The thymus gland is part of the immune system. It is composed of lymphoid tissue and epithelial cells. It consists of two irregularly shaped lobes united by a connective tissue capsule. Each lobe is subdivided by connective tissue septa into lobules, which consist of an inner medullary zone, continuous with the medullae of adjacent lobules, and an outer cortical zone. The cortex is rich in lymphocytes (thymocytes) and the medulla in epithelial cells. This gland is supplied by the inferior thyroid and internal thoracic arteries, and its nerves are derived from the vagus and sympathetic nerves. The thymus is the site of T-lymphocyte production, necessary in early life for the normal development of immunological function. The thymus reaches its greatest relative weight shortly after birth and its greatest absolute weight and maximal development at puberty. It then begins to involute resulting in a slow decline of immune function throughout adulthood.