The structure indicated is the great cardiac vein.

The coronary sinus lies posteriorly on the heart in the coronary sulcus and receives four cardiac veins:

• Great cardiac vein
• Middle cardiac vein
• Small cardiac vein
• Posterior cardiac veins

The great cardiac vein lies in the anterior interventricular sulcus together with the anterior interventricular artery (left anterior descending), and for this reason it is sometimes referred to as the anterior interventricular vein.

The great cardiac vein begins at the apex of the heart and ascends in the anterior interventricular sulcus to reach the coronary sulcus, where it then runs along with the circumflex artery to the posterior aspect of the heart to drain into the coronary sinus.

Learn more about the anatomy of the venous drainage of the heart in this tutorial.