Question: what two veins form the superior vena cava?
Answer: On each side, the subclavian vein, the principal vein of the upper extremity, joins with the internal jugular, the principal vein of the head and neck, here, behind the medial end of the clavicle, forming the brachiocephalic vein. The two brachiocephalic veins enter the thorax and unite, forming the superior vena cava.