The decline and disappearance of …… gland by the middle age is primary cause of ageing.

Thymus is a pyramidal shaped lymphoid organ situated in front of the heart in the upper part of sternum. Thymus is active in young ones but gradually becomes inconspicuous after sexual maturity. Hence, the decline and disappearance of this gland by the middle age is the primary causes of ageing.
Thymus is enveloped by a thin loose, fibrous connective tissue capsule. Septa extending inwards from the capsule, divide the two lobes of gland into a number of small lobules. Each lobule is distinguished into a cortical parenchyma containing numerous lymphocytes and a medullary mass of large irregularly branched and interconnected epithelial cells (reticular cells).