Openings of smallest cardiac veins

The openings of the smallest cardiac veins (venae cordis minimae or Thebesian veins in humans) are small orifices that open directly into all four cardiac chambers, draining the inner third of the myocardium. 

These tiny venous channels represent a unique component of the cardiac venous drainage system, distinct from the coronary sinus and right cardiac veins. The smallest cardiac veins drain blood from the myocardium directly into the cardiac chambers without first passing through larger venous tributaries. Their openings are most prevalent in the right atrium and right ventricle, though they are present in all four chambers.

The foramina Thebesii serve as direct communications between the coronary circulation and the cardiac chambers. These vessels can establish connections between coronary arteries and cardiac chambers (arterioluminal vessels) as well as between coronary veins and cardiac chambers (venoluminal vessels).