Middle cardiac vein

The middle cardiac vein follows the posterior interventricular sulcus and drains into the coronary sinus. Its course is associated with the posterior descending artery on the posterior/inferior surface of the heart.^[ref] It is a cardiac vein, carrying return from heart tissue rather than serving as an outlet from a ventricular cavity.

For specimen orientation, first identify the posterior interventricular sulcus, then trace the middle cardiac vein toward its junction with the coronary sinus. Tracing the neighboring posterior descending artery instead leads back toward an arterial source. The same surface corridor thus supports two different paths through the coronary circulation.

Connections can also cross the simplified boundaries of a schematic drainage map. A primary dissection study reported an approximately 1 mm connection between the anterior and posterior interventricular venous routes in $19\%$ of its 37 specimens.^[ref] This observation illustrates a network property: drawing separate named tributaries does not prove that they are isolated tubes. For an oxygen-balance experiment using a venous sample, the actual drainage connections determine which tissue contributions can reach the sample. Neither the vessel's name nor its surface position alone specifies an exact sampled tissue volume.