphoid tissue is replaced by fat. The t. consists of two irregularly shaped parts united by a connective tissue capsule. Each part is partially subdivided by connective tissue septa into lobules, 0.5 to 2 mm in diameter, which consist of an inner medullary portion, continuous with the medullae of adjacent lobules, and an outer cortical portion. It is supplied by the inferior thyroid and internal thoracic arteries, and its nerves are derived from the vagus and sympathetic nerves. 2. The t. of the calf or lamb.thymus gland; [G. thymos, excrescence, sweetbread]


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