Transverse Sinus

A passage between the two sites of reflected serous pericardium is the transverse pericardial sinus. This sinus lies posterior to the ascending aorta and the pulmonary trunk, anterior to the superior vena cava, and superiorly to the left atrium. It opens laterally into pericardial cavity both side.
It is a space which separates basically venous end and arterial end of heart and used for clamping of those great vessels during heart surgery.