The visceral layer, or epicardium, covers the heart and the great vessels, and from the latter is continuous with the parietal layer which lines the fibrous pericardium. The portion which covers the vessels is arranged in the form of two tubes. The aorta and pulmonary artery are enclosed in one tube, the arterial mesocardium. The superior and inferior venæ cavæ and the four pulmonary veins are enclosed in a second tube, the venous mesocardium, the attachment of which to the parietal layer presents the shape of an inverted U. The cul-de-sac enclosed between the limbs of the U lies behind the left atrium and is known as the oblique sinus, while the passage between the venous and arterial mesocardia—i.e., between the aorta and pulmonary artery in front and the atria behind—is termed the transverse sinus.