smallest cardiac veins
Thebesian veins

The smallest cardiac veins drain directly into the heart chambers.

Additionally, some of the flow through the smallest cardiac veins empty back into the left heart directly.

The openings of smallest cardiac veins (foramina venarum minimarum or foramina Thebesii) are located in the heart.

Here the smallest cardiac veins returns blood into the heart chambers from the capillary bed in the muscular cardiac wall, enabling a form of collateral circulation unique to the heart.

The smallest cardiac veins or Thebesian veins or veins of Thebesius are minute valveless veins in the walls of all four heart chambers and they are often confused with the distinct set of arterial connections, the "vessels of Wearn".