INTRODUCTION: the thymus is a two flask-shaped lobed structure located in the anterior part of the superior mediastinum. It is very prominent in children.
EXTENT: it extends from the 4th  costal cartilage to the lower pole of the thyroid gland.
RELATIONS:
Anterior: pretracheal fascia, the sternohyoid and sternothyroid muscle, the manubrium and superior part of the body of the stenum and adjacent costal cartilages.
Each side: these are overlapped by the pleura
posterior: pericardium, aortic arch and branches (brachiocephalic artery, left common carotid and left subclavian arteries), the left brachiocephalic vein and the trachea.
STRUCTURE: the lobus of the thymusis divided into an outer cortex and an inner medulla.
Cortex: dense and continuous
medulla: pale and contains the characteristic thymic corpuscles of Vassal
ARTERY SUPPLY: from the inferior thyroid artery, anterior intercostal and internal thoracic arteries.
VENOUS DRAINAGE: veins drain into the brachiocephalic veins, internal thoracic and inferior thyroid veins.
LYMPH DRAINAGE: lymph vessels of the thymus drain into the parasternal, tracheobronchal and brachiocephalic nodes. The thymus does not receive any afferent lymphatics.
NERVE SUPPLY: sympathetic fibres entering the blood vessels are vasomotor and there are vagal fibres also.
CLINICAL CORRELATES: Benign tumors from thymic tissues (thymona) may be responsible for retrosternal pain, coughing and dyspnea (shortness of breath) due to pressure on the trachea.