Posterior wall of the pericardial sac, showing the lines of reflection of the serous pericardium on the great vessels. (Transverse sinus labeled at center. Oblique sinus not labeled, but visible below transverse sinus between the right and left pulmonary veins)
There are two Pericardial sinuses: transverse and oblique.
The cul-de-sac enclosed between the limbs of the inverted U of the venous mesocardium lies behind the left atrium and is known as the oblique sinus.
The passage between the venous and arterial mesocardia—i.e., between the aorta and pulmonary artery in front and the superior vena cava behind—is termed the transverse sinus. Also, the sinus that forms in the pericardial cavity where the dorso-mesentary pericardium reside.
Can be used to pass ligature during cardiac surgery.