The smallest cardiac veins or Thebesian veins or veins of Thebesius or vessels of Thebesius or vasa Thebesii are minute valveless veins in the walls of all four heart chambers. The veins are frequently confused with arterial fistula and a distinct set of artery connections eponymously referred to as the "vessels of Wearn". The Thebesian veins are reportedly most abundant in the right atrium and least in the left ventricle. They drain the myocardium and pass through the endocardial layer to empty mostly into the right atrium, but a few empty into the ventricles. This is different from most cardiac veins, which normally empty into the cardiac sinus, which then empties into the right atrium. The Thebesian venous network is considered an alternative (secondary) pathway of venous drainage of the myocardium.
The endocardial openings of the heart chambers that connect to the Thebesian veins are called the openings of smallest cardiac veins. Not every endocardial opening connects to the Thebesian veins as some connect to the vessels of Wearn which are arteries. Therefore, the endocardial opening must be traced to a vein before it is definitively called an opening of the smallest cardiac veins.
^Snodgrass, Brett Thomas (1 July 2012). "Vessels Described by Thebesius and Pratt Are Distinct From Those Described by Vieussens and Wearn". The American Journal of Cardiology110 (1): 160. doi:10.1016/j.amjcard.2012.04.005. PMID22704295.