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 mesocardiabi.e. , between the aorta and pulmonary artery in front and the superior vena cava behindbis 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. [ http://en.wikipedia.org/wiki/Pericardial_sinus ]