The thymus is a flattened, bilobed structure lying between the sternum and the pericardium in the anterior mediastinum. In the newborn infant, it reaches its largest size relative to the size of the body, at which time it may extend up through the superior mediastinum in front of the great vessels into the root of the neck.
The thymus continues to grow until puberty but thereafter undergoes involution. It has a pink, lobulated appearance and is the site for development of T (thymic) lymphocytes.