Central Tendon of Diaphragm
The structure indicated is the central tendon of the diaphragm.
The diaphragm is a musculotendinous structure which seals the inferior thoracic aperture and separates the abdominal cavity from the thoracic cavity. When it contracts, it flattens, increasing the intrathoracic volume and thereby allowing air to enter the lungs.
The central tendon is an aponeurosis which forms the top of the dome-shaped diaphragm. It blends with the fibrous pericardium above, helping to maintain it in place. The inferior vena cava passes through the central tendon at the level of the 8th vertebra.