Middle cardiac vein

Where does the middle cardiac vein run?

It begins at the apex, on the back of the heart, and runs up the posterior interventricular sulcus. (StatPearls, Heart Veins)

It joins the coronary sinus after the posterior vein of the left ventricle and before the small cardiac vein. (StatPearls, Coronary Sinus)

What does the middle cardiac vein drain?

It drains the territory of the posterior interventricular artery, the interventricular septum and parts of both ventricles, and empties that blood into the coronary sinus. (OpenStax A&P 2e, 19.1; StatPearls, Heart Veins)

It has a valve where it joins the coronary sinus, as the tributaries other than the oblique vein of the left atrium do, though the form of these valves varies. (StatPearls, Coronary Sinus)

Which vessels lie near the middle cardiac vein?

The posterior vein of the left ventricle runs between it and the great cardiac vein. (StatPearls, Heart Veins)

Where the sources differ

The sources disagree on where it ends. OpenStax lists it as a branch the great cardiac vein receives; the StatPearls articles on the coronary sinus and on the heart veins have it emptying into the coronary sinus. (OpenStax A&P 2e, 19.1; StatPearls, Coronary Sinus and Heart Veins)