The thymus is a small, soft, bilobed gland located in the upper front part of the chest, specifically in the anterior superior mediastinum. It lies behind the breastbone (sternum), between the lungs, just in front of and above the heart. The thymus extends from below the thyroid gland in the neck down to around the level of the fourth rib. It is positioned anterior to the ascending aorta and rests on the pericardium, separated from major blood vessels by fascia

. In children, the thymus is relatively large and pinkish-gray, but it gradually decreases in size after puberty, being replaced by fatty tissue in adults

. The gland is involved in the development and maturation of T lymphocytes (T cells), which are critical for the immune system